Q: My coolant is boiling out of my overflow tank.

asked by on July 23, 2016

I noticed a problem with my ac blowing warm while idling, once driving, it starts blowing cold again. One day driving fast down the highway the ac was blowing warm and the engine overheated. I noticed I was low on oil and coolant. I got an oil change, and the level of oil has been fine since then (2 weeks) I added coolant and the ac blows fine even when idling. Well I was driving a few days ago, got out and smelled something and popped the hood and noticed my coolant was boiling out the reservoir cap. So today I filled the tank up with water and let it idle for 30 minutes even revving the engine. No water boiled out of the cap and my ac was blowing cold the whole time. So finally I took it for a 10 minute drive and drove it hard and came back and parked it. I smelled the warm smell and had hot vapor coming out the hood. I popped the hood and there was hot water and coolant everywhere under the hood. Looks like it shot out the reservoir tank again. Could this simply be a bad cap?

My car has 165000 miles.
My car has an automatic transmission.

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This is most likely the result of a failing fan switch relay or a failed thermostat. The thermostat is a component that regulates the flow of coolant through the motor based upon specific temperatures. When the engine reaches a certain temperature, it opens allowing coolant to flow through the radiator and then to the motor.

When this is not functioning properly, the result can be catastrophic overheating of your car’s engine. The fact that the coolant was boiling out of the reservoir cap suggests that the thermostat may be stuck closed. When this happens, the coolant is not allowed to circulate throughout the vehicle’s engine as it should. I would suggest having an expert from YourMechanic come to your home to diagnose the overheating problem as this could be possibly a few different issues, all of which will require some testing.

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