Hey there fellow Fiero Fanatics. I have a friend here in El Paso that has an 88 GT with a 4.9 in it. Recently it started to give him fits of overheating. Here's everything he has done so far.

Pulled the radiator and had it rotted and cleaned. Flushed out the coolant pipes and even ran a snake through them just to be sure there was nothing stuck in there. Flushed through the motor and had good flow. He put it all back together and here is an excerpt from the email he sent me today.

"OK, I reinstalled everything and filled the radiator and left pipe and reattached it to the t-stat housing. Then filled the Tpipe on the heater hose till it was full and capped it. I fired it up and let it run till I had coolant flowing. This time I had a hot left pipe and awhile later I had hot water in the right pipe. The t-stat is functoning since I can feel the heat and cooling off at the radiator with the fan on. It ran at normal idle for about 30 mins with the fan on and deck and hood open. It finally hurled and filled the overflow tank. I had just checked the radiator and the right tank was not very hot and the right pipe was hot but I could touch it for awhile so it wasn't superheated. So, WTF is going on? I'm thinking it's developed a bad head gasket and overpressuring the system and here's why. When I fire it up it runs nice and smooth till it get's hot and just before the t-stat opens I can hear the idle deteriorate and begin to sound like misfires, it comes and goes but mostly is sounding like misfires."

Does this sound like he's on track with the head gasket? I am pretty sure he knows how to and has been succesful in "bleeding" the system. The water pump is new - at least it was new when he installed the motor - maybe less than 2 or 3k on the install? I think it's a 180* t stat and it is working. Sounds like he has coolant flow. I am asking those who have gone before us as we have little to no 4.9 experience. Thanks in advance for your response.

Pat

Hi Pat,

Wish I could help, I have an 88 4.9. It sounds like he has done everything I would have done short of trying to run it without a thermostat to see what happens. I think he might be on track with the head gasket. There are several posts on the cadillac forum that mention this. If it was a HG wouldn't he be blowing some white smoke? Suggest he go through that forum for ideas as well.

Fiero or 4.9 t-stat? With a fiero T-stat you can keep in out to make sure everything is fully burped. Following standard fiero procedure its very easy.

With the 4.9 T-stat it get more complicated. First you need to drill a 1/8" hole in the outer ring of the Tstat to allow some coolant/air to flow past when its closed. OR alternately pick up a high flow T-stat. They contain a small check valve that allows some flow when closed (I can provide a part number if needed). Without some type of bypass there it will create a steam bubble in the engine particularly right at the CTS. The steam/air bubble gives false, uneven, readings of coolant temp and can cause the idle problems. The only reason this is a problem in a fiero is cause the radiator is lower then the engine.

The idle issues when warm indicate, to me, a problem near the T-stat/CTS. Since they are located in the same housing the location of the CTS is the highest point in the engine. This is where the air/steam collects if the T-stat completely blocks flow when closed. And since the 4.9 t-stat makes the seal it can't be removed to burp the system. The small hole in the outer ring allows it to pass and go to the overflow bottle.

Also sounds like it could be burped a bit better to me. I have never found any reason to fill the hoses themselves. The one time I tried it I ended up with more air in the system.

Fill from the back with radiator cap off. As soon as coolant starts to come out the radiator cap it and continue filling. Once full cap and start engine for 15-20 seconds with heater switched to HOT. This will purge the heater core lines. Shut off and top off again. Repeat until no more coolant can go in. Then let warm to full temp T-stat opening at least twice (it will open once then almost immediately close again). Fill overflow bottle to max mark then shut off engine. Let cool completely then check coolant level one more time. This is what works best for me.

I fought cooling issues for almost a year in my 4.9. Ended up being the lack of a relief hole in the t-stat and the pinhole leak in the radiator. Instead of sucking the coolant back in from the overflow bottle it would suck air through the tiny hole in the radiator. This air would gather near the CTS and caused many problems for me. It would also cause the overflow bottle to "overflow" spilling the coolant on the ground almost evertime I shut the car off. It was very frusterating, and in retrospect a little funny. It really was an "overflow bottle" for a while. It would overflow constantly!! Sorry for the long winded post. If he's useing a fiero t-stat this entire post is useless. Hope it is helpful either way.

EDIT: Almost forgot. Have him check the heater core also for blockage. When the T-stat is closed its the only place for the coolant to still flow. All the pressure from the waterpump goes there.

[This message has been edited by Fieroseverywhere (edited 04-06-2010).]

Fieroseverywhere, I just had to give you a + for the post. Long, but very informative. Also it makes sense on ANY swap that doesn't use the Fiero thermostat.

