by Keith Hamburger

Created on: March 27, 2010

There are a number of causes of engine compression problems.  A blown head gasket, bad rings, cracked head or block, a stuck or burnt valve or a holed piston can cause loss of engine compression and a poor running vehicle.  Even minor compression issues that are difficult to notice from your engine running can result in loss of performance or poor efficiency.  Engine compression should be tested before buying any used vehicle and periodically when servicing your vehicle.

Checking engine compression is quite easy with the proper tools.  A manual compression gauge is relatively inexpensive and is part of any well equipped tool set.   If you just have a need to test the compression on one vehicle you can often borrow a compression gauge from major chain auto parts stores by giving a deposit equal to the price of the gauge.  The deposit will be refunded when you return the gauge.

To check the compression on your engine you will need to remove all of the spark plugs and disable the ignition.  Since you will be removing all of the spark plugs and wires you will want to mark each wire as to which cylinder they go to in order to make it easier to replace them.  It can be quite frustrating to get the spark plug wires crossed up and then have to track down the firing order of the engine.

You will also need to hold the throttle open to ensure enough air enters the engine for testing.

With the spark plugs removed you will thread the compression gauge into each spark plug hole, one at a time, and crank the engine.  You can use a remote starter switch or have a helper crank the engine from inside the car.  The gauge will give you the maximum compression of each cylinder.  Note the number before releasing the pressure using the button on the stem of the gauge and testing the next cylinder.

You will want to consult a service manual to determine what the compression should be for your specific engine.  The variation between each cylinder should be no more than 10% and, while compression will decrease as an engine ages, all cylinders should be reasonably close to the specifications.

Variations between cylinders or overall low compression can indicate a variety of different problems.  Other symptoms can give an indication as to what those problems may be and engine compression is just one part of a complete diagnosis.

Learn more about this author, Keith Hamburger.
Click here to send this author comments or questions.