Here's one of the best , short explanations of blown head gasket scenarios that I have seen. Add yours or anything you'd like to add.
It comes from user "Professor Sir" over on Toyota Nation in response to an original post (not mine) about an engine putting out cold air from the heater and yet engine overheating spontaneously and cyclically....
QUOTE
Head gasket leaks can vary from one incident to the next.
If the part of the gasket that separates the cooling jackets around the cylinder from the oil passages fails, then the high pressure coolant will enter the oil passage contaminating the oil.
If the part that separates the cylinder bore from the cooling jacket fails, then you get the high pressure combustion gases entering the cooling system causing "air pockets". The water pump cannot move air, so the water flow stops. Almost instant overheating will happen. The coolant in the heater core will cool to the point that there will be little heat, or no heat if there is an air pocket. The temp sensor in the cylinder head will register "hot" if coolant is present, and "cold" if there is an air pocket. That often trips the check engine light. Boiling of the coolant is almost certain, so steam is likely to escape past the cap or any weak point in the system. Unfortunately your car's symptoms seem to point exactly to this scenario.
It comes from user "Professor Sir" over on Toyota Nation in response to an original post (not mine) about an engine putting out cold air from the heater and yet engine overheating spontaneously and cyclically....
QUOTE
Head gasket leaks can vary from one incident to the next.
If the part of the gasket that separates the cooling jackets around the cylinder from the oil passages fails, then the high pressure coolant will enter the oil passage contaminating the oil.
If the part that separates the cylinder bore from the cooling jacket fails, then you get the high pressure combustion gases entering the cooling system causing "air pockets". The water pump cannot move air, so the water flow stops. Almost instant overheating will happen. The coolant in the heater core will cool to the point that there will be little heat, or no heat if there is an air pocket. The temp sensor in the cylinder head will register "hot" if coolant is present, and "cold" if there is an air pocket. That often trips the check engine light. Boiling of the coolant is almost certain, so steam is likely to escape past the cap or any weak point in the system. Unfortunately your car's symptoms seem to point exactly to this scenario.
