This has got to have something to do with my overheating problems. I keep noticing that my coolant recovery tank is overflowing. I'll remove some coolant from the tank, and I wind up needing to fill the coolant level in the engine. I'm starting to think that my coolant fill cap in the back must be leaking air, allowing the coolant to fill up the overflow tank.

Any opinions or experience with this one?

Alright Stimpy, one of yer wonky toothed mates from Blighty here . I had exactly the same problem. Tried the burping procedure as a kick off and it worked for a while and then the problem returnded. Put a new radiator and filler cap on (make sure the rad one is the correct rating), repeated the burping procedure and all fixed.

Ogre's cave has lots of useful info on the correct caps and burp procedure.

Cheers
PK

I would try the new cap first (both). If that doesn't work, then get a cooling system pressure test to rule out anything nasty......Paul

Thanks for the replys, gentlemen! +'s around.

Ya, cap may be all you need. A worse case would be a head gasket leaking combustion pressure into the cooling system and forcing fluid out. Test your thermostat to make sure its opening too.

That would truly suck. But wouldn't a leaking head gasket cause the car to run roughly? The car isn't having any drivability issues other then the heat issues, so I'm optimistic that it's something minor.

If the head gasket was leaking you would know and you would have any constant coolant pressure, Pluss it would spit white smoke out of the exhaust.

Make sure you have the correct radiator cap. The parts books list the incorrect one for the fiero. The cap should have a little plunger type thing on the bottom, on the correct cap the plunger is held shut with a spring, on the incorrect cap, the plunger will dangle. Ogre has a great write up on it in the cave.

-Rick

Been there......done that!
The rad cap is the first place I would start. I tried 3 different caps ( all the same numbers)
before I got one that sealed the correct way.
Go to Advance Auto or a Checkers and get their pressure testers (free to use) do a pressure
check on your car. It will check if you have a bad cap/rad/and for headgaskets.....
do a search for rad caps for the correct # very important or I thing Orge's cave show the correct
# also....
Good Luck,
-Michael

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88GT/Auto/Beechwood Leather/SunRoof/Wing/Sub-Woofer/w-EQ/225/45/17 Michelin Pilots/All Opts Ex.T-Tops
<<< <<< Pix of my GT /- -/ Pix of my Vettes >>> >>>
*** *** "If you appreciate my contribution,..please rate me, Thanks" *** ***

The overflowing coolant bottle was one of the symptoms I had when my head gasket went away, due to a broken head bolt.
It'll be fairly easy to diagnose.
Make sure the cooling system is full. Make a note of the coolant level in the overflow.
Let the engine idle until it is warm.
Open the front compartment.
Put the car in gear.
Hold your foot on the brake and press down on the gas. (Of course, don't give it enough gas to smoke the tires or push the locked front wheels.)
If a head gasket is blown, it should push coolant out of the overflow, into the bottle, almost immediately.

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Raydar
88 3.4 coupe.

Coming soon...
88 Formula, presently under the knife.

I'm going with the radiator carp... Same thing that happened to me bringing the T-top over from GA. Pour coolant in the rear and it pours out the overflow... I don't see how it can be anything else. If the cap is doing it's job there shouldn't be anything going to the overflow (unless there are major mechanical problems in the engine)... Stant caps that are listed in the book do not work on Fieros! Buy another brand and you'll be fine.. I've got a box full of Stants here that let coolant pass through when they were brand new.

------------------

Fiero-Performance.com - New mods for your Fiero from Fiero Performance Bayern in Germany

[This message has been edited by Songman (edited 01-27-2004).]

The radiator cap, or the thermostat cap?

Ive seen head gaskets that leak air pressure out into cooling system, but not let water back into the cylinder. The only symtoms are slowly losing water over a long period or overflowing thru tank, or lots of little bubbles in the radiator while running.

quote Originally posted by stimpy: The radiator cap, or the thermostat cap?

Radiator.... If the radiator cap isn't holding pressure the coolant will leak through to the overflow tank.. For some reason the Stant cap that is listed in the book does not match up with the lip on the radiator. I was about to buy a new radiator, after putting all other possibilities down. Decided to try another brand since I had already bought 4 Stant caps on my way cross country. A cap of a different brand fixed it right up.

stimpy.........
Go to the parts store like Checkers/Autozone an borrow a pressure tester, it easy to use.
You can check the cap/rad/or you can check for a bad head gasket. also checks for any other leaks in your system.
I can't tell you how many guys told me I had a bad headgasket (ok ...3 ) I was about ready to pull the engine
to replace the head gaskets.......My car over heated something BAD!!
All it was .......The dang radiator cap!!!
That pressure tester is free to use from thoses stores . You just have to leave a deposit.

