I have a fiero with a 3800 Buick V6 in it and sometimes when I'm driving, I'll see the temp gauge shoot to max. If I shut the car off, and just turn on the ignition so all the gauges function, it reads about 190 degrees (which is where the car runs all the time). However if I start it back up, it will typically read max temp (250) on the autometer gauge, however I've not noticed any engine sputters/issues (but I usually shut it down as soon as I notice the temp) and if I slowly remove the radiator cap, I don't hear any pressure release nor water trying to push its way out. Also, when it goes to "max", the gauge moves as fast as speedometer sweeps when initially turned on (temp gauge doesn't move at intial power on as its not a factory gauge).
Also, if I let the car sit for a few minutes (engine off) after the max temp gauge, it usually restarts and reads about 190 degrees (usually immediately to 190 or sometimes it takes like 5-10 seconds before the temp goes down).
After it has been running if I feel the top radiator hose, it is warm/hot (I can hold it but it feels hot to the touch) which leads me to conclude coolant is flowing (ie thermostat is opening), and the lower radiator hose is warm as well, but much cooler than the upper hose.
Any ideas if I'm looking at a bad temperature sending unit, a bad gauge, an air bubble or possibly a thermostat? I can say my thermostat sending unit is on the intake closest to the water pump side of the engine if that matters. And I don't notice any odd smells/colors/smoke/liquids coming out of the exhaust.
Seems odd that it will run fine for quite awhile (not a consistent timeframe), and then eventually shoots up. I'm going to look at putting a moroso inline filler by the thermostat housing, and will put a high pressure (27-29psi cap) so that I can ensure better coolant fills. I will plug the overflow valve, and still rely on the 16lb cap and overflow in the front of the car to handle any pressure relief. Once I install it, I'm planning to pop the front cap and make sure its completely full, then will finish filling at the rear filler cap, and run the car through a couple of warm up/cool off cycles with refilling at the back (which will be the highest point in the system) after each cycle.
Any thoughts/ideas on what could be the possible cause?
Just an update - I did two things today, first I parked with the front end lower, and swapped the front radiator cap. I went from ST-16V (Stant 10331) to a ST-16 (Stant 10330) which isn't vented so holds pressure better based on my understanding. Also, I put the moroso filler and a 27-29lb cap (overflow vent is plugged solid - still have functional overflow on the cap on the radiator up front) in the top hose where it comes out of the thermostat housing and topped it off, bounced car, topped off, for a few times until it seemed all air was out of the system.
After that I took it for a test drive, and didn't even get 5 miles from home on the highway and the temp gauge went from 180-190 and immediately pegged full right. I pulled over, verified my cooling fan was running and then shut the car off. Waited about 30 seconds, turned ignition on and temp gauge read 180-190, and then when I started it up, the temp stayed in the 180-190 range. Continued on about the ride, and the rest of the ride (about 20 minutes more) the car temperature stayed in the 180-200 range which is what I'd consider reasonably normal.
When I got home, I can say I did notice a difference in the hoses, as they all were warm, however they seemed "firm" when I tried to squeeze them, whereas before the new cap, they would seem kinda soft. So I'm hopeful that it was just something as simple as the front cap not keeping a tight seal and potentially letting some air bleed into the system. I plan to take off the rear cap tomorrow and top off any needed coolant, but I'm hoping this takes care of the issue.