------------------
-Brian

My 87 GT Poly Suspension Upgrade (all pics) thread https://www.fiero.nl/forum/Forum2/HTML/094633.html

Removing the roof panel
https://www.fiero.nl/forum/Forum2/HTML/106413.html

quote Originally posted by MulletproofMonk: Fieroseverywhere, I just had to give you a + for the post. Long, but very informative. Also it makes sense on ANY swap that doesn't use the Fiero thermostat.

Thanks! I try to make my posts short but I guess I just talk...er.. type too much!

If it was a blown head gasket (coolant into combustion chamber), then as soon as the engine starts there would be pressure on the coolant system vs. building pressure as the coolant becomes hot. It would also produce alot of white smoke.

The coolant will take the path of least resistance which may or may not be through the radiator. Obvoiusly some it making its way through the radiator, but is is 50/50 with the heater core vs. something like 95/5 (95% to radiator). The heater core is the coolant bypass when the thermostat is closed and on several occasions I have seen it cause an overheating condition. Easiest way to check, let it idle and get above the thermostat temp and then take some vice grips and pinch the heater core line shut and see what happens. If there is no change, it is something else. If it starts to cool down, you need to put a restrictor in the heater core lines (many GM vehicles come with them now - the 1999 4.3 I am working on has one).

A non-contact inferred heat gun is a handy tool to check temps of hoses and such to help troubleshoot cooling issues.

[This message has been edited by fieroguru (edited 04-06-2010).]

quote Originally posted by Fieroseverywhere: Once full cap and start engine for 15-20 seconds with heater switched to HOT. This will purge the heater core lines.

The heater switch does not control water flow through the heater core - the water will flow through the core regardless of the control setting (the control just opens/closes the air damper).

When does it over heat? While at idle, highway driving, city driving. Does the fan turn on? Is the front rad cap good? Does the car have the front air dam on (or was it removed)?

Try a new radiator cap.
Good Luck
Gary

When you did the conversion did you use Dexcool or regular stuff?
Switching causes issues to come up. With me it seems to run fine then the thing would over-flow from too much combustion pressure getting into the coolant system. Eventually it will only take 5 minute, not the 30 it does now.

Any 4.9 swap should involve a re-gasketing of the heads and intake and abandonning dexcool with a quickness.

The least resistence is the over-flow tank. Combustion pressure would force coolant out of the combustion chamber back into the coolant system along with exhaust, hence the pressure build up. White smoke would only be on startup until all residual coolant was burned or pushed back into the coolant system.

That's what seems to have happened to me...

[This message has been edited by lou_dias (edited 04-06-2010).]

Thanks to everyone for your replies plusses to all - more for Fieroseverywhere if I could. I'll send him a link to this thread. I am pretty sure he is using standard anti freeze and I know for sure that he is using the 4.9 T-Stat. Thanks again everyone - I think we just may have the answer here.

Pat

Mine used to puke coolant out shortly after I shut it down.

I ended up doing several things....
Replaced the cap.
Installed a 180* thermostat.
Drilled a small hole in the thermostat to allow trapped air to escape.

i'm not sure which one fixed it. I'm guessing the cap.

BUT... I had a blown head gasket on my 3.4.
It would easily idle for 1/2 hour without losing any coolant. Never had any "run" issues.
I could drive it "easy" and not have any issues.
As soon as I accelerated even moderately hard, the combustion pressure would blow the coolant out through the overflow bottle.

I used my air compressor and the hose from my compression tester to do a quick test.
I turned the crank to TDC on each cylinder, then applied ~100 lbs of air. (Don't worry. Compression is ~140, so you're not going to hurt anything that isn't already hurt.)
When I hit the leaky cylinder, I got bubbling from the coolant. (Remove the thermostat and cap for this test. If it bubbles, you'll hear it.)

------------------
Raydar
88 4.9 Formula IMSA Fasback..........................88 3.4 coupe -soon to be something other than red

Read Nealz Nuze! Praise the Lowered!

Thanks Raydar! Every tip helps. Here's a little more background from the owner. Thanks again everyone for the help.

"Thanks everyone for the good info, it helps to get some other perspectives and ideas to try out concerning this problem. So you all will know the car's history I'll give you some helpful details to go on.

Since the V8 swap was done the car has had episodes of overheating. The weird thing about it is that I've been able to drive it in town and on short trips of about 70 miles and it has behaved just fine, we did one of those 2 weeks ago with no issues. Then for no reason it'll hurl coolant after just a short drive. It never has any white vapor out the exhaust other than the normal winter warm up and it goes away shortly. I know what water getting into the exhaust system in quantity looks like but this is a normal exhaust.

