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Firebird overheating issues

I recently bought a 1991 Pontiac Firebird Formula 5.7L TPI with 124k. We was driving it home when noticed it was getting really hot (240-250). We pulled over at a gas station and gased it and let it cool off. We had been driving in the city at about 4 o clock for about 7 miles if that makes a difference. When it cooled we tried again. We got about another mile and the temperature would stay down so we pulled back over. We inspected it. The oil pressure showed really zero but we checked the dipstick and it looked okay. The engine sounded normal and the radiator is fairly new. The fan was blowing. We noticed that after it cooled off if you hit the gas the oil pressure would go up (similar to a tachometer…i thought that was weird). After if got cooled and we got on the open highway the temperature stayed normal and the car performed like it should. We think its the thermostat but are not sure. any advise? thanks.

I think you just found out why the car was for sale!

Any coolant in the oil or vice versa? I would do a compression check to see if the head gaskets need to be replaced.

Otherwise, do a coolant flush and change the thermostat.

no coolant in the oil. haven’t check the radiator yet. letting that thing get aood and cold before i touch it. where going to change to the thermostat today. I’ll do a compression check but the engine was absolutely normal.

no smoke either by the way

Are the fans working?

Not sure why it’s overheating, but you need to have the oil pressure checked with a mechanical gauge when the engine is warmed up. What you describe with the oil gauge is a classic sign of an engine with worn bearings.

Sounds like a stuck thermostat and a worn engine. The overheating issue you experienced is classic stuck thermostat symptoms. If a new thermostat clears it up, which I suspect it will, you are good to go. Regarding the oil pressure issue, the tachometer oil pressure gauge issue is very common to aging GM engines, and based on my experiences, is not really cause for concern as long as you monitor the oil level and not let it run out. I have had many an old GM vehicle with the tachometer oil pressure gauge syndrome and never lost an engine to it.