Engine Compression Test Cost Estimate

An engine compression test is used to determine if one or more of the cylinders in an engine is producing insufficient or low compression. A basic engine compression test is performed by allowing the engine to turn over without starting, while other tests may be done while the engine is running. All together, these engine compression tests can help a technician: locate damage to piston rings, pistons, or cylinder walls; identify bent, worn, or stuck intake and exhaust valves; diagnose a deteriorated head gasket; or even point to exhaust problems, such as a clogged catalytic converter or muffler.

To perform a compression test, a technician will make sure the oil level is normal and warm up the engine before disabling the ignition and fuel systems. The spark plug wires are disconnected and the spark plugs are removed. A compression tester is installed in one of the spark plug mounting holes. The technician will have someone turn the key to engage the starter for a short time. The gauge on the compression tester reveals the pressure in the cylinder. The technician will record the reading and repeat with the rest of the cylinders. The process is more complex for a test with the vehicle running, but it reveals more information about the condition of the engine.

Variations from one vehicle model to the next require access to vehicle-specific information and repair procedures. It is recommended that a qualified technician diagnose and repair symptoms of low compression.

The ignition system and/or the fuel system need to be disabled prior to performing an engine compression test, unless the test is being done while the engine is running. In the latter case, a professional technician should perform the test. Damage or injury can occur if excess fuel is sprayed from the engine during the procedure.