I have a 2000 deville base. Can someone tell me, if there is air in your coolant
system, could this cause your thermostat to not open?
Air would not be introduced into the cooling system unless there was some work done to open up the cooling system, or if it overheated and pushed all the coolant out, or a leak as to cause you to add a lot of coolant to the system without purging the air out. If this is the NorthStar engine, they are notorious for bad head gaskets. You could try and change out the thermostat, but also the water pump could be bad also, the impellers on the water pump erode away so it can not circulate the coolant in the system.
As these cars have a backwards cooling system air around the thermostat is almost impossible as it is in the lower hose supply housing, so if you the no the thermostat is stuck then better replace it and get one that has a fail safe option so if it fails it pops open and stays open so you don't overheat the engine. And Rowefast is right about these engines blowing headgaskets, so unless it has had the block redone to prevent this your at a high risk of that happening if you get even the slightest bit overheated.
Okay, I have no leaks, no coolant on the dipstick, no white smoke, and I'm not losing any coolant. What could it be, besides the head gaskets, did a block test.
If you did a block check and the fluid did not turn green then it is not head gaskets, that fluid is very sensitive to combustion gasses. I can remember one vehicle that was brought into my shop with a lot of air in the cooling system and I also though head gasket until I did a block check and got a negative on the test, so then I pressurized the cooling system to the limit and noticed the water pump just seeping ever so slightly so I replaced it and no more air in the system. Just a something found in all my years, might be worth a check.
Looking for a Used DeVille in your area?
CarGurus has 20 nationwide DeVille listings and the tools to find you a great deal.
Search Cadillac DeVille Questions
Cadillac DeVille Experts