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Engine overheating and thumping sound under hood

I have a 1999 Oldsmobile Intrigue, w/ 127,200 miles. Recently the car overheated, I replaced the thermostat, I had already replaced the dual fans last year. The car’s temperature gauge is still cycling up to hot and the warning light still continues to come on. Now there is a thumping sound under the hood. I took the car to the dealer, and of course, according to them the car’s temperature gauge did not run hot and they heard no thumping sound, they also stated that no abnormalities are showing on the computer.

Please help, I do not want to blow a head gasket and have to buy a new car.

Waiting Patiently for answers…

Does this happen all the time or just occasionally??

Once I start driving it happens constantly

Then drive it into the service bay when it’s thumping and bumping and the gauge is pegged and say “fix it”…

Would love to but I work split sifts and it does not happen when the service bay is open. That’s why I let the dealer keep the car for a several days to try to recreate the problem

Why did you replace the fans?? Was it doing this last year?? Do the fans come on? They should both come on automatically when you turn on the A/C…Are you losing coolant? More info please…

Fans were replaced when one of the fans ceased to come. Both fans come and run with no problem, Not loosing any coolant, Hoses are not hard, no radiator leakage. The temperature gauge is cycling to hot and back to normal. checked sensors no problem, no one can tell me what the thumping noise is. Checked engine head gaskets and seal seem find.

“Checked engine head gaskets and seal seem find.”

How was this done?
I only ask because people often confuse head gaskets with valve cover gaskets.

I recommend a cylinder leak down test. The cylinders are pressurized then look for bubbles in the coolant, which would indicate a leaky head gasket.

The thumping could be coolant starting to boil inside the engine or puffs of combustion gas slipping past a head gasket and bubbling into the coolant.

Has the engine overheated in the past? That could cause a head gasket failure later.

The dealer said they performed a pressure test on the coolant system, they did not mention a cylinder leak down test. Someone else suggested it could be a flap between the heating/cooling system. The engine has overheated three times in the 10 years I have had the car. Once we replaced the water pump, then 4 years later the cooling fans. This time we replaced the thermostat.