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coolant temp not constant


Full Member
Buick Ownership
1999 Park Ave Ultra; 1985 Olds Delta-88 Royale
Coolant temp on my 99 Ultra is not constant. On a cold day on the highway it never gets to 180. This morning (1°F) it was only 160 even after a 30 minute commute. If I idle for 10 minutes it'll drift up to just over 200. Is this just a thermostat problem?



Full Member
Buick Ownership
2007 Lucerne CXL. and 1995 Lesabre currently, past 1973 Riviera, 1968 Riviera
Thermostat would probably be the first item to check and what temperature was installed, coolant age / temp should be another option to check. Could be air in the system as it kind of depends on when this started to showed up and when the other items have been done last, there may be others on the forum who can help with suggestions.


Full Member
Sustaining Member
mid michigan
Buick Ownership
Buick park avenue ultra
Kinda sounds like the thermostat is partly working, but coolant temp is related to outside temp. Get a new thermostat in there before going any further.......


3.8L Member
The Gem State
Buick Ownership
1996, 92 Buick Century
Get a good thermostat from a GM dealer or a premium, 'high accuracy' thermostat. Cheap thermostats will act like what you have, even when they are new.
Buick Ownership
1999 Buick Park Avenue Ultra (Supercharged)
It sounds like you may have one of the optional lower temp thermostats. 180. My car is a 99 ultra and its doing the exact same thing. If i idle it gets up there. But my coolant is DISGUSTING(going to flush that bad boy), and it did run low when I first got it so possibly air, and the thermostat is definitely getting changed.


Full Member
Southern Utah
Buick Ownership
2003 Park Avenue Ultra
Sounds like the thermostat is either missing entirely, opening way to early, or stuck open.

The cooling system works like this:
Engine is started cold, starts running and producing heat. Thermostat is closed so no coolant flows to the radiator. The coolant circulates inside the block and into the heater core inside the cabin(even when using max AC). If the engine is running, the coolants is flowing. The radiator and hoses are full the entire time, the coolant is just not moving as the thermostat(heat controlled valve) is closed. Some radiant heat goes into the coolant from the lower radiator hose as there are not 2 thermostats, just 1 to block the flow. The fluid is touching so heat goes into it.

As the thermostat approaches it's operating temp, it starts to open up, allowing coolant to flow into the radiator. On a 180* Tstat it will be fully open at 180*, 210*, at 210. Most thermostats start opening about 10* cooler than the rated temp, so you can see 170* partial open on the 180* one, and 200* partial open on the 210*. It is designed this way so you don't get a sudden thermal shock from hot coolant hitting ice cold coolant. Instead it starts to slowly mix before a full valve opening. Once the thermostat opens the now warm coolant flows into the radiator and starts to circulate through it and back into the engine with cooled coolant.

The radiator is a heat exchanger, changing the heat from the engine to heat in the air and cooling back down by airflow across the fins. The airflow comes from either fans or air movement at speed. So the return to the block should be 50-100*cooler than the inlet depending on ambient temps.

If it's cold outside the thermostat(heat valve) will close back up when the coolant drops below the 170* or 200*, again circulating the coolant through the block and into the heater core. Once it reaches opening temp it again cracks open and circulates through the rad. This lets the engine be at operating temperature for either max power(180*) or max efficiency(210*) no matter the weather. Even if it drops to -40 the engine should still get to operating temp if the entire system has no issues(correct fill, not gunked up lines or cooling passages, no air bubbles, good coolant, operating tstat, no leaks, etc)

Thermostats usually fail one 1 of 2 ways, stuck open, or stuck closed. Stuck open never warms up. Stuck closed overheats like crazy as the coolant never gets a chance to dump excess heat except through the heater core. A lot of people just remove them entirely to mask overheating issues caused by plugged up radiators, blocked lines, degraded coolant, air bubbles, and leaks.


Top Dog
The thermostat sits in a rubber ring with the edge of the thermostat in a slot on the inside of the ring. Some people install the rubber seal with the thermostat not sealed into the middle and coolant can get around the edge. Another factor can be if the thermostat has not been changed it may be erratic in operation OR the rubber ring may have hardened and cracked with age allowing coolant to get through it. Replacing the thermostat would require a new rubber ring to use as the sealer.