Well I had this problem before and I though it was the head gasket. Now that the engine is rebuilt and the cooling system has been flushed and the problem remains I'm thinking major air bubble but I hope somebody can help me pinpoint the problem.

I've got a new water pump, new tstat, new correct rad cap and new correct tstat cap. The tstat doesn't sit very sturdy though in the housing, it seems to be sort of loose in there.

The problem is the temp gauge rises VERY fast while idling after initial warmup. But just a little rev and it goes back to the second mark in 2 seconds. On the highway it pretty much stays dead on the second mark but just a short stop and it rises very fast to almost red. Even the idiot light blinked on me for a second but revved the motor and it's all good for 10 seconds.

The heater is set on max and it doesn't heat the cabin very good at idle. But when I rev the engine or keep it at ~1200 rpm now I get proper heat in the cabin and engine temp goes back to the second mark. Let it idle and cabin goes cold and engine goes hot... My overflow tank also spilled some coolant on the ground seconds after engine shutdown.

What can I do to diagnose this? Been trying to burp the system via old threads and Ogre's cave. Haven't yet to try to burp the heater itself, maybe there's the problem.

Any ideas? Many thanks.

[This message has been edited by cebix (edited 10-31-2019).]

See my Cave, Coolant Fill but already read that.
Crush pipe(s) or block anything can still cause this.

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Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should.
(Jurassic Park)

The Ogre's Fiero Cave

[This message has been edited by theogre (edited 10-31-2019).]

Yeah I read it and basically did the whole thing... except burping the heater itself. Will try it tomorrow morning and see if it helps but looking for other ideas in the meantime.

EDIT: I can't see one thing though. Should I burp the heater with the engine running or not? Hot or cold?

[This message has been edited by cebix (edited 10-31-2019).]

Check if your water pump has enough flow maybe the impeller is no good .

quote Originally posted by claude dalpe: Check if your water pump has enough flow maybe the impeller is no good .

Would that still apply if giving the car some revs will get the temp down and it does stay cool on the highway?

Depending on weather... Standard T-stat can often behave like that but shouldn't turn on Temp light. (Tstat isn't design to be full on/off instantly but many are too fast to open/close and in cold weather can slam the engine w/ cold coolant.)

bleed heater likely isn't your problem if you follow direction in cave.

block intake manifold output pipe/hose to heater maybe

Heater core and/or radiator can be very blocked and still fill the system w/ about same amount of coolant.
All it takes is to block very small sections of them to stop coolant flows down many tubes. Or tubes are heavily coated w/ crap and "rust" causing little heat transfer to air.
Might see block tubes w/ FLIR, Cheap IR temp, etc. Start at D side and look for cold tubes relative to D side tank and other tubes at same distant from D side tank. All radiator tubes should be close to same temp at ¼ ½ ¾ of total distant between tanks.
Might see block tube at bottom of either using a cheap bore scope w/ mirror tip to look into them. (Empty the rad before trying this.)

Bottom tubes are more likely to block then upper tubes because how both slows down flow in them on the way to their outlets. Too the point top rad tubes you see thru fill port may look new and bottom in completely block in them and/or in D side tank.
Block tubes may not care when you flush them ether. It depended just what is blocking them and often how you flush.

Bent pipes are crush pipes. Small bending can hind big blockage.

[This message has been edited by theogre (edited 10-31-2019).]

So you're saying my problem is less likely air in the system but more likely a flow obstruction? The rad is old, yes. I was told it was flushed during engine rebuild but who knows. Two of my previous t-stats wouldn't open when dropped into boiling water. One had a little hole drilled out and it seemed to work the best as in slowest temp creeps but also a longer time to get up to temp during cold starts.

quote Originally posted by cebix: I've got a new water pump, new tstat, new correct rad cap and new correct tstat cap. The tstat doesn't sit very sturdy though in the housing, it seems to be sort of loose in there. Any ideas? Many thanks.

