Technical Bulletins

Trapped Air In Cooling System Causes Engine Overheating

Over the last couple of years the auto manufacturers have continually lowered the hoods on cars to achieve better aero dynamics.  In many instances the radiator has needed to be shortened to the point where the filler neck is lower than some of the radiator or heater hoses.  This can lead to air getting trapped in the cooling system when a rebuilt engine is installed or the coolant replaced as part of normal maintenance.

On in-line engines this has lead to scuffing of the rear piston since an air pocket in this area prevents coolant from removing combustion heat.  An overheated piston will scuff on the sides of the skirt and not in the center.  The industry refers to this as four corner scuffing.  If the piston was scuffed in the center of
the skirt, the most likely cause is insufficient piston to wall clearance.

Another symptom of air trapped in the cooling system is a general and rapid overheating condition.  Most often the engines temperature rises sharply as soon as the engine is started.  Eventually the cooling system boils over.

Filling a cooling system must include bleeding any trapped air from it.  Be sure to turn the heater control to its highest temperature setting (hot) to assure that the heater core is filled at the same time.  Some trucks and vans have rear
passenger compartment heaters that could also trap additional air. 

Locate the highest heater hose near the rear of the engine and either loosen the clamp or temporarily disconnect the hose.  Slowly fill the radiator until coolant comes out of the hose.  Replace the hose or tighten the clamp and top off the radiator with coolant.

After the engine has reached operating temperature and then cooled down again, check the coolant level and refill if necessary.

The AERA Technical Committee