Engine temps all over the place (Page 1/9)
Cliff Pennock JUN 14, 03:07 AM
Like the subject says, my engine temperatures are all over the place. "Normal" temperature seems to be between 175F and 195F but most of the time it shoots up to 230F in 20-30 seconds, stays there for a few minutes only to drop back to 175 within 10 seconds. Rinse. Repeat.

There seems to be enough coolant in the system. If I remove the thermostat cap, coolant is right up to the edge. Same thing with the radiator. If I remove the cap, coolant is up to the edge. I don't hear any air going through the system. I checked the overfill bottle after driving the car for about an hour and it was empty. So I filled it up and checked it after another hour of driving. The coolant inside the overflow bottle wasn't warm. I'd say around 75F so I'm guessing the radiator cap needs to be replaced.

I had already replaced the thermostat and the thermostat cap recently. But could a defective radiator cap cause the violent temperature swings?
Patrick JUN 14, 03:43 AM

quote
Originally posted by Cliff Pennock:

I checked the overfill bottle after driving the car for about an hour and it was empty.



When did you last check it? The coolant must be going somewhere.


quote
Originally posted by Cliff Pennock:

So I filled it up and checked it after another hour of driving. The coolant inside the overflow bottle wasn't warm. I'd say around 75F so I'm guessing the radiator cap needs to be replaced.



Why would the coolant reservoir be warm? The engine was already warmed up when you re-filled the reservoir, so there wouldn't be any hot coolant flowing into the reservoir. However, when the engine cools down, coolant would be drawn from the reservoir into the main system.

Unless you didn't fill the coolant system (including the reservoir) properly in the first place, your coolant is either leaking and/or is being burned.
Cliff Pennock JUN 14, 08:07 AM
My reasoning for why the coolant in the overflow button should be hot is because the engine reaches temperatures of about 240F which should cause pressure to rise above 15/16 psi causing hot coolant to enter the overflow button (and sucked back into the radiator when temperature drops). Apparently, the radiator cap valve didn't open at all during the time I drove the car.

And yes, I do have a coolant leak somewhere. Just can't find where. Although it seems it´s has been a bit less lately.
Cliff Pennock JUN 14, 09:41 AM

quote
Originally posted by Cliff Pennock:

My reasoning for why the coolant in the overflow button should be hot is because the engine reaches temperatures of about 240F which should cause pressure to rise above 15/16 psi causing hot coolant to enter the overflow button (and sucked back into the radiator when temperature drops).



Reading this again, this sounds a bit condescending. It was in now way meant like that. You probably know how the coolant system works better than I do. 😄

But I just checked coolant levels with a cold engine and when I opened the thermostat cap, it was filled to the edge still. Checked at the radiator and saw the same thing. So it doesn't seem like it's loosing coolant (anymore).
theogre JUN 14, 10:19 AM
Engine Temp Gauge "Bouncing" is often 100% Normal. More So w/ Standard Thermostat in Fiero and many others and "dumb" gauges.
Fiero has ~ 3 times the coolant the T-stat tries to handle and depending on weather and time engine runs and loads, may take a long time or never reach a stable temp w/ OE type T-stat.

Get Stant SuperStat
See my Cave, Thermostat

When you have open the system, can take hours to days for system to purge air out even assuming overflow tube and tank is working.
That even if you follow Burping crap methods used by many others.

Just replacing T-stat shouldn't let much air in so should purge easy. Open the radiator etc lets more air in and take longer to purge. more so if you don't drive enough to heat cycle the system.
See my Cave, Coolant Fill

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

Doug85GT JUN 14, 03:51 PM
The last time I had coolant disappearing like that my water pump was leaking. The next time you get your car up to operating temperature, slide a flattened cardboard box under the engine and let it idle for 2 minutes. Pull the cardboard out and look for any fluid that dripped on it. The cardboard can be used to figure out where the leak is located too. It is also possible that something else is leaking when the system is up to pressure.
Patrick JUN 14, 04:48 PM

quote
Originally posted by Cliff Pennock:

But I just checked coolant levels with a cold engine and when I opened the thermostat cap, it was filled to the edge still. Checked at the radiator and saw the same thing. So it doesn't seem like it's loosing coolant (anymore).



Instead of removing either cap, especially the radiator cap, you should be able to determine coolant level by simply looking at the level in the reservoir. It should be at the lower mark when the engine is cold, and at the higher mark when the engine is at operating temperature. The reservoir should never be empty, and/or never be boiling out of the top.


quote
Originally posted by Cliff Pennock:

...my engine temperatures are all over the place. "Normal" temperature seems to be between 175F and 195F but most of the time it shoots up to 230F in 20-30 seconds, stays there for a few minutes only to drop back to 175 within 10 seconds. Rinse. Repeat.



Of the four Fieros that I've daily-driven and/or autocrossed (two 2.5s, two 2.8s), not one of them has ever had coolant temperature swings. I suspect you've got pockets of air within your GT's cooling system.

Yes, the Ogre scoffs at any sort of "crap methods used by many others" to burp the system ... but nevertheless, if you wish to try my method, Here it is.

[This message has been edited by Patrick (edited 06-15-2022).]

Raydar JUN 15, 01:30 PM

quote
Originally posted by Patrick:
...
Yes, the Ogre scoffs at any sort of "crap methods used by many others" to burp the system ... but nevertheless, if you wish to try my method, Here it is.




I should add, that this is precisely how I burp my system.
With the added step of parking my car on an incline, with the back end higher than the front. Probably not necessary, but it certainly doesn't hurt.
Patrick JUN 15, 02:54 PM

quote
Originally posted by Raydar:

With the added step of parking my car on an incline, with the back end higher than the front.



Raydar, that step was definitely implied. "Position rear end of Fiero higher than the front." I back my Fiero(s) up onto ramps if/when I needed to burp their cooling system.
Raydar JUN 15, 03:03 PM

quote
Originally posted by Patrick:

Raydar, that step was definitely implied. "Position rear end of Fiero higher than the front."



Ah. I missed that. Must have "head-spaced".
You've "known" me a long time. This should not be a surprise.