Give it a try....takes maybe 20-25 mins.

Good Luck,
-Michael

Well, I replaced the thermostat cap with an AC Delco model. The new one is MUCH tighter then the old one, and so far the level is holding. Knock on wood, but I think I got it right.

quote Originally posted by stimpy: This has got to have something to do with my overheating problems. I keep noticing that my coolant recovery tank is overflowing. I'll remove some coolant from the tank, and I wind up needing to fill the coolant level in the engine. I'm starting to think that my coolant fill cap in the back must be leaking air, allowing the coolant to fill up the overflow tank. Any opinions or experience with this one?

It's almost certainly a bad radiator cap. (front one) Check the Ogre's cave for the correct part number - the parts book at the auto parts store is WRONG.

This is copied from the cave:

What radiator/thermostat caps do I use? Why?

I broke this out of the coolant fill article hoping more people would be made aware of the Stant catalog error.

Warning! There is an error in Stant listings, which means all catalogs listing Stant parts share the same error. The correct caps for Fiero are Stant part numbers 11230 or 10230 non-vented caps.

You want to use a 15-16psi cap as recommended by OE specs. There are 2 very important reasons for this. (We'll get to vented vs non-vented in a minute.)

The radiator cap along with the coolant product determines the boiling point of the filled system. A 15psi cap increases boiling point about 45oF vs no cap. Lower system pressure and the coolant will boil at a lower temperature.

The cap effects water pump efficiency and life by controlling cavitation. When you spin the water pump it forms areas of low pressure around the impeller. This low pressure makes air dissolved in the coolant, and even the coolant itself, explode into bubbles. These bubbles cause turbulence in the water so it's harder to pump. Cavitation also damages the water pump by giving the impeller a rough surface that reduces its efficiency permanently. Coolant under pressure is less likely to cavitate and is therefore easier to pump around the system. Life of the water pump is also increased.

There are low pressure, usually 13#, caps out there. They often get installed on old weak systems to prevent popping the radiator tubes, etc. These caps often cause other problems, like easy over heating, and should only be used as a temporary item. Long term use of these caps can cause damage to the water pump impeller. They are only a little better than a bad cap.
GM & AC Delco caps.

Thanks to "Two88GTs," the current GM and AC Delco cap numbers for Fiero are:

Rad Cap

AC Delco: RC27

GM : 10409635

Thermo Cap

AC Delco: RC40

GM: 6410941

"The p/n for the RC27 in my parts book is 10036879 which is still a valid number in the GM parts system but it is for the OLD design cap, and the remaining stock is indeed OLD. Get the 10409635."
Stant Catalog Error

Somewhere along the line the Stant catalog got messed up. This **** happens in the auto parts business. Whether the mistake originated from GM or Stant I doubt anyone really knows. Unless GM tells Stant the catalog is incorrect, which isn't likely, the error will probably never be fixed.
Vented vs. Non Vented

The Stant xx231 cap is what everyone has been calling a "vented" cap. The xx230 is what has been called a "non-vented" cap. The difference is the check valve, that dime size metal disk on the cap. On the vented cap, the check valve is simply hanging loose. On the non-vented cap, the valve has a small spring holding it tight to the seal. (This article about DEX-COOL at IMCOOL.COM has pictures of the two caps.)

The 2 caps are otherwise identical. In fact on cars with DEX-COOL, GM recommends all vented caps be replaced with non-vented. (In that case it is intended to help reduce coolant contamination...)
A bit of background

In the Fiero cooling system, the radiator and more importantly it’s filler neck, are not the highest point in the system. Some coolant in the engine is above the radiator neck. Exactly how much coolant is above that level depends on which engine is involved.

When the system is cool, this layout creates a slight vacuum in the engine. In a perfect system this isn’t a problem but what happens when there is a microscopic air leak or the rear of the car is jacked?

The problem with the vented cap is that if there is even a tiny air leak in the system then coolant can drain through the cap and out the overflow line to the reserve tank. This happens because the flow rate is too slow to push the check valve shut.