I'm sure you know there's suppose to be an O-ring on the T-stat, right? And it should fit snug in the T-stat bore. If not then you may be bypassing the stat to some degree.
Everybody covered the basics so you have to look deeper. I had those same symptons on a non-Fiero and it was a partially clogged radiator. The coolant was taking the path of least resistance thru the radiator. Goose the gas pedal and the heater gave warm air and temp gauge would drop.

I pulled the radiator and took it to a professional radiator shop. They dropped what they were doing and hooked the radiator inlet up to what looked to be an 1-1/2" hose & pump and threw a switch. With just a visual of the discharge flow he knew the % of blockage. When I went back to pick it up he gave the same demo and even I could see a huge the difference.

That radiator was from 72 Dodge van V8 and it was worth cleaning. The aluminum stuff they make today it's probably cheaper to replace it with a Champion if a flow check justifies it.

Spoon

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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

Yes, there is an oring. I think the shop might have polished the tstat housing inside a little too much, the thermostat isn't as snug as I remember it to be before the rebuild. Anyway bypassing the tstat a bit I guess basically contributes the same thing as drilling a little hole in it? I guess it shouldn't cause these problems and the car gets up to temp very quickly on initial startup and the temperatures here (~32 degrees Fahrenheit).

I'll try to burp the system some more tomorrow as I still see coolant needs to be added after cooling down (no leaks found). If that doesn't get me anywhere I'll look at the radiator. Thanks.

[This message has been edited by cebix (edited 10-31-2019).]

If everything is in good shape...
If you just fill the system, often takes a few heat/cold cycles to remove all air out regardless of "Burping" methods posted here etc.
Even fill w/ cave method can trap air for days to allow system to do it's job pushing air out.
Tstat may act "weird" while system purges itself of air too.

And that's if the overflow tank and tube and rad cap are good and sealed. They may not leak coolant out but any air leak will suck air back in when cooling after engine shutdown.

But Sadly is possible for above reasons...
Flush a rad is a electric circuit. Most water flows where is easy to do so.
Block tubes are like big resistors that flow very little to nothing.
Radiators and heater cores in a car, heavy crap often sinks to bottom and often stays there even ignoring corrosion problems hard blocking tubes.

Could try to take it out of car then put water in both side taking turns. Warning: If car is auto trans keep water out of oil cooler or you have big problems "drying" the heat exchanger guts. Water get in the trans will cause big problems even wrecking that.

If you try flush products...
Some times flush products helps but often doesn't. Worse some might "wash" crap away that's sealing weak/leaking tubes.
Some products eat aluminum parts so can't use most strong flushes. Most of these should have warnings that say so but don't know labels where you are.

So If you can get tools above so can see inside of both tanks, look at temps across the rad, etc. might save you headaches trying to flush a "dead" radiator.
That's assume either tank isn't dying w/ cracks starting etc like in my cave. If you see small cracks is likely time to get a new one soon. More so w/ D side because gets a lot hotter.

Flushing heater core is about the same but ½ of tubes flow in opposite direction because inlet and outlet is on same side.
But is small so could try flushing both directions in the car w/ a water hose and a extra hose so wast water get out of car.

I want to chime in here with a problem I have not heard of with our Fieros but happened on my '73 mach 1. I rebopped the cooling system- new radiator, hoses, and heater core. The car would do fine but every time I got on the freeway it would overheat.....Finally figured out that the LOWER radiator hose was collapsing under suction- The hose had a spring in it to keep it from collapsing- but this spring was short!

Ended up cracking the Cleveland heads (They are known to be weak) I swapped in a FI 5.0 and AOD trany...

My question here is, do any of the Fiero hoses have anti-collapse springs in them? This is a bizarre set of circumstances- It took me a while to figure it out.

A lower radiator hose with a spring to prevent collaspe is on the suction side of the water pump. I don't recall the Fiero having one. Nonetheless at idle it is less likely to collaspe than at a higher rpm. Your Fiero is overheating at the lower rpm.

