# Thermostat replaced, overheating now



## dgallmeier (Nov 5, 2009)

I have a 1999 Nissan Sentra, which has had a history of thermostat problems. I replaced the thermostat myself a few years back, which solved an overcooling problem. It was replaced again this February after overheating issues - also, the heater stopped functioning intermittently.

After the heater began failing intermittently again, I decided to replace the thermostat again. Now the heater is not working at all and the car overheats as if the thermostat is closed. The upper radiator hose is hot, while the lower one stays cool. 

I've replaced the thermostat twice now, same results on both. Before I installed the the second thermostat, I tested it in a pot of boiling water and it would open up at at 180-190 degrees F. I also drilled a bleeder hole into the second thermostat (none of the off-market ones seem to have this) and both times I bled the coolant system by raising the car on jack-stands and running the engine with the radiator cap off.

Does anyone know what the problem could be?


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## azkicker0027 (May 31, 2002)

did you check for any restriction in the cooling system? maybe the radiator? is the water pump working? does coolant seem to flow at first start when the radiator cap is open? can you check with an infrared thermometer where the restriction and great temp difference be? is the thermostat facing "up" in the correct direction? air pocket in the heater core?


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## dgallmeier (Nov 5, 2009)

The problem turned out to be with the one thing I neglected. On my car, there is an air-bleed screw for the cooling system on the engine block, which I did not remove.

I removed this screw and filled the radiator until fluid came out of the screw. Then I filled the radiator full, put the cap back on and took it for a test drive. It took a few minutes, but the car wasn't overheating and eventually the heater began blowing hot air.

The airbleed screw is near the distributor cap on mine. There is a larger screw above it, which I believe holds the distributor assembly to the engine. But the airbleed screw is smaller and just below this one.


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## jdg (Aug 27, 2009)

Assuming you have a 1.6L, there's a brass air bleed screw right near the thermostat housing itself.
I've got a '97 Sentra (wrecked) and a '98 200SX and neither have an air bleed screw next to the distributor cap that I know of...then again, I never looked over there. Maybe it'll punch me in the face next time I go out and look at it...Could happen...


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