Air pocket in the cooling system

Symptoms

• Gurgling sound from the dashboard or engine bay, especially after a coolant service
• Heater blows weak or intermittent heat
• Temperature gauge fluctuates rather than staying steady
• Coolant level in the reservoir seems to drop and needs repeated topping off shortly after a service

Likely causes

• CommonIncomplete bleeding after a coolant flush or repairMany cooling systems have a specific bleed procedure or bleed valve that needs to be used to fully remove trapped air.
• CommonAir introduced during a hose or water pump replacementAny time the system is opened, air can enter and needs to be purged during refill.
• PossibleLow coolant level allowing air to enter at the reservoir or pump inletRunning low can draw air into the system even without a repair having been done.
• Less commonMissing or incorrectly closed bleed valveSome vehicles have a specific bleed screw that must be closed properly after use or it can let air back in.

What to do

1. Check the coolant level and top off if needed, since air pockets often coincide with a slightly low level.
2. Look up whether your vehicle has a specific bleed procedure or bleed valve, since many do and it's often overlooked after a service.
3. With the radiator cap off and the system cool, some vehicles can be bled by running the engine with the heater on full hot and the front of the car slightly elevated, per the owner's manual.
4. Watch for the gurgling or heater performance to improve as air is purged during the bleed process.
5. Avoid closing up the system and driving normally until the proper bleed procedure is followed if one exists for your vehicle.
6. Have a mechanic bleed the system properly if you don't have access to the specific procedure or a bleed valve.
7. Recheck coolant level after a day or two of driving, since it often needs a final top-off once trapped air fully works its way out.

Tools and parts

• Gloves
• Owner's manual

When to call a pro

See a mechanic if you're unsure of the correct bleed procedure for your vehicle, since some systems need a specific tool or process to fully purge air.

Safety