Blown head gaskets usually occur when a car overheats. When this happens, the head that is bolted to the engine block expands at a different rate than the engine block. This expansion breaks the gasket seal and engine compression inside the cylinders then escapes, causing permanent damage to to the gasket. In most cases, this also allows engine coolant to invade the cylinders. Inspecting the spark plugs for failure signs is a viable test for a blown head gasket.
Remove the spark plugs from the engine using a 3/8-inch spark plug socket and ratchet. Lay the plugs out in the order in which they were removed to properly reference the cylinder they represent.
Inspect each spark plug for failure signs. Dark sooting from misfiring, water droplets, brown spotting, and rust around the threads are all signs of head gasket failure.
Inspect each of the spark plug boots. In some cases, where severe gasket failure has occurred, condensation and moisture will accumulate on the plug boots.
