How Do You Complete an Engine Compression Test?

To perform an engine compression test, disconnect the big wire leading from the distributor cap to the coil, disconnect the fuel injection, remove the spark plugs, and place the compression tester into one of the spark plug holes. Crank the engine several revolutions to ascertain whether the tester registers over 100 psi per cylinder, which is healthy.

When disconnecting the big wire leading to the coil, remember to also move the metal connector away from the spark plugs, leaning it on an unpainted metal surface. In cars lacking distributors, locate the control module, and disconnect the electrical connector there. After disabling the fuel injection system, remove the fuel pump fuse, and run the car until it uses up all the fuel and the vehicle stalls. Next, remove the spark plugs, and insert the compression tester into the spark plug hole.

If you have a remote starter, connect one clip to the battery's plus terminal and the other to the starter solenoid's small terminal. Otherwise, have a friend sit in the driver's seat, turn over the ignition, and crank it about six times, while you keep the compressor tester plugged into the engine. Repeat these steps for each cylinder, moving the compressor tester into the corresponding spark plug cavity, and write down the results with pen and paper. The highest and lowest of all registered values should not vary by more than 15 percent.