yes - during winter months I have to hang cardboard onto the front of the radiator to get any heat at all once it has opened

One of two things can cause this...

1. Coolant is leaking past the thermostat.(due to installed incorrectly/missing gaskets)
2. Thermostat is incorrect or failed again.

Because it'* done this with more than one thermostat, maybe there'* something more at work. It'* probably not the case here, but is the edge of the thermostat seated into the groove in the rubber o-ring visible in this picture? And Is the large end of the thermostat down into the engine?

Is the radiator full of coolant and has the air been bled from the heater after the car has warmed up fully? Is the heater giving consistent temp when the temp variation is occurring? Just leave the HVAC on heat and see if there seems to be air in the heater and it won't give out strong heat part of the time.

Is the underside of the radiator cap gunked up keeping it from sealing?

And another question for you, Bonne99, is the car losing any coolant?

I'm sure it is finally closing without leaks overnight, as the temp climbs easily till it hits the 'open' threshold - it just never closes until the coolant is way cool

I always thought that they would slowly close part way or somesuch as the temp when down, but these seem to open with a pop & close the same way back at room temp.

Are There diff versions of these or markings of some kind to ID functionality? I've just been buying whichever is cheap at autozone.

Last 15 or so years, I've not lost any coolant.

If he was loosing coolant, we would be looking at a overheating condition, not an extreme cooling effect.

The temp at which the thermostat opens is usually stamped on the bottom of the t-stat.


Yours should say something like 195.

I always recommend with the 3800'* to run a 180 in the summer time and a 195 in the winter. Try switching to a better brand and see what happens.

Yeah, was just hoping there might be some kind of marking that would show whether it moved progressively or not. When I put these into water & heat them, they open suddenly with enough of a popping sound to splash water all over - close with a snap also.

Maybe I'll try a Stant.

I just took out my second Stant thermostat--the higher quality/priced model even. It was running warmer than others and tended to go warm, e.g., 207, 213, 217 even, and stay
there for a minute or more while the car had cooler water in the lower radiator in the return hose to the water pump. I called it sticking. The consistent sign was I drive a mile of side road and then I get on the interstate: the temp goes from 198 to 205 and stays for a few minutes and then drops to 200.

I put in a Murray $6 stat from OReilly auto parts ( Checkers in parts of the country). It runs at 194-198 instead of 199-207. And it rarely goes to 203 and moves right back down with higher revs or/and car moving through the air.

And the temp is very stable. It reaches operating temp and varies very little from there. Years ago in my Ford Ranger it was constantly going from 1/4 to 3/4 up the temp gauge and back down again every few minutes or so until I put in a Motorcraft thermostat. After that experience I only use OEM thermostats.

***constantly