Thermostat Bypass Valve

[frank2644]

My 1994 Cutlass Supreme has a thermostat bypass valve that is mounted on the water pump housing. I can't find any information in the shop manual about it.

Does anyone know what this valve does?

I am having some overheating problems. Basically it looks like the thermostat is not opening (top hose cold). I have changed the thermostat and it made no difference. Also the old thermostat tested good. Although I don't get much air out, bleeding the coolant seems to help somewhat but the engine still runs hot (maybe 230-240 degrees). And I have to bleed again almost everyday, again not much air escapes. The coolant has been checked for hydrocarbons and the oil is clean.

It seems to me that the coolant just doesn't flow good especially when cold (per above). I am thinking that the water pump is weak, but I believe that is a rare event with most cars.

Back to the original question, could the thermostat bypass valve cause this kind of problem? If not, any suggestions?

The car has 60,000 miles and has been well maintained.

Thanks.

[GTP Dad]

I seriously doubt that the bypass valve would cause this issue due to the fact that it shouldn't move enough water to prevent the thermostat from opening especially since the temp goes as high as it does. I would have to believe that the issue is with the water pump and air. The pump can be damaged and if worn internally it may not be pumping enough water to move it through the system. Of course air will do the exact same thing.

I would replace the pump just in case it is bad. Then to bleed the air from the system, leave the radiator cap off and allow the engine to heat up to where the thermostat opens. This should get the majority of the air out. You will then need to refill the system. I don't know what size engine you have but if you have a 3.1 you should be able to insure the block is full by removing the thermostat housing and filling it to the level of the bottom of the thermostat. Reinstall the thermostat and then refill again. This should get the water to a level where the above will work. This would be impossible on a 3.8 due to the location of the housing.