Compression test

LS9 Cutaway, revealing the valves and piston.

Since the essential function of an engine is pumping air, it is crucial to have good compression. Testing the compression of an engine is a good way to tell if it is healthy. If an engine has poor compression it will run inefficiently (if it runs at all), rough, and have issues starting.

Generally most piston driven gasoline engines should have 120 to 160 lb. of cranking compression with no more than 10% difference between the cylinders. Although due to variances in engine design, the exact range should be looked up or calculated for the specific engine.

How to check compression

Average compression tool set.

Checking the compression manually with a gauge is a fairly simple procedure. It requires removing all the spark plugs, unplugging the ignition coil, and holding open the throttle. With the hose from the gauge threaded into the spark plug hole, the engine should be cranked and then the pressure noted.

While unplugging the ignition coil is the most popular and simple way to do a compression test, it is recommended to kill the fuel system as well to ensure more accurate readings. The fuel injected each time the engine is cranked is washing down the bores, which can make the compression readings abnormally decrease from the first to the last cylinder tested. This can be done numerous ways such as initiating the fuel cutoff switch or removing the fuel pump fuse while the car is running to ensure no pressure is left in the fuel rail, or unplugging fuel injectors to prevent them from spraying.

Some older cheap gauges will require having an assistant to look at the gauge and note the pressure. However most decent compression gauges come with a one way valve that allows you to capture the reading and then release the pressure manually.

Determining the source of low compression

Testing the compression of an engine is a great method to diagnosing the problem in an engine without having to go through dis-assembly.

• Low compression in one cylinder is caused by a leaking exhaust or intake valve, typically caused by pitting on the mating surfaces and requires replacement or lapping of the valve.
• Low compression in two adjacent cylinders is caused by a blown head gasket, typically as a result of being crushed from heat expansion.
• Low compression in all cylinders is a good indicator that the cylinders and rings are worn out and require an engine overhaul.
• Squirting a small amount of motor oil into the cylinder through spark plug holes and repeating the test can help determine if the compression loss is due to leaking valves or piston rings. If it is the rings or cylinder walls, the oil will temporarily seal them and give quite a higher reading during a second test.