air in the cooling system. here is a sure fire test to confirm compression. will work on any pressureized cooling system typical radiator caps anywhere from 7 to 10 to 13 lbs. if the head gasket is leaking compression it will over power the cap and force it to vent. take the over flow tube (may need to put a longer one on for the test)and run it into a bucket of water about 3/4 full now take an empty coke or gatorade bottle and fill it up with water in the bucket and turn it upside down in the bucket and put the over flow tube in the neck in the bottle far enough that it will stayu there. start the engine and let it run. if the heag gasket is leaking, it will force air into the bottle and the water out. with in a few seconds the bottle will float up.