Is my thermostat stuck closed?

[dnguyen1963]

Hi,

The last few days the temp is dropping down to below 30 and I had the heater setting to max. My temp gauge has been fluctuating between 1/2 and 3/4 and settled down to slightly above 1/2 mark. Last night the temp gauge almost got to the red mark. I checked the car and found the upper radiator hose warm, but the lower one was ice cold. Is my thermostat stuck closed or is there an air bubble flushed from the heater core and trapped inside the engine somewhere? The car put out good heat when I set it to max. Any help would be appreciated.

Thanks.

[paperplane94]

Same problem...

I'm thinking a leak in the heater core somewhere is introducing an air leak when the heater is on. Perhaps through the O-rings in the heater core piping?

Otherwise, the car will run well within the normal temp if the heater is not on, but once the heat is turned on the temp starts to creep slightly higher. The next day after turning off the heat, the temp will return to its previous state of well within normal, probably because the heater valves seperate the core from the rest of it. OP do you concur?


I see a giant hole in the dash in the near future...for me and the OP.

[ttrousdell]

Until the engine warms up the coolant will curculate in the engine only. Operating temp is determined by thermostat. For example at 80c, an 80c thremostat will open and allow coolant to radiator to maintain the temperature. In example if temp climbs past 80c and the thremostat hasnt opened it is defective. dnguyen i would suspect the thermostat. Try bleeding the system first though. Bleeder is at top of thermostat housing.
Paperplane-is the radiator hot at operating temp. If so the tstat is opening to allow cooling and is not defective. if so your overheating may be from air in system or not enough heat exhange at radiator possibly caused by bad fan clutch, dirty radiator, etc
-the tstat can be tested in a pot of hot water on a stove.

[KenC]

If your gauge is above half, and the lower radiator hose is cold, then your thermostat is closed.

Regarding the second post, if you have a leak in your heater core, your cabin is going to be steamy and sweet smelling... and you'll need a new carpet.

[paperplane94]

How does this relate to the heater being on in both of our cases?

If there is a hole in the core can it not just put air into the system without any signs of a leak, but also cause a bubble to travel to the thermostat housing which in turn keeps the thermostat closed because there is no coolant around the thermostat?


^longest question ever...

Maybe I am over thinking and the thermostat is just slow to react.
BTW my lower hose was cold too during the 3/4 mark event. thermo closed.

[dnguyen1963]

How does this relate to the heater being on in both of our cases?

If there is a hole in the core can it not just put air into the system without any signs of a leak, but also cause a bubble to travel to the thermostat housing which in turn keeps the thermostat closed because there is no coolant around the thermostat?


^longest question ever...

Maybe I am over thinking and the thermostat is just slow to react.
BTW my lower hose was cold too during the 3/4 mark event. thermo closed.

Ok, I am totally convinced that my thermostat is stuck closed. I bled the heck out of the system so there was no air in the system. The upper radiator hose is cold up to the thermostat cap so it is definitely stuck closed.

With regard to a small hole in the heater core, I can't see how it would introduce air into the system because the internal pressure is much higher than air pressure so any small hole will cause a leak in coolant. I can understand that as you cool down the car and the heater core, air can be sucked into the system through a small hole. However, you should still see coolant leakage when the heat is turned on.

Thanks to all of your inputs. New thermostat ordered.