A cylinder head gasket is a mechanical seal that prevents engine oil and coolant from escaping the engine and from entering the wrong compartments inside.
A cylinder is a cylindrical compartment in which a piston travels inside your engine. Most modern engines feature four to eight cylinders. The cylinder head mounts on top of the engine, creating a cap on the cylinders. The cylinder head also houses the spark plugs, intake valves, exhaust valves, and, in most cases, the camshaft(s).
Four-cycle internal combustion engines, like the one used in your non-electric vehicle, do four things in succession. First, each piston in its turn moves downward in the cylinder, creating a vacuum and drawing in a mixture of air and fuel from the fuel injector. Second, the piston moves upward toward the cylinder head, compressing the air and fuel. Third, at just the right time, the spark plug fires, setting off a combustion event that drives the piston downward. Fourth, the piston travels upward again, this time expelling the exhaust gases that are the byproduct of combustion.
In order for this sequence of events to happen, each cylinder is fitted with one or more intake and exhaust valves that open and close in response to the action of the camshaft. Since an engine can have anywhere from four to eight cylinders or pistons, and each cylinder can have up to four valves, it is possible for a camshaft (multiple camshafts) to actuate thirty two valves at a time. All of this activity and more happens inside the cylinder head.
A bad or “blown” cylinder head gasket is one that no longer seals as it should. Engine oil or coolant might escape the engine. Or they might leak from one area of the engine to another, into places where they do not belong. For instance, coolant could mix into the oil through a blown head gasket. Or it could get into the combustion chamber where air and mix with the air and fuel inside.
Signs of a bad cylinder head gasket include:
