Coolant boiling over

Symptoms

• Coolant visibly boiling or bubbling in the reservoir
• Coolant pushed out from under the cap or overflow tube
• Temperature gauge reading high or in the red
• Gurgling sounds from the engine bay

Likely causes

• CommonFailed radiator cap not holding pressureThe cap maintains system pressure, which raises coolant's boiling point; a bad seal lets pressure escape, causing it to boil sooner.
• CommonEngine overheating from another causeAny of the common overheating causes (low coolant, bad thermostat, failed fan) can push coolant temperature past its boiling point.
• PossibleAir pocket preventing proper circulationTrapped air can create hot spots that boil locally even if overall temperature seems moderate.
• PossibleBlown head gasket allowing combustion gases into the cooling systemCombustion gases pressurize the cooling system abnormally, pushing coolant out and causing boiling.

What to do

1. Pull over safely and shut off the engine if you see coolant boiling over; do not continue driving.
2. Do not open the radiator cap or reservoir while hot; wait at least 20–30 minutes for things to cool down.
3. Once cool, check the radiator cap's rubber seal for damage or hardening.
4. Check for any other overheating causes: coolant level, fan operation, and visible leaks.
5. If white smoke from the exhaust or a bubbling/foamy look to the coolant is present, suspect a head gasket issue and stop driving.
6. Have a mechanic pressure-test the cooling system and check for head gasket failure if a simple cap or coolant issue doesn't explain it.
7. Replace the radiator cap if it's found to be faulty, since this is an inexpensive and common fix.

Tools and parts

• Gloves

When to call a pro

See a mechanic if a new cap doesn't solve it or if you notice white exhaust smoke, since that points to a more serious head gasket problem.

Safety