Cooling issues (continued) (Page 1/3)
Cliff Pennock FEB 27, 06:10 AM
Yes, I have posted about this problem before but I don't want people having to read through a lot of posts that are no longer of any importance (because I don't think I have a major leak anymore, and I know I don't have a leaking head gasket) so I'm starting anew.

TL;DR

Engine overheats within minutes with thermostat, hardly reaches operating temperature without thermostat. Tried two thermostats with the exact same results.


The problem

Ever since my engine was serviced, my Fiero has had a strange cooling issue. On normal drives, the temperature gauge quickly (within minutes) hits the red zone (around 230°F), though it usually stays there without fully overheating - even on longer, 30+ mile trips.

After not driving it for a few months (since it’s no longer my daily driver), I had to use the Fiero when my main car broke down three days ago. On a short 10‑mile trip, the gauge behaved as usual, quickly rising to red. However, on the return, the temperature climbed well past red. With no safe place to pull over immediately, I slowed down and drove about 1 mile to the nearest parking spot. Suddenly, I heard a loud pop, the needle shot to the maximum, and the heater switched from blowing hot air to cold. When I parked, I discovered that the overflow bottle was dumping coolant, with no other leaks visible.

After the engine cooled completely, I refilled the system by bleeding it (I always have coolant with me) - filling the thermostat housing, opening the radiator cap to let out air, and repeating until no more air escaped (it took about 5 quarts). I then drove home (about 2 miles) without the thermostat, and the engine stayed cool.

Yesterday, I took a 20‑mile trip. The thermostat housing was full of coolant, and the engine took a long time to warm up; after 20 miles (mostly highway), the gauge read only about 140°F, with similar results on the return trip.

Assuming the thermostat might be at fault, I replaced it today with another 195°F unit and made a short 1‑mile trip into town. Again, within minutes, the temperature quickly reached 230°F and stayed there. I’m not sure what would happen on a longer trip with the thermostat installed (nor do I really want to find out and get stranded somewhere), so for now, I’m driving without it.

What could be causing this? A slight coolant leak might have lowered the coolant level during the few months it was parked, and lead to the overheating, triggering coolant dump through the overflow bottle - but that doesn’t explain why installing the thermostat makes such a big difference. I suspect the thermostats aren’t faulty (both open, as indicated by the hot radiator and pressure in the hoses). It might be that the water pump is damaged; a worn or broken impeller could prevent adequate coolant flow through the thermostat, accounting for the drastic temperature differences.

BTW, my water pump has a metal impeller, not plastic.

And yes, I checked the coolant pipes. They are not crushed. Besides, crushed pipes would also cause overheating without the thermostat installed...

So any ideas?
olejoedad FEB 27, 06:51 AM
I had an issue one time that the car would overheat at highway speeds (70 mph) but not around town.
It turned out that the water pump inlet hose was sucking shut at high rpm. I replaced the hose and put a spring inside it - problem solved.

What brand of thermostat? Are you buying from the same place? Have you checked function and opening temperature on the stove with a thermometer and a pan of heating water?

Is the hose from the radiator to the coolant expansion tank secure and not leaking?

Are you using a 50/50 mix of antifreeze?

Is the timing correct? Advanced timing will make the engine run hot.

Is your radiator fan kicking on at the proper temperature?

Is the fan turning in the proper direction and pulling air through the radiator?

If you turn on the A/C does the radiator fan turn on?

Is the radiator original or has it been replaced?

Is the front surface of the radiator clogged with debris?

Are the plastic air ducts and lower spoiler below the radiator support intact and in place?
cartercarbaficionado FEB 27, 09:44 AM

quote
Originally posted by Cliff Pennock:

Yes, I have posted about this problem before but I don't want people having to read through a lot of posts that are no longer of any importance (because I don't think I have a major leak anymore, and I know I don't have a leaking head gasket) so I'm starting anew.

TL;DR

Engine overheats within minutes with thermostat, hardly reaches operating temperature without thermostat. Tried two thermostats with the exact same results.


The problem

Ever since my engine was serviced, my Fiero has had a strange cooling issue. On normal drives, the temperature gauge quickly (within minutes) hits the red zone (around 230°F), though it usually stays there without fully overheating - even on longer, 30+ mile trips.

After not driving it for a few months (since it’s no longer my daily driver), I had to use the Fiero when my main car broke down three days ago. On a short 10‑mile trip, the gauge behaved as usual, quickly rising to red. However, on the return, the temperature climbed well past red. With no safe place to pull over immediately, I slowed down and drove about 1 mile to the nearest parking spot. Suddenly, I heard a loud pop, the needle shot to the maximum, and the heater switched from blowing hot air to cold. When I parked, I discovered that the overflow bottle was dumping coolant, with no other leaks visible.

After the engine cooled completely, I refilled the system by bleeding it (I always have coolant with me) - filling the thermostat housing, opening the radiator cap to let out air, and repeating until no more air escaped (it took about 5 quarts). I then drove home (about 2 miles) without the thermostat, and the engine stayed cool.

Yesterday, I took a 20‑mile trip. The thermostat housing was full of coolant, and the engine took a long time to warm up; after 20 miles (mostly highway), the gauge read only about 140°F, with similar results on the return trip.

Assuming the thermostat might be at fault, I replaced it today with another 195°F unit and made a short 1‑mile trip into town. Again, within minutes, the temperature quickly reached 230°F and stayed there. I’m not sure what would happen on a longer trip with the thermostat installed (nor do I really want to find out and get stranded somewhere), so for now, I’m driving without it.