If the cooling system were perfectly sealed then this wouldn't be much of an issue. In reality there is often some tiny spot that leaks air even if no coolant is getting by. The reason this problem usually doesn't show up when you open the thermostat housing is that the sudden flow pushes the check valve of a vented cap shut and holds it there. The problem will only show up under a slow leak condition.

When the car is tipped nose down, we add another issue. Coolant is heavy and wants to run down hill. This increases the vacuum back in the engine. Even if the cooling system is perfect with no leaks to let in air, we run into a design problem with the water pump seal.

The water pump seal is designed mainly to keep pressure in the system. It’s often not designed to deal with the amount of vacuum that the Fiero cooling system can develop when it pointing nose down. Air is literally sucked past the pump seal by the coolant running down hill. Again, we have a slow flow of coolant, so the vented cap check valve may not close when this happens.

The non-vented cap prevents this simply because it has a sprung check valve. All the shifting coolant can do is push it tighter against the seal.
Why is there such a big load with the nose pointed down?

Well, one reason is that for a small car, Fiero has a big cooling system. About 14 U.S. Quarts, or 3.5 Gallons of coolant, compared to 6-9 quarts used in many other cars and light trucks. That’s a lot of weight. Measure it yourself, put a gallon of water and a gallon of antifreeze on a scale at the same time to simulate a 50/50 mix. The Fiero system has nearly twice that weight in it.

The other big factor is the length of the system.

Keep in mind that a fluid always wants to seek level height. If it can’t find level where it is, it will run down hill to someplace where it can.

Normally, the difference between the radiator cap and the highest point in the system is measured in inches. When parked nose down the height difference can get pretty big. Depending on the angle, you could have most of the volume of the cooling system pulling a vacuum in back. If the seals in the engine can’t take the vacuum, the water runs downhill and out the vented cap.
The thermostat cap keeps sticking...

This is annoying to say the least. To prevent this, clean the thermostat neck and coat the rim with a silicone product like Brake Grease. Coat anything the cap touches or covers. Brake Grease won’t wash out easy and it won’t damage the rubber seal on the cap.

Copyright 2003 The Ogre’s Fiero Cave, D.W.Lane. All rights reserved. Not for reproduction in any media without written permission.

Last Revised Tuesday, January 27, 2004 22:01:34.

quote Originally posted by stimpy: Well, I replaced the thermostat cap with an AC Delco model. The new one is MUCH tighter then the old one, and so far the level is holding. Knock on wood, but I think I got it right.

Good deal! Please keep us posted.

bump

No apparent leaks. I haven't driven it any distance, as I'm taking apart the interior to do some restoration on the peeling pieces. (Keep an eye out for my how-to thread on that.) I did run the temp up idling in the driveway, and it stayed right at 180.

quote Originally posted by stimpy: No apparent leaks. I haven't driven it any distance, as I'm taking apart the interior to do some restoration on the peeling pieces. (Keep an eye out for my how-to thread on that.) I did run the temp up idling in the driveway, and it stayed right at 180.

"Feels good... hey"

------------------

88GT/Auto/Beechwood Leather/SunRoof/Wing/Sub-Woofer/w-EQ/225/45/17 Michelin Pilots/All Opts Ex.T-Tops
<<< <<< Pix of my GT /- -/ Pix of my Vettes >>> >>>
*** *** "If you appreciate my contribution,..please rate me, Thanks" *** ***

quote Originally posted by stimpy: No apparent leaks. I haven't driven it any distance, as I'm taking apart the interior to do some restoration on the peeling pieces. (Keep an eye out for my how-to thread on that.) I did run the temp up idling in the driveway, and it stayed right at 180.

Good for now - but please do yourself a favor and replace the front cap too. If it wasn't leaking, you wouldn't have had excess coolant in the overflow bottle.

As long as your system is air tight, it won't leak - but if you develop even a tiny leak anywhere it'll start running out the overflow tank again if you don't get a good (and the right one) cap on the front.

Just keep one thing in mind. the pressure rating of your radiator cap MUST be lower than the rating of your collant fill cap in the engine compartment. It's a closed system and you want the radiator cap to open before the engine cap does for obvious reasons.

I run a 16 lbs radiator cap and a 20 lbs fill cap.

I didn't think that the thermostat housing cap was a type that opened under pressure, therefore it has no pressure rating. I suppose that it could blow off if the pressure got too high for whatever reason, but there are several other weak links in the coolant system that would likely fail before the rear cap did. It is critical that the front cap be rated for 15 PSI, though.

