Skip the auto shop - Our mechanics make house calls

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

asked by on

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.

A: This is most likely the result of a failing...

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.

Was this answer helpful?

Need advice from certified mechanic? Get help now!

Over 1000 mechanics are ready to answer your question.
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Ask a Mechanic
(100% Free)

Have a car question? Get free advice from our top-rated mechanics.

Ask A Mechanic
Over 10,000 questions answered!

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Save up to 30%

Get a quote

What others are asking

Q: Car died after running the AC

Hi. Running the AC with the engine off will definitely drain your battery. Most automotive batteries are designed to last last about 3-5 years. Being that your vehicle is a 2002, it should be on its 3rd or 4th battery...

Q: Engine overheating - 2009 Buick Lucerne

Hello. The thermostat is located inside the housing opposite the engine water pump. The radiator hose and housing must be removed to gain access and new coolant should be added when the work is done. I recommend having an expert...

Q: How do I check the engine coolant level?

Checking the engine coolant level is very important. Regular checks and maintenance is a good habit to get into over the life of the vehicle. To check the coolant level. Inspect to see that the coolant level reaches the...

Related articles

How Long Does a Heater Control Valve Last?
Keeping the right amount of coolant in a car is essential in keeping the engine at the right temperature. Failing to have the right amount of coolant or even bad elements...
P0052 OBD-II Trouble Code: HO2S Heater Control Circuit High (Bank 2 Sensor 1)
P0052 code definition HO2S Heater Control Circuit High (Bank 2 Sensor 1) What the P0052 code means This code is seen when the Engine Control Module (ECM) tries to control the...
Rules of the Road For Iowa Drivers
Driving on the roads requires knowledge of the rules, many of which are based on common sense and courtesy. However, even though you know the rules in...