Hey Guys I just got my car today which is a 1987 Pontiac Trans Am GTA. It needs alot of body work but I got it for $1500. The engine is the l89 350-TPI and has 200,000 miles on it. When I drove it home it had no problems but when I took it out for a ride at about 10pm I had the car at idle for a while without watching it. When I went out to the car the temp gauge read over 260f. I immediately shut the car off and waited for a couple of minutes. When I turned the engine back on the temp was good. I took it to the gas station to get gas and I saw some steam coming from the engine. I popped the hood and found that the coolant bottle was completely empty so I filled it up with water. Thinking I solved the problem I took it out on the freeway and about 5 miles in the temp gauge shot back up past the 260f mark. about another 2 mins it went back down to normal temp. it continued to fluctuate like that the whole time I was driving so I immediately took the car home and parked it. I was really afraid that the thermostat was sticking and not letting coolant in but the car kept a constant idle in traffic so I dont think it was overheating but I could be wrong. Does anyone have any ideas?

If it kept a constant idle, it may have just been hot but not hot enough to cause detonation yet. Let it cool completely off. Open the radiator and see where the level is in there. If low, top it and the overflow bottle back off. Fire it back up and let it warm up. Check for leaks. Look around for any evidence that coolant/water may be leaking anywhere...including the heater core. It would leave water in the pass. floor area. It sounds like either you have leak and it's low on water or maybe the fans aren't coming on maybe and at low speed the car gets hot. Does it still have the front air damn/plastic piece under the front bumper that directs air into the radiator opening?

Forgot to mention could be bad sensor as well. And most thermostats stay open when they fail so it will just be open flow. But do check all the above first.

Yea, Berlin makes another good point...once you check fluid levels and such, make sure the gauge is even reading correctly. If you have one of those infrared temp. guns or a manual temp gauge you can plug in or check with to verify the correct temp., that would be your next step.

Well I am pretty sure when I bought the car there was no coolant in it to begin with and I just noticed that it was low when I checked it at the
gas station. The car does not just get hot (according to the gauge) when
at low speed because the second time it happend I was at 60mph on the
freeway. the gauge just kept jumping from vertical to horizontal past the
red line within 5 mins. I am going to flush the coolant system and check the
sensor today so I will see if that solves the problem.
Thanks for the help so far guys.

Well I tried to flush my coolant today but guess what...
it was already empty I tried putting coolant in but
it just spat it out the bottom of the car. I quickly looked
under the car and there was a stream of coolant coming
from the lower radiator hose so I thought "oh well its the hose"
and I proceeded taking up the whole day replacing the hose.
when I got it all done I tried putting more coolant in and guess
what...there was still a leak. After taking a closer look I determined
that it was the water pump throwing a crap load of coolant out.
How hard is it to replace one on a TPI? I have done one on a regular
350 before but never on a trans am or a TPI.