So after replacing the head gasket on my car I started it up and it runs great. But now the temp gauge jumps into the red after the cars been running for only seconds. I've replaced the thermostat and bleed the cooling system. I if anyone might know what else to look for please let me know.

 

Are you sure the cooling system has proper flow? If it takes only seconds for your engine to overheat, perhaps the coolant is not moving which will build up heat in a hurry. A siezed up water pump could cause this problem and also a clog somewhere in the cooling system. The passages in the heater core are pretty small so if you used any type of stop leak that could have clogged up the heater core.

 

only a few seconds ?

sounds like the coolant temp sensor is defective, or the wire going to it becomes an open circuit.

 

^Thats a good possible solution. I was assuming that the engine was actually overheating. To test the coolant temp sensor, start the engine and check the voltage at the sensor. If you give me some specifics on your car (year, model, trim, engine) i can try and look up what voltage corresponds to what temperature. I would think that 0v means really cold and 5v is really hot.

 

thanxs - that makes more sense (open circuit is zero).

 

It's a 93 dx.