The radiator is clean and pipes are unobstructed and hoses are not collapsing. A little over a week ago I drove it on a 35 mile run and it did fine till I pulled in the driveway and shut it off. It overflowed the bottle but showed no signs of getting hot on the drive. The radiator cap is good (almost new) and factory style not a lever action type. There are no external leaks anywhere in the system. I had a 180 T-stat in it but the other 4.9 we did on a friends car is fine and he's getting a bit better mileage so I put the 195 in it (brand new) since that's what is in his. It seemed fine while driving it. The coolant wasn't fresh so I flushed the system till it ran clear and drained it. I filled it back up with green coolant, I never use the DEX JUNK in anything.

I backed the car up on ramps to do the filling so I had the T-stat housing as high as possible and filled it till it just ran out of the radiator and then capped it up. I removed the upper hose and filled it to capacity, I've never tried this method before and it did help it to fill faster that way over the T-flush. I put it back on the housing and then opened the T-flush on the heater hose off the housing and filled it the rest of the way. Then I fired it for about 20 seconds and added more to the T until it was filled as much as possible. Then I fired it up and let it idle till the T-stat opened and the top hose flashed hot. I then checked the left side pipe to the radiator and it was hot all the way forward. The right side of the radiator was cold, cap was cold so I turned on the fan and waited a little longer and then checked the right side pipe and it had nearly hot water in it all the way back to the engine so I know the system was operating as it should. I could tell the coolant was flowing from the cycles of hot and then cool air from the fan blast so the radiator was working fine. All hoses were hot on the engine and the heater core is ok and hot on both sides. I let it idle with the hood and engine cover open for about 20-30 min's. Then I saw a geyser from the overflow bottle and I shut it down just shortly after I'd checked the fan blast to see how hot it felt and it was in a cooling cycle at that time. No typical percolating noises of overheating from the engine or hot smells or run on at shutdown.

Back when it was a 3.2 I had drilled a 1/8" hole in the T-stat but I didn't do that on the 4.9.....just forgot to I guess. Maybe the head gaskets are ok and I just have a giant air bubble trapped behind the T-stat side of the housing that becomes superheated and blows the system limit of 16 lbs. When it's running fine I'm not losing any coolant or having to add more all the time and on shutdown it's tight. I put the GM coolant pellets in the system, they came with the kit I bought from Ed. Yesterday I bought a bottle of Bar's Leaks copper liquid permanent cooling system block seal intake and radiator stop leak but I haven't put it in yet as I want to see what you all think at this point about this problem.

Why can I drive it a long distance one day and then I cant go 3 miles without a problem the next? I had the radiator done because it had never been serviced while I've owned it. When I checked the temp in the side pipes a week ago I found the left was hot and the right was all cold. Now I'm thinking I just didn't give it enough time to circulate fully and in any case I still have the problem. I also had removed the cradle hoses during the flush and everything was open, I had water coming out the water pump and side pipes and then ran a fish tape through the side pipes and there's nothing blocking them, the side pipes have no damage anywhere. The water pump was brand new and is the good one with the metal impeller and I'm not using the Fiero T-stat housing pipe, just the factory 4.9 system.

The overheating episodes have been while driveing or in the driveway so it doesn't seem to make any difference either way nor with the 180 or the 195 T-stat. So, now what? Load my clips for a live fire exercise? The green stuff is getting to be costly as I've gone through so much of it. I hope a solution is in sight before I go postal on the car. I'm going to drill the T-stat for sure and clock it at 12 so all air can get out. Thanks again for the help! "

Pat

quote Originally posted by katatak: Pat

hey pat, did i do your chip for you, i dont remember....? if i did, it was designed to be used with a 180 t-stat. using the 195 will cause your fans to be running all the time and never shut off. also, is the radiator a 3 core or a two core? obviously 3 core would be the better choice. i think you just have air in the system, either its not entirely being bled right, OR you have a loose line that is sucking in air...BUT you could have a head gasket leak too.. get some bar's leak head gasket sealer, not the copper radiator stuff, and give that a shot.

another thought: is the water pump working properly? you tested your flow with the hoses off and the t-stats removed, yes?

check for leaks at the heater core as well, it could be a source of the air coming into the system.....

one last thing... possible bad coolant temperature sensor? maybe you arent actually overheating?

[This message has been edited by stickpony (edited 04-07-2010).]

Pat,

If the drilled thermostat doesn't fix it, I'm kind of at a loss.
It's really strange.

But then again, my blown 3.4 head gasket was really strange too.

If it's not air trapped in the system, I'm still thinking it might be a head gasket.
Are you certain the coolant hoses aren't collapsing anywhere?

Just for grins, you might try hooking up a scan tool if you have one.
Pay attention to what the lean/rich flags are doing.
If it suddenly goes "full lean" it might make it overheat.
This is, admittedly, far-fetched as hell. But when you've done all the easy stuff...

