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Q: Over Heating

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I have a 2004 chevy impala 3400 and it is overheating and the heat isn't working. Some of the hoses are cold and some are hot it just depends it changes. There is no leak in the system (that I can see) but it seems that there is air that's getting in the system. When I open the radiator cap after its ran and cooled there is a lot of pressure that releases. So I burped the system thinking that was the problem, and a lot of a it came out and the car ran well and the heat worked for about an hour or so of driving, and then it started heating up and the heat would start going in and out until the temp. guage would max out. We replaced the thermostat because it was bad we tested it, and we replaced the water pump because we noticed that if the temp. guage was going up and we put the petal down the guage would go down, but replacing he thermostat, the water pump, and burping the system did nothing. We also had the radiator flushed (so our mechanic said). The fans are also working. Anything?

My car has 126500 miles.
My car has an automatic transmission.

I would test for exhaust gas in the coolant which, if present, would mean that there is a head gas leak. If the coolant level is falling slowly over time, that suggests a leak as well and a pressure test will be useful in determining that. If pressure falls during a pressure test, and there is no evidence of an external leak, the leak could be through the head gasket. As far as trapped air, there are purge points high in the system that should be opened to expel air as you add coolant. If you prefer that a certified Mechanic diagnose this overheating problem, please simply request a coolant is leaking diagnostic.

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