Spoon

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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

Still suspecting it's got air in it. After multiple burpings I can hear now fluid sloshing around in the heater core at startup and some hissing while revving up and down.

Thinking of installing a T fitting to the heater hose to help with burping but for now I want to purge it the way it is.

How should I burp the core? Hot or cold? Engine running or not? Thanks.

The core will self burp.
Coolant flows through it constantly when engine is running.
Check undercar lines on passenger side of fuel tank for kinks or crushed area.
(Not very common)

Im thinking you have a small air pocket that will work its way out.
Keep the expansion tank level at the correct level and be sure the hose does not leak.

I usually fill the system and run it for several miles without the stat. That way coolant will circulate hot or cold. after it cools off I check level in bottle and add if needed. Then I add to t-stat housing if needed and put the t-stat back in. Start engine and let it warm up til fan kicks on and back off. Drive and watch the temp and check for heat at the vents.

Seems to work for me.

Spoon

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"Kilgore Trout once wrote a short story which was a dialogue between two pieces of yeast. They were discussing the possible purposes of life as they ate sugar and suffocated in their own excrement. Because of their limited intelligence, they never came close to guessing that they were making champagne." - Kurt Vonnegut

All good ideas so far, but one issue hasn’t been raised. What condition are the radiator and thermostat housing caps? Does coolant keep rising in the overflow when you add? That normally means a bad rad cap seal.

Radiator is old, condition unknown. Rad cap is new but isn't hot to touch, just warm when engine is hot. Thermostat cap is new and seals great.

I've been trying to burp the system today a few times and I keep adding coolant to the thermostat housing and it just seems to want more and more. Also today my expansion tank overflowed coolant on the ground a few minutes after the hot engine was shut off. You think the rad cap may be bad? It's new and the correct type and pressure rated, non vented.

After shutting down the hot car I opened the hood and the rad cap was hissing. I watched the overflow tank expand and expand and it overflowed about 3 minutes after shutting down the engine.

[This message has been edited by cebix (edited 11-02-2019).]

quote Originally posted by cebix: Radiator is old, condition unknown. Rad cap is new but isn't hot to touch, just warm when engine is hot. Thermostat cap is new and seals great. I've been trying to burp the system today a few times and I keep adding coolant to the thermostat housing and it just seems to want more and more. Also today my expansion tank overflowed coolant on the ground a few minutes after the hot engine was shut off. You think the rad cap may be bad? It's new and the correct type and pressure rated, non vented. After shutting down the hot car I opened the hood and the rad cap was hissing. I watched the overflow tank expand and expand and it overflowed about 3 minutes after shutting down the engine.

Should be right cap but could be bad or leaking.
Cap can be 100% good but rad neck may be damage and cap and tube can have problems.
If good there then system still has problems which is likely.

Rad cap hot might not mean much. check D side tank and see if is hot or not at same time.

Cheap USB bore scope via amazon etc can look inside of rad tanks and often have mirror tip to look at tubes too. Often w/o taking rad out of car but you must drain the system before using. try to anchor USB end to laptop etc so you can't stress that end and break USB plug or port. Auto Trans rad's maybe hard to find spot to get by trans cooler to look at bottom of D side tank. Either way is easy to stress USB end as you bend/twist the other end.

Check oil.
If engine keeps taking coolant, possible someone F'd the rebuild and leaking coolant in #4 cyl from bad intake gasket then "burning" coolant or in the oil thru iffy head etc.
If you have some shop neerby w/ exhaust tester... See if tester see exhaust in T stat tube. Do not put tester on coolant, just sniff the opening w/ cap off. If tester see exhaust there then have a rebuild w/ big problems. But problem is if you have coolant flow problems too then might not get enough exhaust there to test.

quote Originally posted by cebix: Radiator is old, condition unknown. Rad cap is new but isn't hot to touch, just warm when engine is hot. Thermostat cap is new and seals great. I've been trying to burp the system today a few times and I keep adding coolant to the thermostat housing and it just seems to want more and more. Also today my expansion tank overflowed coolant on the ground a few minutes after the hot engine was shut off. You think the rad cap may be bad? It's new and the correct type and pressure rated, non vented. After shutting down the hot car I opened the hood and the rad cap was hissing. I watched the overflow tank expand and expand and it overflowed about 3 minutes after shutting down the engine.

