A simple question but... no idea if it's supposed to work like this. '85 duke auto w/AC.

My temp gauge fluctuates pretty badly since yesterday when the car is up to temp and standing still. It stays on the second mark all day when driving even crawling.

But when I come to a stop it rises pretty fast to the red - even my idiot light turned on today but just give it a little gas and it pulls down to the second mark in a matter of 3 seconds. No real sings of overheating though I did blow some coolant out of the reservoir tank some days before - but I guess that's because I added too much to the tank earlier.

All hoses are hot, normally hot I guess so there's flow through the radiator and the t-stat opens up. The radiator cap is dead cold though and I hear some hissing when I shut off the engine from the cap. It's the proper non-vented cap. No leaks anywhere found.

What could this be? A bad gauge, sensor? Air bubble in the system? Something worse? Driving 80mph all day and it stays dead on the second mark. Temperature's here are ~70 outside. Gauge shows proper cool when cool.

EDIT: My radiator fan works both using the A/C buttons and on its own but it rarely comes on by itself. Mostly only after I shut off the car and turn on the ignition after a few minutes.

EDIT2: I had the temp gauge fix done years ago that swaps the bulb test from the gauge to the light so that works too. Gauge doesn't peg and the light comes on when it should according to the gauge. Thermostat cap is brand new and the thermostat is a few years old. I do have a brand new one lying around just in case.

[This message has been edited by cebix (edited 04-27-2018).]

The cap and even parts of radiator Can be cool/cold w/ engine warm to hot. more so on Fiero because no engine heat soaking the front of car.

Rad Cap is on cold side plastic tank that's a poor conductor of heat.
Many times Rad Overflow parts are cool/cold too.

Weather can effect front heating and most of front parts like HL lift assemblies can be cool/cold when weather is cool/cold.

------------------
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 04-27-2018).]

Is your overflow tank 1/2 full cold?
When warm, does the level of the overflow tank rise?

When cold it's almost full - that's my bad, I filled it up too much. Yeah, it rises when heating up and I guess that's why it overflowed a few times before, now it's a little over full when hot but not pouring out.

When cold, if you loosen the front cap, does fluid come out, or do you hear air coming out?

No, when cold it all sits there still.

EDIT: Oh wait, you mean the rad cap, not the reservoir cap? Didn't check that. Will do that tomorrow since the engine's still hot now. It's 10pm here right now.

[This message has been edited by cebix (edited 04-27-2018).]

Replace the front cap and pull the thermostat and fill the system.

Thanks, that sounds reasonable. I get the idea of filling the system with the thermostat out but why do I need to replace the front cap? It was replaced ~2 years ago. What can make it go bad and which of my symptoms tell the story that it has gone bad?

I will replace it anyway since it seems to have rusted a little but a little knowledge in what to look for would help. Thanks!

I meant reinstall the cap.

If you fill from the back, it will over flow if the front cap is off.

I will wager you already knew that!

Cool, now I get it. I thought you meant the cap was bad and it needs replacing with a new one. Will try that tomorrow, test drive and let you all know. Thanks for the suggestions.

Any time the coolant temp gage starts fluctuating rapidly, it's a good sign of an air pocket or low coolant.
You are very welcome!

So I guess it's solved. Think it was an air pocket. Not a big one though, it only took about ~200ml of coolant until it topped off. Now the gauge's steady and works how It should I guess.

Burping the system was impossible though. Keeps foaming up on me after starting cold for ~30 seconds and the coolant level doesn't drop so there's no way to add more.

Though I do hear the rad cap hissing just about when the radiator fan kicked on. Could this be where some air is getting in? Or is it normal for it to "hiss" and burp a bit?

As the coolant heats up, it expands and also the system pressure increases.
At some point the pressure exceeds the rating of the cap, it opens slightly and coolant (and air, remember the top of the radiator is designed to be an air trap) is forced into the overflow tank.
Upon cooling, the coolant contracts, pressure drops to the point of a slight vacuum and the coolant is pulled back into the system. The tank discharges through the bottom to keep air from being introduced into the system.

Thanks, the cooling system was always some kind of mystery to me - especially on the Fiero since it's a little "different".

Thank you for your guidance, this means a lot.

You, sir, are very welcome.
I'm glad it worked out and I was of some assistance.