It is the job of the Head Gasket to make sure that none of these respective parties: Engine Oil/”Sharks”, Coolant/”Jets”, and air, fuel and exhaust/”rest of the party” ever mix with each other. When the head gasket leaks or fails – then one or more of these elements can mix – which leads to a myriad of issues. Here are a few examples: Low combustion pressure – poor performance. Exhaust gets into the coolant – leads to overheating. Coolant gets into the oil – poor lubrication and possible engine damage.