The only thing that I can think of at the moment why it will let you go on a long trip one day and then only a short the next is that maybe you have an issue with your fan with it working sometimes and not the others - loose conntector?

Nevered answered - so I will assume the air dam is still ON the car - the 4.9 WILL overheat with the air dam missing - but not always. I found with my car I could sometimes drive all day with no issues and then the odd time it would overheat - problem solved when I put the air dam back on. I have also had the fan not working issue with mine just this year after it's winter sleep (looks like mine was a bad EPROM though, the car is fine with a spare EPROM that I have).

This sound like exactly the problem I was having. Mine ended up being a pinhole leak in the radiator. This hole was small enough that the little coolant that leaked out while the car was running would evaporate before hitting the ground. I never saw a single drop and it held pressure well when tested. However this same hole allowed air to be sucked in when the car was shut off ( from the vacuume created while the car was off and cooling down). This would force the coolant into the overflow and allow the air bubble to gather in the engine compounding the problem. As soon as the car is shut off there is also an expansion of coolant as the remaining heat is transfered to the coolant. Making matters worse yet again. I found the leak by starting the car and, quite literally, laying down under the radiator and watching it for an entire cooling cycle.

The leak was tiny and located near the top of the radiator, opposite side as the cap, back side of the radiator, at the joint of center section and end tank. Because of its obscure location and small size there were no real indications that it was there. Once I layed there though I actually SAW it start to leak. This corrected the problem completely for me and I have not lost any coolant out of the overflow since. I took a very long time to find it and had I just replaced the radiator from the start I never would have had a problem. I probably would have spent less then I did on coolant over that year.

The thing is it could do the same for a small leak anywhere, radiator, cap, and any gasket or hose in the whole system. Air fits through smaller holes then liquid does so even if you never see anything leaking it could still be sucking air.

Since his cap has been replaced have him double check that it is the correct type also. Many auto parts stores and catalogs list the incorrect ones for a fiero. Check the Ogre's cave (powertrain > cooling > radiator caps) for a complete explaination. This would also cause the same problem. EDIT: MUST BE A NON-VENTED CAP!!! Don't use the vented cap most places recommend. (Thanks to Mr Moose for pointing that out. )

Another long post....

EDIT: Going to add a little more detail...
When the car is shut off there is a period where the coolant expands more. It is from the remaining heat of the engine transfering to the coolant. It pushes a bit more coolant to the overflow. When it cools enough and starts to create the vacuume it would attempt to suck the coolant back in but was sucking in air instead. The next start up would start the process over again. Basically it would overflow every 2-3 starts because it took that many for enough air to replace the coolant to cause it to overflow. Made it even harder to track down, inconsistant, and more frusterating. Longer drives seemed to make it better. I suspect because the T-stat opening allowed some air to go into the overflow. The shorter the trips the quicker it overflowed. For me it was this short period of heat soak that actually pushed the coolant out. The overheating while driving was just a side effect of the air getting into the system. Topping off the coolant before every drive helped but did not fix the problem.

[This message has been edited by Fieroseverywhere (edited 04-08-2010).]

quote Originally posted by Fieroseverywhere: Here is the part numbers for the "High Flow T-stat". I prefer these to drilling the hole in a standard t-stat. Main reason being instead of just a hole that is always open it contains a small check valve. When coolant is pushing against this check valve it acts like a standard t-stat and diverts all coolant to the heater core. This lets the car come up to temp faster. When air is behind the check valve it opens and acts like a drilled t-stat letting the air through. You get the best of both worlds and don't have to modify anything. MotoRad part numbers.... 2000-180 (180 degree) 2000-195 (195 degree) I can post some pics tonight I took of the one I bought. The motorad site does not have a pic. I highly recommend it for any 4.9 swap using a 4.9 t-stat. Since it has a larger opening the temps drop much faster also. Its a bonus in every aspect.

I had trouble locating where to buy these, at least in Colorado. Found it at Checkers (O'Rielly?) Auto under the name "Murray". In the rather long description of their Ultrastat Thermostats it says it allows air to escape AND lists the name Motorad Fail-Safe Thermostats. $7.99 - Here's a link:

http://www.oreillyauto.com/...2&pt=03442&ppt=C0331

quote Originally posted by Fieroseverywhere: Since his cap has been replaced have him double check that it is the correct type also. Many auto parts stores and catalogs list the incorrect ones for a fiero. Check the Ogre's cave (powertrain > cooling > radiator caps) for a complete explaination. This would also cause the same problem. MUST BE A NON-VENTED CAP!!!

FIXED...

Must be non-vented as the rad sits lower than the block and the coolant will drain out through the overflow bottle if the cap is vented. You only use a vented cap if the the cap is the highest part of a cooling system - a vented cap is open to atmosphere when cold.

