First, are you absolutely sure you're overheating. A simple test would be to leave the AC turned off, start the car and let it sit and run. Raise the front hood and watch for the cooling fan to come on. When it comes on, look at the temp gauge to see if it's up in the 220-230 range. The cooling fan should quickly bring it back down, but the important thing is where the fan comes on relative to the gauge reading. It could simply be that your temp sensor circuit is giving an erroneous reading. If it all turns out to be correct, then try removing the thermostat for a while and see how it reacts. We can check it out at the meeting or maybe this weekend if you'd like.
Second, one of the common problems with 134a conversions on the Fiero is that people tend to put in the same amount of 134a as was originally required of R12. The expansion rate for 134a is much greater than the R12 and the same volume of 134a into a Fiero system will overpressure and reduce cooling efficiency. It could possibly even damage a weak system. You should use only 80% the required R12 refrigerant when converting to R134a. The system required 32 oz. of R12. Multiply that by .80 and you get 25.6 oz. So, 2 12 oz. cans is very close. Not enough difference to open a 3rd can in my opinion.
After experiencing poor cooling on my 87 which was filled using a gauge, I slowly drained it, being careful not to loose any of the oil. I then put 2 cans (total 24 oz) of 134a back into the system and it cools just great.
