Starting just this morning, the car warms up to the normal indicated
190F but from then on slowly drops to about 150F, slowly climbs back to
190F, and continues to do so, apparently unaffected by any external conditions.
Until now, it warmed up to 190F and just stayed there.
In addition to the above (personally I don't think it is the sensor, but the thermostat was my first thought) check coolant level and make sure there is no entrapped air in the system.
With the heater on, do you notice these temperature swings affecting the heat from the heater?
Coolant level's OK and the heater behaved as well as ever though the recent nasty winter. No noticeable fluctuations in heater output, although the weather's finally starting to get to the point where the heater's not on full all the time.
There's one thing I only just noticed, which wasn't obvious because of the way I usually use the car.
If the car's been allowed to cool by being parked for a few hours or more, or if it's only been stopped for a few minutes (like when buying gas), the temperature seems to behave as always, rising to 190F and staying there.
It's only when it's been stopped for an intermediate period, maybe between a half-hour and an hour, that these temperature swings occur.
Since I don't usually do the sort of multi-stop errands that involve these intermediate-length stops, this problem might have existed un-noticed for quite some time.
Anyway, I'll look into replacing the thermostat when I have the time.
The thermostat is a fairly simple replacement. But, make sure the engine is full cold before removing the plastic housing. Less likely to warp if all is cold.
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