This has been talked about many times, see Ogre's Cave for part numbers (he also says non-vented).

NOTE: all rad caps have the pressure valve in them, it is just the vented cap opens to atmosphere when coolant is cold and has no pressure (ie: self bleeds). The cap on the Fiero thermostat housing is just a cap - nothing more. ALL rad caps have the pressure valve as protection for overpressure - it what it does when the system is cold that defines the type of cap it is.

[This message has been edited by Mickey_Moose (edited 04-10-2010).]

quote Originally posted by Mickey_Moose: FIXED... Must be non-vented as the rad sits lower than the block and the coolant will drain out through the overflow bottle if the cap is vented. You only use a vented cap if the the cap is the highest part of a cooling system - a vented cap is open to atmosphere when cold. This has been talked about many times, see Ogre's Cave for part numbers (he also says non-vented). NOTE: all rad caps have the pressure valve in them, it is just the vented cap opens to atmosphere when coolant is cold and has no pressure (ie: self bleeds). The cap on the Fiero thermostat housing is just a cap - nothing more. ALL rad caps have the pressure valve as protection for overpressure - it what it does when the system is cold that defines the type of cap it is.

Crap!!! See how easy it is to mix them up!!! Thanks for the correction Tim. I corrected the above post also.

Thanks everyone for your replies. I will talk to the owner this weekend. Something I am wondering about is that instead of using the modified duke tstat housing, he is using the stock Caddy setup and has a flush "T" installed in the heater hose coming off the tstat housing. Do you think it's possible that it is sucking in air at this Tee? I have had toruble with these tees in other vehicles in the past. It is basically the highest point in the system on this GT. I would think that if it is sucking air, it should be leaking there too? If we figure it out, I will post what we find. Thanks again for all the help.

Pat

OK, I didn't read every reply but I scanned most of them and most of the information was good, accurate, and correct. I'm going to run down a list of things to check and make sure you eliminate all of them.

1) I drill two 1/8" holes in the thermostat to aid bleeding. It has little effect on the thermostat operation and helps a LOT getting the air out.
2) Get an aluminum "T" with a radiator cap neck on it to bleed out of. Put that near the thermostat housing.
3) The aluminum "T" will have a small 1/8" Pipe thread hole. I plug that with a 1/8" Pipe Thread Allen Plug (optional, really doesn't make much difference after the next item)
4) Get a racing 26 or 30 lb radiator cap for the "T". Do NOT use this on the radiator. We don't want the cap in the rear to open, every. That's the job for the front cap.
5) Make sure you have the proper cap from GM or Stant on the front.
6) When bleeding, it helps to have the back end jacked up enough that there is no doubt that your new fill point in the rear is the highest point in the cooling system.
7) Add coolant to the system with the front radiator cap OFF. Add coolant until the radiator is full, then put the cap on and leave it alone.
8) Fill the overflow tank to the full cold point NOW and make sure it stays full, especially the first few weeks.
9) Add coolant some more until you can't add anymore. Start the engine and run it for no more than a minute. Shut it off, wait a minute, and check level.
10) Repeat 9 as often as it takes to not be able to add anymore coolant, running just a minute or so at a time. (Probably 3 or 4 times.)
11) Start engine and watch your temps. Make certain you run it until the cooling fan comes on, then shut it off.
12) Let it cool off completely, monitor the overflow tank and keep it full. It WILL draw coolant into the system displacing more air as it cools.
13) When cool, check the coolant at the back again. If it isn't full, and I mean completely full, do steps 11-12 again. (this will also probably take a few times)
14) By this point, you should have the system nearly completely air free. If not, you're sucking air somewhere. Check your overflow hose (a common spot, draws air in with the water from the tank), your radiator hoses, and your heater hoses. Make sure they are all on and tight.
15) Make absolutely sure you have the proper size belt on the engine. If your tensioner isn't about 1/2 way on it's stroke, then you don't have enough tension. The water pump WILL SLIP on the belt if you don't maintain enough tension, especially if the alternator is working hard and the a/c is on. I can't stress this enough. I commonly hear that the 4.9 overheats when the a/c is on when it should actually be better in stop/go traffic because that forces the cooling fan on. If this is a symptom, CHECK THE BELT!

Best of luck to your friend.

John Stricker

Doesn't work. From experiance when parking on a hill with a vented cap the coolant leaks out. Without constant pressure there is nothing stopping it from flowing out the overflow unless the engine is running and the waterpump is creating the pressure. The coolant in the engine is higher then the overflow tank and the only way for it to find its level is to move to the overflow tank. The vacuume created while the car is cooling has no problem opening the valve on a non-vented cap and sucking the coolant back in. My experiance obviously differs from yours yet again. With all due respect.