It has been my experience that if you keep adding coolant and it keeps going into the overflow tank, it’s the Rad cap. If I recall, parts stores actually list the wrong part for the Fiero. Ogre can probably clear this up.

quote Originally posted by fierobear: It has been my experience that if you keep adding coolant and it keeps going into the overflow tank, it’s the Rad cap. If I recall, parts stores actually list the wrong part for the Fiero. Ogre can probably clear this up.

Already did... has New non-vent cap as in cave. He's read my cave.

"Boil over" and most of coolant system is cold to barely warm is blockage somewhere or rebuild engine has big problems like messed up head or intake gasket(s).

4 cyl Intake gaskets are Dry install. standard ones are rubber/plastic over metal and sealer or wrong bolt torque will leak coolant out or far worse leak into #4 runner.
just iffy 1 head bolt and head gasket can leak coolant, cyl pressure or both and can be internal, external, or both.

That intake gasket got me thinking... I do have occasional white smoke at idle and a lot when cold starting. I figured it's the weather but not so sure now... The intake manifold torquing being messed up is of some probability. Gonna have to take off the manifold then to check what's going on there. Guess I'll have to flush the cooling system to do that? Or can I just loosen the nuts and retorque them to spec? Are intake gaskets reusable or throwaways once torqued?

quote Originally posted by theogre: 4 cyl Intake gaskets are Dry install. standard ones are rubber/plastic over metal and sealer or wrong bolt torque will leak coolant out or far worse leak into #4 runner.

I don't know how long it would take to become obvious... but if that's occurring, wouldn't #4 spark plug look "cleaner" than the other three?

Gonna look at those spark plugs but I suppose they're all gonna be pretty clean cause of the mileage. I've only done about 500 miles since the rebuild and changed the oil two times. Going to order new rad and thermostat caps just to be on the safe side and probably go to a specialized shop that deals with cooling systems if I can find any...

EDIT: Should I buy the AC Delco safety release rad cap or the standard AC Delco?

EDIT2: Ok, I see the safety caps are not recommended so there you go. I currently have the safety cap. So that might be the problem. Rockauto says it fits but now I believe it's the wrong type and might be venting...

[This message has been edited by cebix (edited 11-04-2019).]

Wasnt there a problem with some duke intake manifolds.cracking internally and letting coolant into the intake air?

quote Originally posted by Patrick: I don't know how long it would take to become obvious... but if that's occurring, wouldn't #4 spark plug look "cleaner" than the other three?

Most times yes. Problem is new/rebuilt engine plugs may not be driven enough to see this.
I usually tell people to remove all plugs and keep them in order and look for any plugs cleaner then rest for this or bad head gasket etc.

Changed the rad cap. Still acts the same but now it doesn't overflow the reservoir so that's one thing down. These nonvented but safety caps should really be taken down from rockauto.

Maybe now it's just a matter of burping?

quote Originally posted by cebix: Maybe now it's just a matter of burping?

The Ogre will come along again and say "burping" isn't necessary ... but Here is the procedure I follow.

quote Originally posted by cebix: Changed the rad cap. Still acts the same but now it doesn't overflow the reservoir so that's one thing down. These nonvented but safety caps should really be taken down from rockauto. Maybe now it's just a matter of burping?

Sounds like there were multiple problems, and the rad cap was one. I’m glad you are making progress. Best of luck.

