Q: Overheating, blow cold air

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So it started off that my car wasn't blowing any hot air. Not consistently I should say. If I drove over 40mph it seemed to warm up. If I am idle it's cold again. Recently the engine started overheating. I would see the temperature gauge creep up to the HOT area and if kept driving, it started to cool down and the temperature gauge points in between Hot and Cold. Then it repeats the process and goes back up. I had lots of guesses but I don't know much. I was thinking it's the thermostat needs replacing. I read it could be the water pump but I just got that replaced 2 years ago. Someone said it could be a large air bubble stuck.
My car has an automatic transmission.

The heater not working in some cases may be related to the overheating problem you are also having. This may be due to a faulty heater blower motor or potentially a bad heater core. As you may know, the heater core is a small radiator like unit that circulates the hot coolant from the engine through the heater core which then uses this warm coolant to heat the inside of the car with the help of the blower motor that blows the warm air through the vents inside the car. When the cooling system is not properly circulating coolant through the motor, this results in the warm coolant not being pumped through the heater core properly in order to warm the inside of the vehicle. In some cases there may be air pockets in the heater core which can cause the heating of the inside of the vehicle to fluctuate between warm and cool air erratically.

Engine overheating can be caused by a number of things such as low coolant levels, a faulty thermostat, or a failing coolant fan switch. As you may know the coolant fan switch helps to maintain the proper coolant temperature by turning on and off at specific temperature thresholds. When this switch is not working properly, this can cause the fans to come on intermittently, all the time or sometimes not at all. When this happens you will notice a temperature spike and drop occasionally as the fan comes on and off. When your thermostat is not working properly or is stuck closed, this will not allow the coolant to properly circulate through the engine, which may cause the engine temperature to fluctuate erratically or in some cases just remain hot. As mentioned above, this also restricts the warm coolant from flowing through the heater core which uses this to blow warm air into the cab of the vehicle. I would suggest having an expert from Your Mechanic come to your home to diagnose your cooling system.

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