Pontiac Grand Prix 2007

yeah sometimes it's your coolant temperature sensor that's on the outside of the radiator maybe you could unscrew it clean it off and put it back in don't forget to put the white pipe dope around the threads additionally I think your coolant looks pretty funky it doesn't look very clear looking looks kind of cloudy you might want to change your coolant but it could just be that your temp sensors clogged with a bunch of junk around it so it's not very accurate?

Ive owned 3 3800 powered vehicles. First thing you need to do is get rid of dexcool. Completely flush it out and refill with distilled water and add some preston cleaner or TSP drive the car for about 3 days then drain then refill with water idle until warm cool down drain. Then refill with green antifreeze and don't forget to bleed the system there is a small 8 mil bolt on top of the thermostat housing. Once you have this done change your thermostat to a 180 degree thermostat. You should see a cooler running car unless in stop in go traffic during this time it will still climb to 190. Also 04+ grand prixs do not have coolant temp sensors located on the radiator.

This is perfectly normal in cars that actually idle (my last two vehicles have had idle stop/start system, and a hybrid system, respectively, so I haven't seen this behaviour in the last two vehicles).

When idling, there is no air current on the radiator. Thus, the system continuously heats up, until the radiator fan(s) kick in. The thermostat opens at usually bit below 90 degrees Celsius or so, which is the normal temperature of the engine when driving. However, the fan(s) kick in at much higher temperature, probably around 100 degrees Celsius or so.

Thus, you are close to the hot red zone when idling, but you never reach the hot red zone if everything is working properly.

When driving, the air current in the radiator ensures that radiator is adequately cooled even without fans rotating, and thus, the thermostat opening temperature (usually bit below 90 degrees Celsius or so) dictates the engine temperature.

If your engine temperature stays normal when driving at highway speeds for extended periods, your water pump and thermostat are probably working just fine. The engine temperature when driving at highway speeds fo extended periods should stay constant. If there's variation in the engine temperature (apart from asymptotically reaching the steady-state level), something's not right. A bouncing temperature reading at these speeds would be a cause of concern.

I would check the coolant level (please don't open the radiator cap when engine is hot!) and the tightness of the radiator cap (again, don't loosen it when engine is hot!), though. It buys you peace of mind to do this check, but it costs you practically nothing apart from few minutes of your time.