1991 Plymouth Sundance with new thermostat and sporatic heating

Please help if you can! A few days ago, I was driving home and noticed I had no heat, despite my warm engine. By the time I got home my car began smoking and overheating. I drove immediately to the shop and had both my water pump and thermostat relaced.
When I picked up my car, the mechanic told me that my thermostat sensor was acting wonky and may need to be replaced.
Since then, the engine heat gauge has been bouncing from cold up to hot and back again every few minutes. This wouldn't bother me since it's clearly inaccurate, except that every time it goes up to hot, my heater stops blowing warm air and begins to blow cold air.

HELP! I can't make it through winter with no heat, but I don't want some mechanic to rip me off (possibly again).

 

Your gauge may be acting up because of a grounding problem or the sensor for the gauge is bad which, most of the time, is the same thing. However, a problem with the gauge won't cause an overheating problem.

But, two things you said goes together. When the temp on the engine goes up and the heater starts blowing cold air, it's an indication the thermostat is sticking and not allowing the water to circulate through the heater core. There's always an outside chance something is plugged, etc, but the thermostat is usually what causes that problem.

Hope this helps,

Bob

 

Could also be a case of an air bubble in the system. I'm assuming the engine has beeen checked for something more serious like a blown head gasket?

 

well, right after the water pump and the thermostat were replaced, I noticed the gauge acting up so I waited until the engine was cold and then poured cold water in the radiator in case there was an air bubble. So now I'm even more confused
I'm not sure if they checked the head gasket, would a mechanic normally check that if the engine was overheating?

Thanks for your help, I really appreciate any advice I can get!

 

Well, I would. Modern engines do NOT tolerate overheating and the older and more mileage they have, the more susceptible they would be to blowing a head gasket and/or warping a head. For the possible air bubble you can try one more thing. Park up-hill on as steep an incline as you can find, ideally car ramps if you have them. Remove the radiator cap and start engine. Let run until the thermostat opens (upper hose will get hot and you should see waterflow in the radiator). Until it gets to that point you will get some overflow from the filler neck due to expansion, that's normal. As it approaches operating temp and the thermostat opens the level in the neck should drop somewhat. Immediately top off the level to the top and put cap on. This should burp out any air bubble. Some cars are equipped with a special fitting, often labeled, for doing the bleeding operation, but I'm more partial to this technique.

 

I doubt if an average (I did say average) mechanic would automatically check for a blown or leaking head gasket since the first stop is usually the thermostat/water pump/belts, especially with no heat in the heater. If there were some other symptoms given like air blowing out of the radiator, they would probably follow that lead.

Usually a blown head gasket will force air into the cooling system. The latter runs with about 10 lbs of pressure, while the combustion pressure is many times that. A head gasket can leak coolant either to the outside or internally to the oil and be void of any contact with combustion pressure.

So the general rule is if the coolant is frequently low check for outside leaks, water in the oil = milky color, and finally into the combustion chamber = a lot of air in the cooling system.

 

Thanks for the advice, as soon as the weather clears up (hopefully tomorrow) I will try the parking on the hill and I will check the oil to see if it's milky!