No real way for me to get the rear higher right now so I'm trying burping without that. No real success though, still hear hissing from the heater core when decelereating and with light load. Think I'm gonna install a bleeder valve there and see where that gets me. If nowhere then the radiator is next. Checked the bottom and no crushed pipes.

Can't get those heater hoses off so I'm gonna cut the bigger, upper one and install a bleeder valve. I have an AC car. I bought this one:



Wondering where to cut the hose... I guess the higher the valve is the better. Does it hurt if it won't be pointing upwards but rather at an angle? Or will it not do its job at all?

Say if it was installed like shown below with the blue arrow being "up":

[This message has been edited by cebix (edited 11-09-2019).]

I wouldn't add that fitting.
Pull the thermostat and drive it a bit.
The heater core and the rest of the system will work the air out and trap it in the top of the radiator so that it will self-burp.

It's just not burping itself. I don't know why but it doesn't. Driving it regularly and still hear hissing from heater core on deceleration and coolant sloshing around there on startup. Still all my symptoms remain except for the reservoir tank - that doesn't overflow anymore I guess thanks to the new rad cap.

In fact I think with every drive I see the temp gauge go higher and higher a little. Why wouldn't you add that fitting?

Hissing from the heater core.

I bet that is air being sucked into the system. Apparently the leak is so small that coolant does not come out, or comes out in a spot you have not found yet.

[This message has been edited by Gall757 (edited 11-09-2019).]

Is it possible it's just air that is already trapped in the system? No leaks whatsoever, checked and rechecked. Or if there's one it's so small I can't see it.

Because the cooling system does purge itself of the usual air pockets, it appears that your cooling system is somehow gaining air. It could be a head gasket, but that does not explain the hissing sound. A pin hole in the rubber hose at the heater core (or a loose hose clamp) might do it.

[This message has been edited by Gall757 (edited 11-09-2019).]

Damn. That's gonna be a tough one. Might have to take the car to a shop for a pressure test, that's gonna be expensive I guess because nobody here knows those cars. The hissing sound doesn't actually come straight from the heater core, it's just all around if I walk around the car. It even sounds like it's coming from the engine when revving up and down so that must be air. And it's always straight in pitch, doesn't change with the revs or anything. And it only happens when throttling down or moving at part throttle. It's a deep hiss though, not like a tire inflating sound, kinda Darth Vader if you will.

Still don't understand how that fitting can hurt anything? Is it not a good idea to try it anyway? What I'm trying to achieve with this is burping the heater core as the cave states but without removing the hose and using a bleeder valve.

[This message has been edited by cebix (edited 11-09-2019).]

The heater core has constant coolant flow whenever the engine is running, not dependant on thermostat position. It has no choice - it has to push the air out due to flow, unless that flow is greatly restricted by a crushed supply tube, a kinked hose or a hose that is delaminating internally.

Even if there was a bit of air in the core, it would have absolutely no effect on the overheating of the engine, if indeed that is still happening.

I would not add the fitting because it is not needed.

Damn. No idea what to do now. Maybe there is a blockage somewhere but just a slight enough one to be overthrown by the pump when at moderate throttle?

Would a pressure test show that?

I do get a lot of heat from the heater and it is HOT but only while at some throttle. Idling for a minute or so and it gets cold.

[This message has been edited by cebix (edited 11-09-2019).]

quote Originally posted by cebix: I do get a lot of heat from the heater and it is HOT but only while at some throttle. Idling for a minute or so and it gets cold.

Obviously not normal.

If it was my Fiero, I'd remove one of the hoses from the heater core and flush and then back flush the system with a garden hose. This would at least tell you if you have a blockage somewhere in that part of the system, and maybe it would also push out whatever the blockage is.

Keep in mind that you don't necessarily need to disconnect a hose up front from the actual heater core. If it's easier to disconnect a heater hose in the engine bay, do it there instead.

[This message has been edited by Patrick (edited 11-09-2019).]