JazzMan

I used a 35 pnd Moroso racing cap on the engine end of my V8 car and an OEM Fiero on the radiator. It worked out fine.

i'll agree with Jazzman on the fill cap - there is no pressure release system on any of the fill caps I''v ever seen , radiator yes fill cap no. Don't think there is enough room for one, it's just a flat cap

I noticed one thing that's been missing from this thread:

No one has mentioned the thermostat. If your coolant overflows BEFORE the car is up to temp, it's a headgasket problem, because the pressure build up and forces the thermostat open, and the coolant out any place it can.

If you coolant is fine until the car warms up, but then a steady stream flows out the overflow tank (or into), it's the radiator cap.

The thermostat won't send pressurized coolant to the radiator until the coolant reaches a certain temp. This is the exact problem I have with my car. 10 minutes idling, than a constant flow of coolant.

My only question is: what happens when the coolant temp goes down? Does it suck the overflow back in, or does it suck air in?

quote Originally posted by iluvmacs: I noticed one thing that's been missing from this thread: No one has mentioned the thermostat. If your coolant overflows BEFORE the car is up to temp, it's a headgasket problem, because the pressure build up and forces the thermostat open, and the coolant out any place it can. If you coolant is fine until the car warms up, but then a steady stream flows out the overflow tank (or into), it's the radiator cap. The thermostat won't send pressurized coolant to the radiator until the coolant reaches a certain temp. This is the exact problem I have with my car. 10 minutes idling, than a constant flow of coolant. My only question is: what happens when the coolant temp goes down? Does it suck the overflow back in, or does it suck air in?

Read my post and you will see the flaw in this theory... With the Stant cap installed, my car was pouring coolant past the radiator cap with the car completely cold... You have to have a good sealing cap for this theory to work... Change the front cap, Stimpy.. It's only a few bucks.

Ok, ok, I'll do it!

quote Originally posted by Songman: Read my post and you will see the flaw in this theory... With the Stant cap installed, my car was pouring coolant past the radiator cap with the car completely cold... You have to have a good sealing cap for this theory to work... Change the front cap, Stimpy.. It's only a few bucks.

Then there was something else wrong with your car, because the whole point of the thermostat is to send coolant at or above operating temp to the radiator. Even if the cap is bad, which on my car it is, it won't send coolant before the coolant circulating through the engine reaches 160F. (stock is 190?)

No, there was nothing else wrong with my car. I have been driving it for a year and a half since and the temp gauge sits steady on the 1/4 mark and no coolant has been lost until my original plastic radiator busted a few weeks ago.

My comments had nothing to do with a thermostat because with me standing at the back of the car with the cap off and the thermostat out pouring coolant into the back and it runs through the system, into the radiator, past the cap into the overflow and out onto the ground... That really is a bad thermostat that can cause that when it is sitting on top of the plenum! haha

Gravity and a bad radiator cap were my problem. The purpose of a thermostat is to sort of dam up the water at a point so that the water in the radiator can stay there long enough to cool down. Then when the water at the thermostat reaches the set temp of the thermostat, it opens and lets the water pump push the water... The cooler water from the radiator goes into the engine to cool it, the hot water from the engine goes to the radiator to be cooled. The thermostat doesn't actually 'send' water anywhere... it stops water. But.. as I said, my thermostat wasn't even in so that has nothing to do with what I was talking about.

[This message has been edited by Songman (edited 01-30-2004).]

Mine did it.It was caused by a leaking head gasket.
Do a compression check.

I think Songman is right.
The stat has nothing to do with it, in this case. The stat does not totally seal off the coolant supply to the engine. It has a small vent hole in it and with out a persision seal around the side it will leak a little around the stat too. In this case the front cap is not making a good seal ( the little metal disk in the center of the cap should be spring loaded for our Fieros. If the little disk is loose it will cause the coolant to just flow out at will) and just alowing coolant to flow through the cap and into the overflow tank. Any pressure in the system will push coolant through the bad cap. Or just take off the rear cap and you can see the coolant drain out of the rear filler neck and into the over flow tank, just with gravity.

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Rickady88GT QuadCam 3.5 V6

Songman - Dude, I completely missed that. Of course your stat won't do crap when it's not installed! My bad. I kept thinking your car was running...

Back to the original topic:

get a new radiator cap, and if it doesn't solve your problem, you spent $5 to get a better looking one at least