First time owning a legendary Supra really could use some help.

[mirage83]

Sounds like air pocket(s) in the cooling system, probably been hiding in the heater core. Couple of ways to deal with it. Park the car on an embankment with the nose much higher than the tail, pop the radiator cap and let the engine run for a while to evacuate the air trapped in the system. And/or, loosen the bleed screw on the coolant line on the front/left (looking at the engine from the front) side of the engine to allow air to leak out of the system.

Once you get all the air evacuated your temps should stabilize and not dance all over the place.

[Lopt]

I changed the thermostat last night and haven't had an overheating issue to or from work.

[Busted Knuckles]

I'd also consider replacing the radiator cap. Cheap insurance.

[mirage83]

How'd the coolant look? If it's been sitting there could be a fair amount of trash in the system, especially if the previous owner(s) didn't maintain it well.

[Lopt]

So far so good.. been driving it around the phoenix area with no issues. I think it might actually be maintaining a cooler temp than when I first got it.

The coolent is clear of any debri and isn't murky by any means. Just needs a good tune up and some seals replaced now.

[mirage83]

I'll keep my fingers crossed for you.

[Green7mgte]

standard cheap troubleshooting when dealing with temp issues is to replace the thermostat and the rad cap and burp the system. as stated above the fluctuating temp could be a result trapped air pockets.

In the summer time I always use a low temp fail safe thermostat. between 140 or 150 degrees. When and if it fails it just sticks in the opened position so no harm will be had to your engine. Costs a little more but were talking maybe 5 additional dollars.

Yes I know (to low thermostat) can cause issues also, by not running at recommended temp. I disregard this information because I'm a loose cannon and a lightweight masochist.