If your not sure which one to try, try them both. They're cheap. Park on an icline, nose down, and see what happens. Only one will work properly.

[This message has been edited by Fieroseverywhere (edited 04-09-2010).]

No, you misunderstand.

There is an unvented cap.

There are TWO types of vented caps, but BOTH are vented.

The common vented cap has a disk in the center of the bottom that just hangs there, it relies on positive coolant system pressure to force it closed. With no pressure, there is an open hole to the overflow tank.
The less common vented cap has a spring loaded disk in the center that is ONLY open when there is negative pressure in the cooling system. (I don't remember how much, but it's less than 2" of vacuum IIRC). When the system cools and if there is air, it will allow coolant to be sucked in from the overflow.

A truely non-vented cap will have NO disk in the center, common on older cars, trucks, and industrial equipment that do NOT have an overflow tank at all.

It doesn't find a "level", it purges air and replaces it with coolant from the overflow tank and the tank is never pressurized. How does it purge air if it's not the lowest point? The air is normally (if the system is properly bled) not held in large pockets in the engine but is mixed in a type of froth in the cooling system, I'm sure you've seen it. If pressure is great enough to force anything out, it will move the air with the coolant into the tank, the air will float to the top, and as it cools it will draw in air free coolant. Once you have it properly bled, little will be forced out because it won't be running enough pressure to open the cap, but because air is a compressible fluid and coolant is not, it will draw a significant amount back in from the tank as the system makes negative pressure which, BTW, it will not do without SOME air in the system someplace.

Since the Fiero and most conversions don't have a pressure tank as a fill point, it's nearly impossible to get all the air out of it but the draw back will happen for some time after you've had the system open and bled it.

The Fiero has a vented cap, just not the commonly vented cap. It does NOT have an unvented cap. If you don't believe me, then try this test. Stick a piece of toothpick under that little disk and see what happens. If you don't want to do that then please, by all means, explain the purpose of that little spring loaded disk.

That disk is there for a reason, it's not a decoration. If the system didn't need a vent it would use an unvented cap, and it does not.

John Stricker

quote Originally posted by Fieroseverywhere: Doesn't work. From experiance when parking on a hill with a vented cap the coolant leaks out. Without constant pressure there is nothing stopping it from flowing out the overflow unless the engine is running and the waterpump is creating the pressure. The coolant in the engine is higher then the overflow tank and the only way for it to find its level is to move to the overflow tank. The vacuume created while the car is cooling has no problem opening the valve on a non-vented cap and sucking the coolant back in. My experiance obviously differs from yours yet again. With all due respect. If your not sure which one to try, try them both. They're cheap. Park on an icline, nose down, and see what happens. Only one will work properly.

Obviously there is a teminology misunderstanding here. I am not speaking of a truely unvented (notice I said unvented not non-vented) cap found on older cars, ect, that do not have an overflow tank. These use a true unvented cap because there is no where for the coolant to go. The cap is sealed tight under all situations except over pressure. Overpressure happens when the system pressure excedes the rated pressure of the cap. Under this situation even a turely unvented cap has a vent and blows off the excess pressure.

Vented vs Non-vented (not unvented) (as they are commonly called nowadays)

Most radiator caps for cars nowadays come in 2 varieties depending on how the system is setup. Technically, both are vented caps because both vent to the atmosphere under certain situations. However they are not referred to as such anymore because of this exact type of confusion it causes.

Non-vented
Disc type check valve cap that contains a spring. This spring keeps constant pressure on the check valve keeping it closed tight with or without pressure. Once negative pressure is applied the valve opens allowing coolant to be pulled from the overflow back into the radiator.

Vented
Also a disc check valve type of cap. This type of cap DO NOT contain the spring. Unless pressure is applied to the disc the valve remains open. When under normal operating conditions there is constant pressure on the cap keeping the valve closed. Without pressure the valve is always open.

Both of these caps will release excess pressure in the event of overheating/overpressure of the system. This has nothing to do with the disc valve. Thats a story for another day.

Liquid always tries to seek its level. Always. You can test this easily (Not you personally but as an example). Grab a clear section of hose and put some liquid in it. Raise and lower one end of the hose slowly. What happens? The water in both sides of the hose will always be level.

This is where the fiero cooling system has some issues. The coolant in the radiator and overflow tank are always lower then the most of the coolant in the engine. This is unavoidable because of the low hood line in a fiero and location of the radiator. This is also why a vented cap (disc valve not using a spring) does not work in a fiero. Without pressure behind it there is nothing keeping the coolant from flowing in a low flow situation. Being that liquid will always seek its level the coolant will flow to the overlfow tank until the level in the engine is even with it. The weight of the coolant itself will not always be enough to close the valve.