What could be causing this? A slight coolant leak might have lowered the coolant level during the few months it was parked, and lead to the overheating, triggering coolant dump through the overflow bottle - but that doesn’t explain why installing the thermostat makes such a big difference. I suspect the thermostats aren’t faulty (both open, as indicated by the hot radiator and pressure in the hoses). It might be that the water pump is damaged; a worn or broken impeller could prevent adequate coolant flow through the thermostat, accounting for the drastic temperature differences.

BTW, my water pump has a metal impeller, not plastic.

And yes, I checked the coolant pipes. They are not crushed. Besides, crushed pipes would also cause overheating without the thermostat installed...

So any ideas?


describing the Tangerine menace here cliff. flush out the cooling pipes and install new springs in all the hoses and backflush the radiator a few times..
that ended up fixing that 88 and my 87 gt that was parked 20 years so it seems the cooling pipes like to build up alot of sediment inside and suck the hoses closed when they close up in diameter
whats weird is ours wouldnt even get to 120 degrees without a stat. you would comfortably hold any of the hoses and it wouldnt be painful even after 20 minutes.
also maybe check the fins are clear and clean (this was something we did a few times before using vinegar to clean them just to make darn sure any deposits were disolved and loosened up and got quite a bit of rust out) maybe check the fan is spinning at highway speeds too? i know a bad fan relay on our upgraded 84 (using a 86 fan and shroud bypassing the 2 speed crap.) wouldnt make it spin strong enough to help with flow but just enough to basically block it completely which was very cool.
fierobear FEB 27, 12:42 PM
I have a similar issue, at least on the gauge, in my 88GT. The needle often shows that it is running not (in or near the red). I have an OBD-1 scanner, and often have it connected to the ALDL when I'm doing diagnosis and test drives. The scanner showed the temperature reported to the ECM as normal while the gauge showed temps in the red. I changed the gauge, the temp sensor to the gauge, checked the wiring and so on. If the ECM is seeing a temperature in the normal range, and it doesn't overheat, then it likely isn't really overheating. Does the red temperature warming light come on? That does seem to work properly, as it once warned me that the engine was overheating which turned out to be a failed radiator fan. I suspect that the funky "wiring" on the back of the gauge pod has corrosion.

Another problem area is the radiator cap. Often when you get coolant over running the overflow, the rad cap is the wrong pressure (too low) or it is failing. You know that irritating problem when you're refilling the coolant and it keeps flowing into the overflow? Bad rad cap.

Hope this helps.
Cliff Pennock FEB 27, 01:16 PM
Thanks all for the answers.

The fan comes on sometimes. Which kind of makes sense because temperature is hovering around 235°F. It can spin freely and when it spins, I can feel it's spinning at full force because it's moving quite a bit of air. And yes, it's sucking air through the radiator.

But that also means the temperature gauge sender is showing the correct temperature. The "engine hot" light also comes on when the needle creeps into the red.

I replaced the stock radiator by a non-stock 3 layer radiator a few years ago. Since then, it's been cooling a lot better than before. Like I said, these problems only started after the engine was serviced so the radiator isn't the cause. Also, it's clean and clear. Nothing is preventing air from flowing past the radiator.

The rad cap is okay. This was actually the first time I've seen it throw coolant into the overflow.

I haven't looked at ALDL data yet. I would need to make a cable first.
Vintage-Nut FEB 27, 01:56 PM

quote
Cliff Pennock:
But that also means the temperature gauge sender is showing the correct temperature. The "engine hot" light also comes on when the needle creeps into the red.



Two separate circuits in the sensor/switch so believe the gauge and the light!

BTW - Fiero uses a non-vented cap at the radiator, not a vented cap.

It is strange with the thermostat in and out, I would test the thermostat to SEE it opens at 195-degrees


quote
olejoedad:
Have you checked function and opening temperature on the stove with a thermometer and a pan of heating water?

[This message has been edited by Vintage-Nut (edited 02-27-2025).]

cartercarbaficionado FEB 27, 02:22 PM

quote
Originally posted by Cliff Pennock:

Thanks all for the answers.

The fan comes on sometimes. Which kind of makes sense because temperature is hovering around 235°F. It can spin freely and when it spins, I can feel it's spinning at full force because it's moving quite a bit of air. And yes, it's sucking air through the radiator.

But that also means the temperature gauge sender is showing the correct temperature. The "engine hot" light also comes on when the needle creeps into the red.

I replaced the stock radiator by a non-stock 3 layer radiator a few years ago. Since then, it's been cooling a lot better than before. Like I said, these problems only started after the engine was serviced so the radiator isn't the cause. Also, it's clean and clear. Nothing is preventing air from flowing past the radiator.

The rad cap is okay. This was actually the first time I've seen it throw coolant into the overflow.

I haven't looked at ALDL data yet. I would need to make a cable first.


what got serviced anyhow? it could be a rag or something if anything cooling system related
olejoedad FEB 27, 03:11 PM

quote
Originally posted by cartercarbaficionado:

what got serviced anyhow? it could be a rag or something if anything cooling system related



I almost left a rag in a coolant tube one time!
It can happen!
Cliff Pennock MAR 01, 05:25 AM
I tested the two 195° thermostats I have here and they opened but one only barely. Surely not enough for unobstructed coolant flow. The other one opened a bit further but still not as much as a third thermostat I tested. This third thermostat is a 180° thermostat and it opened much further than the other two.

So the best performing was the STC 180°, then the STC 195° and the worst performing by far was the AC/Delco 195°.




I haven't driven with the 180° thermostat yet, so I will try that today and see if that makes a difference.
Vintage-Nut MAR 01, 07:55 AM

quote
I haven't driven with the 180° thermostat yet, so I will try that today and see if that makes a difference.



It will either solve the problem or another data point....I hope that the shop didn't leave a rag in a coolant tube!

BTW
Water boiling point is 212*F and your water looks like it is barely boiling....did you use a thermometer?