I have a 1991 240SX and for some time now when I first start the car up and it's warming up the temperature gauge climbs to almost hot and stay there for a few miles then drop down to normal, only if I'm going above 50 MPH. If I'm driving at slower speeds it begins to rise. Also when the temperature is high the heater quits working. So at slower in town speeds I don't have any heat. I've changed the thermostat a couple of times and that doesn't fix it. Any suggestions?
Pull the radiator cap and start the engine. If radiator fluid begins to churn and bubble and splash out the top, and/or if you get bubbling noises in the overflow tank, then chances are you have an internal head gasket leak. Get it fixed immediately before it gets worse and you blow your motor. If not, then it's safe to say the radiator needs cleaned and flushed, along with the heater core, or worse yet a damaged water pump, but usually you encounter immediate and severly obvious problems if the pump is bad. Try flushing the radiator by draining it and using a garden hose through the top, then spray the hose through the radiator from the inside out to remove the junk stuck to the front.
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