I am starting this topic to give people with overheating problems some important info that I have recently verified. It is not that this info is not in other threads, but none of the other threads seem to end in a conclusion. Here I will give you what symptoms I experienced, and what the problem turned out to be.
Just to give away the ending, my problem turned out to be that I had the incorrect radiator cap. However, I experienced a number of the symptoms that I have seen attributed to other problems, in other threads.
I have an '88 formula with the 2.8 v-6.
The whole think started when I replaced my cracked radiator. I had never replaced the radiator cap, so I tossed the old one (without looking at the part number) and bought a new one having Stant part number 11331 as this was the number listed in Stant's catalog. This is a lever-vent cap. Only the last 3 digits of Stant's part numbers refer to the actual cap. The first two refer to the packaging, so I will refer to this cap as a 331 from now on.
Immediately, I started to experience overheating. The car would run fine under light loads but I would always find that some coolant had leaked ot the overflow. Every time I parked the car, I would notice a big puddle of fluid on top of the overflow tank. By coincidence, I had only water in the system at the time (I had hissed out all the antifreeze while trying to find the crack in my radiator). During these times, I would see the temperature gauge fluctuate wildly after just a little bit of driving. Also, the cooling system could not recover after reaching 212 degrees. The coolant would all come spilling out of the overflow tank and I would overheat.
I got my final clue to the problem while refilling the system. I noticed that the water level at the thermostat housing would go down after I let the car sit with radiator cap on. This really semed strange because it seemed as if there was a leak, but I could find no water dripping under the car. I realized that the water was leaking into the overflow tank. I bought a ew radiator cap having Stant part #230, a non lever-vent cap.
This cured the problem.
I called Stant's tech line and asked them about this problem. They said that 331 and 231 caps are NOT "constant pressure" caps. In other words, they do not start retaining pressure until they reach a certain temperature. They said that this is done to reduce wear and tear on the hoses. The 230 and 330 caps ARE "constant pressure" caps. They maintain pressure at all times. I explained to her that theFiero must be filled from the thermostat housing ,which is higher than the radiator cap. She said that such a situation definitely requires a constant pressure cap. She said that Stant gets their recommended part numbers by cross referencing the AC Delco catalog.
I was just about to pull the heads off looking for a blown head gasket. I hope this helps someone.
Bill Temple

Bill good follow through! I also learned all about the Fiero cooling system on my first one and built a homemade tool that is indispensable when working on Fiero cooling problems. As a matter of fact, it would have helped to identify both your root causes very quickly.

Here it is:

I bought a new thermostat cap drilled a 1/2 inch hole in the center and pulled a new tire valve through it. You simply replace your regular cover; use a hand pump and pump until the radiator cap belches (approx 15psi). Use a tire pressure gauge to verify the 15lbs. If you don't have 15lbs or coolant is pouring from somewhere you've got a problem (or maybe two). Now this is not 100% fool proof as the system characteristics will change at temp such as a head leak but it will get you 95% of the way there. Good post and again good follow through.

I was very impressed by your low-buck cooling system pressure tester. I actually spent about $80 on the professional version that includes a pressure gauge and everything. I must point out, that I did a pressure test on the system and found (to my surprise) that the system held pressure without problems. It is easy to see why in the case of the radiator cap. The part of the system replaced by the tester was the problem - the radiator cap. In the case of the cracked radiator, I am assuming there was some cooling system stop leak in the system that made it hold pressure whil cold. I only found it when I experienced a catastrophic failure of the seal on one of the tanks.
Gotta like that tool though. I have actually done the same thing for the cap on my brake master cylinder to make a one man bleeding setup.

I did a quick skim of your post, so if I am in error I apologize.

From what I gather you bought a vented cap per the stant catalog, rather than a non-vented cap which is the correct part. If so, yes this is a known issue, but not everyone knows, so its a good heads-up.

quote The part of the system replaced by the tester was the problem - the radiator cap.

That is why I used the cap on the thermo so I could see the overflow working.

quote In the case of the cracked radiator, I am assuming there was some cooling system stop leak in the system that made it hold pressure whil cold. I only found it when I experienced a catastrophic failure of the seal on one of the tanks.

Very true. I realize that any pressure tool will only catch certain issues on a cold system. But hey every little bit helps.

quote Gotta like that tool though. I have actually done the same thing for the cap on my brake master cylinder to make a one man bleeding setup.

What do you know... I did the same thing. I don't know about your results, by my bleeder wasn't as effective as I wanted it to be.

PS Bill this topic should go in the 1st time Fiero owner's user manual. Anyone want to start one ... Ogre... anyone?

[This message has been edited by 87GTZ34 (edited 06-27-2003).]

My apoligies,
I did not notice that you had put it on the thermostat housing cap. You are right, then, because your device will will catch radiator cap problems that my "professional" setup will not. I will put this on my list of handy tools that I need to make. I wonder if I can fit a cheap pressure gauge on it somewhere?

Some cheap bicycle pumps have a gauge built-in.

pressure test gauge.... you need a low pressure gauge, 20-30PSI max, for radiator work. A good one. cheap ones can be several PSI off. Enough to damage the cooling system.

The cap issue has been covered in my cave... The error in the Stant catalog has caused many people a pain. I think I'll break that part out of the coolant filling article. It's sort of become a subject of it's own...

The answer from Stant was interesting... To bad it's at least partly wrong. If it was correct they would have the right cap as the GM parts books do list the non vented (constant pressure) cap.

Being that the numbers are only one digit off... I expect that someone at Stant made a typo and they simply don't seem to know about it or they don't care. That or they got the wrong info from GM and we know GM doesn't give a rat's ass.

The whole thing being odd anyway since Stant makes the caps GM installs on new cars and AC Delco sells. You can tell what ones are Stant made. There is a letter S in a specific style stamped into them.

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Nobody loves me. Everybody hates me. Guess I'll just go eat worms.