Compression Test

A compression test is a way of testing an engine’s condition without tearing the engine apart. A compression tester or tool is used to check the compression pressure. An engine compression test will let you know if there is excessive wear between the cylinder and piston. When the seal between the cylinder and piston rings wear out the engine’s compression will drop. This sealing failure will also cause the vehicle to use excessive oil and the car may start to smoke out of the exhaust. A four stroke engine, usually found in cars, will vary per car but should be around 170-200+ psi depending on the static compression ratio of the vehicle. With high performance cars having higher psi numbers with the exception of turbocharged cars having low psi numbers. Two stroke engines and small garden type engines will have considerably lower compression. Generally these engines will be around 80-140 psi. These are the steps to check your engine’s compression.

 

First remove all the spark plugs from their hole, then click them back into the plug wire and lay them on a metal part of the engine. The compression tester is then screwed into the spark plug hole until snug, do not over tighten. Spin the engine over with the throttle in the wide open or full throttle position. The needle will jump up to a psi number, then will steady out quickly. Take the number and write the psi number down for each cylinder. The cylinders should all read within 5-10 psi of each other. The less variance the better. If some cylinders look alot lower or differ alot, the engine will probably need to be rebuilt. The only other possibility would be that the cylinder head valves are leaking. This test is called a valve leak down test.