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.

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?
The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
  1. Home
  2. Questions
  3. My coolant is boiling out of my overflow tank.

Get an instant quote for your car

Our certified mechanics come to you ・Backed by 12-month, 12,000-mile guarantee・Fair and transparent pricing

Get a quote

What others are asking

Q: My 2002 Saturn Vue's rpms started revving into 4-5k when up shifting. Later that day the clutch went out. Now the starter is out.

Hello, thank you for writing in. The decision to invest in an older car is difficult, even more so when you do not have the car in front of you. A failed clutch can interfere with the flywheel causing the...

Q: 1997 Lincoln Town car coolant leak

A lot of these engines had plastic intake manifolds that crack and leak. First the system needs pressure tested to isolate the leak and then a replacement intake may need to be installed. To give a price without seeing and...

Q: I have a severe engine coolant leak

Hello, the overheating issue needs to be fix as soon as possible, the vehicle should not be operated or you can damage the engine. An overall inspection of coolant system components needs to be done. Some visuals may have to...

Related articles

How Do Power Car Windows Increase Passenger Safety?
Power windows are responsible for approximately 2,000 emergency room visits every year. When a power window closes, it exerts enough force to bruise or break bones, crush fingers, or restrict an airway. Though...
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...
How Much Does a Mechanic Make in Vermont?
Automotive technician jobs in Vermont have an average mechanic salary of $37k, with some mechanics earning a salary of $53k.