The non-vented (disc valve with spring) solves this problem and everything else functions the same. It will still push coolant to the overflow when the pressure is higher then the rating on the cap.

Most other cars have the radiator higher then the engine. This makes everything work better IMO. With the radiator cap high the cap can be non-vented (disc valve with no spring). The coolant in the system is already at its level so its not going anywhere.

I understand what you are saying. The problem is, it is nearly impossible to get a cooling system to seal completely. Even microscopic holes around gaskets, waterpump seal, hose clamps, ect can allow air to be sucked in during a negative pressure situation. Any air sucked in can push coolant out the other side. The non-vented cap keeps this coolant from flowing while the car is off.

EDIT: Reading your post again it sounds like we are arguing for the same cap. Hehe. Vented, non-vented, or unvented they're all just caps. unvented works for cars without overflow tanks. The others work with overflow. The internet at its finest.

[This message has been edited by Fieroseverywhere (edited 04-09-2010).]

quote Originally posted by Fieroseverywhere: I understand what you are saying. The problem is, it is nearly impossible to get a cooling system to seal completely. Even microscopic holes around gaskets, waterpump seal, hose clamps, ect can allow air to be sucked in during a negative pressure situation. Any air sucked in can push coolant out the other side. The non-vented cap keeps this coolant from flowing while the car is off. EDIT: Reading your post again it sounds like we are arguing for the same cap. Hehe. Vented, non-vented, or unvented they're all just caps. unvented works for cars without overflow tanks. The others work with overflow. The internet at its finest.

Exactly right. We do agree that it takes the same cap, the one with the spring loaded disk on the inside. I'm just trying to clarify that despite what it's called it DOES vent. Because it's nearly impossible to keep air out there MUST be a vent and in a system with the radiator lower than the engine, there MUST be a way for it to more easily draw COOLANT back into the system than it is to draw air back in. You're also correct that the reason it takes the spring loaded version is because the radiator and overflow tank are not the highest points in the system and without that spring the coolant WILL seek a level that will leave air pockets in the engine in back.

My argument isn't with you, MIckey, or Ogre as much as it is with the terminology itself. These are not unvented caps even though they are commonly called that. There ARE unvented caps, but these ain't it.

I think we're both on the same page now though. My concern is when we use this term and it's incorrect (even though commonly accepted) it makes people think that overflow tank is just that, an overflow and nothing else. The truth is keeping the tank full on a Fiero, before there was such a thing as a pressure tank to fill coolant and bleed the system with, is very, very important and people neglect that, then wonder how that air got in their system since they have an "unvented cap".

It's interesting to me that we've come full circle in cooling system designs. Most cars now have a system with a pressure tank to fill coolant and bleed the air from the system. Many don't even have fill points anywhere BUT there, and these tanks have only an overpressure system that will keep the hoses and tanks from rupturing, there is no "catch tank" beyond the pressure vessel that we commonly use now to fill coolant so we are in effect right back to the old days of a truly unvented system except there's an expansion tank at the highest point, IOW, a divorced radiator top tank.

John Stricker

Please note: in my post: I refered to "vented" as being the one that is open to atmosphere when cold vs the "non-vented" which is not open to atmosphere when cold. All rad caps for modern cars use the 2-way valve, I guess since the 'old' style does not really apply anymore people have started to refer to the 2 new types as vented and non-vented - to keep jstricker happy (no offense) , using what is the correct terminology, use a vented CLOSED system cap on the Fiero (vs a vented open system one).

...bottom line - use the part number posted on Ogre's web page and all is good, use the other one and coolant will piss out when parked...

[This message has been edited by Mickey_Moose (edited 04-10-2010).]

That's better, ya' big lug.

As I said, my biggest problem is with the terminology. I've had to go around with at least 3 different fiero owners that swore up and down it didn't matter if there was coolant in the overflow tank because their system was "unvented". I don't think I ever did convince one of them that they would indeed draw coolant back from the tank or air if they didn't keep it full (their tank was broken and would leak coolant). I wish the terminology was such that it actually did what it was called.

John Stricker

quote Originally posted by Mickey_Moose: Please note: in my post: I refered to "vented" as being the one that is open to atmosphere when cold vs the "non-vented" which is not open to atmosphere when cold. All rad caps for modern cars use the 2-way valve, I guess since the 'old' style does not really apply anymore people have started to refer to the 2 new types as vented and non-vented - to keep jstricker happy (no offense) , using what is the correct terminology, use a vented CLOSED system cap on the Fiero (vs a vented open system one). ...bottom line - use the part number posted on Ogre's web page and all is good, use the other one and coolant will piss out when parked...

