Remove the spark plugs and rotate the engine to put the cylinder to be tested at top dead center. Tip: insert a long screwdriver or extension into the spark plug hole and turn the engine by hand with a socket on the crankshaft. When the screwdriver stops rising or falling you're at TDC. Don't turn the engine backwards if TDC is missed. Go around again. In a four-cylinder engine cylinders one and four and two and three are at TDC at the same time.