Chevrolet Repair: thermostat, blown head gasket, coolant levels

Question
My car has overheated and I thought it was a thermostat. I took the thermostat out, which I thought was supposed to temporarily fix it, and it still overheated. I checked the coolant levels and they are OK. What could be making it overheat?

Answer
Hi Joe,
The engine uses a water pump to circulate the coolant through the engine, where it absorbs heat, and then through the radiator, where it dissipates that heat to the atmosphere.
The water pump is driven by a belt, which also usually drives a fan to draw more air through the radiator.
The fan often has a fan clutch that engages or disengages the fan depending on how hot it is.
If your car has an electric fan, the operating system for it might be malfunctioning.

I wish you had said a little more information about your vehicle, like year, type, engine size, etc. It would make it easier to be more specific.

Most any of those could be the problem, assuming there is not an extreme source of heat, such as a blown head gasket.

If you removed the thermostat, then you were in a great area to check the operation of the water pump, by operating the engine while the thermostat neck was off. Or depending on what type of vehicle you have, you could still be in great shape to check it.

If you can remove the radiator cap, and see some of the tubes down inside the radiator, operate the engine, and see if you can observe coolant flowing through the tubes. Some later radiators you can't see down in there.
If that is the case, removing the upper radiator hose, and operating the engine, should pump a large volume of coolant out of the hose. Catch it if it is antifreeze, or just dump it if you are using just water. You may want a garden hose inserted into the radiator during this test to replenish the dumped water. It is just to see if the impeller of the water pump is moving coolant, so don't run it to the point where you run the engine out of coolant, and get it hot. But several seconds would be fine.

I should have mentioned to check the belt first. If it is gone, or real loose, that could be your only problem.

The fan clutch needs to be turning the fan, assuming you don't use just an electric fan, so make sure there is some pull on it, and it isn't just free wheeling.

The radiator fins need to allow air to pass through, so make sure they are not obstructed with debris.
And internally, the radiator tubes need to allow coolant to flow. If plain water has been used in the engine, the tubes can become obstructed by mineral deposits, not allowing the radiator to do its job.

Another possible problem is the guage, or light. Are you absolutely sure the engine is overheating, or are you just going by a light or gauge, which could be incorrect?

Normally, a blown head gasket will cause loss of coolant, with no apparent leak. Are you needing to add coolant occasionally?

And lastly I will ask if you are checking the coolant level inside the radiator, or just the coolant recovery tank? If there is a blown gasket, it can push the coolant out of the engine and into the tank, leaving the engine with a large air bubble inside.

Van