quote Originally posted by jstricker: No, you misunderstand. It doesn't find a "level", it purges air and replaces it with coolant from the overflow tank and the tank is never pressurized. How does it purge air if it's not the lowest point? The air is normally (if the system is properly bled) not held in large pockets in the engine but is mixed in a type of froth in the cooling system, I'm sure you've seen it. If pressure is great enough to force anything out, it will move the air with the coolant into the tank, the air will float to the top, and as it cools it will draw in air free coolant. Once you have it properly bled, little will be forced out because it won't be running enough pressure to open the cap, but because air is a compressible fluid and coolant is not, it will draw a significant amount back in from the tank as the system makes negative pressure which, BTW, it will not do without SOME air in the system someplace. John Stricker

I would just like to say that air is not the only "reason" the cap purges. Water expands when heated. The more radiator hose you have the more difection there will be, or the more volume will be added thus the lack of purging. But IF the water gets hot enough to purge to the expansion tank it must come back when cooled and condenced. IF steem is produced it will take up vast amounts of voulume in the cooling system. This will cause much greater purging. That also must be stored and returned thru the cap.
The spring loaded disk cap was one of the first things I learned as a Fiero owner I had overheating problems very early in ownage.

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Rick,

Absolutely right. ANYTHING in the system that makes it compressible- air, rubber hoses, whatever, will cause the system to want to "breathe", so to speak.

John Stricker

quote Originally posted by Rickady88GT: I would just like to say that air is not the only "reason" the cap purges. Water expands when heated. The more radiator hose you have the more difection there will be, or the more volume will be added thus the lack of purging. But IF the water gets hot enough to purge to the expansion tank it must come back when cooled and condenced. IF steem is produced it will take up vast amounts of voulume in the cooling system. This will cause much greater purging. That also must be stored and returned thru the cap. The spring loaded disk cap was one of the first things I learned as a Fiero owner I had overheating problems very early in ownage.

Just wanted to let everyone know that it looks like my buddy got his overheating problem corrected - all thanks to everyone that posted replies. He drilled a hole in his T stat and while it was out, he filled he block. He said he was amazed how much coolant he was able to pour into the block, Once he had the block full, he installed the T Stat wth the new hole clocked to 12 oclock. He continued to fill through the hose and said that he still got air bubbles out of the T Stat. When the bubbles quit, He put the hose on and finished filling it through the "T". Once he got it nearly full, he let it set for a few hours while he ran some errands. Just about dark, he was able to get a little more coolant in it then he started it. He said it sat in his driveway idling for over an hour and never overheated. Thanks again to everyone that posted. It was a big help!

Pat

Cool. I love it when a plan comes together.

Do remind him to make CERTAIN his tensioner is about mid stroke. It's easy to check especially if the battery is out of there. I've had three different problem overheating conversions that have been fixed by simply putting a shorter belt on because the tensioner that appeared to be holding the belt tight really wasn't. The 4.9 seems to be particularly finicky about this.

John Stricker

quote Originally posted by katatak: Just wanted to let everyone know that it looks like my buddy got his overheating problem corrected - all thanks to everyone that posted replies. He drilled a hole in his T stat and while it was out, he filled he block. He said he was amazed how much coolant he was able to pour into the block, Once he had the block full, he installed the T Stat wth the new hole clocked to 12 oclock. He continued to fill through the hose and said that he still got air bubbles out of the T Stat. When the bubbles quit, He put the hose on and finished filling it through the "T". Once he got it nearly full, he let it set for a few hours while he ran some errands. Just about dark, he was able to get a little more coolant in it then he started it. He said it sat in his driveway idling for over an hour and never overheated. Thanks again to everyone that posted. It was a big help! Pat

quote Originally posted by jstricker: Cool. I love it when a plan comes together. Do remind him to make CERTAIN his tensioner is about mid stroke. It's easy to check especially if the battery is out of there. I've had three different problem overheating conversions that have been fixed by simply putting a shorter belt on because the tensioner that appeared to be holding the belt tight really wasn't. The 4.9 seems to be particularly finicky about this. John Stricker

Correct - that is why I prefer to install the a/c compressor (there is an a/c delete pulley available, but I am too cheap to buy it when the compressor is free with the motor) - I like having the extra wrap of the belt on the water pump.

quote Originally posted by jstricker: Cool. I love it when a plan comes together. Do remind him to make CERTAIN his tensioner is about mid stroke. It's easy to check especially if the battery is out of there. I've had three different problem overheating conversions that have been fixed by simply putting a shorter belt on because the tensioner that appeared to be holding the belt tight really wasn't. The 4.9 seems to be particularly finicky about this. John Stricker

Thanks John,

Checked it yesterday and all seems to be good - this is an AC car as well. I do not think we could get a shrtoer belt on it than it has. We will know for sure later today after he takes it for a cruise. Thanks again!

Pat