Air Pockets in the Heater Core
A blown head gasket results in combustion gases entering the engine’s cooling system. It makes a noticeable gurgling sound under the dash as the coolant flows past the air pocket in the heater core. Air pockets in the coolant system contaminate the coolant and damage aluminum parts. As coolant breaks down, electrolysis occurs and aluminum parts like the heater core begin to leak. A leaking heater core will result in wet carpet in the passenger’s compartment. It will also affect the ability of the heater core to properly heat the cabin.
Today's EATC systems use blend doors to blend ambient, heated and cooled air to achieve a desired temperature. With this type of system, a gurgling noise may occur whenever the engine is running. Improper bleeding of the system can also result in air pockets getting trapped in the core. On certain models, manufacturer's recommend lifting the back or lowering the front end of the vehicle to remove air pockets whenever refilling after a coolant flush. If unsure, check for special procedures before performing the flush.