Compression Test
Checking theengine compressionin your car can tell you many thinks about the mechanical condition of the top end of your engine. This comes really handy when you have noticed popping noises on the intake and exhaust systems, too much blue smoke coming out of the tail pipe, or a rough idle you can't seem to get rid of. A check engine compression can tell you if worn-out pistons, piston rings, burned valves, pressure leaks or a damaged head gasket are to blame for your engine problems.This test uses a compression gauge you can connect to in place of the spark plugs to measure the pressure in the combustion chamber to measure the compression stroke of each piston. Then you can compare your results against your car manufacturer specifications in your vehicle service manual. You can buy this manual online, through one of your local auto parts stores or even consult one for free in the reference section of your local public library.Preparing for the test1. Warm up the engine, first. You can drive your car for about 20 minutes on the highway and then go back to your garage or wherever you want to perform the test.2. Open the hood and clean the wells around the spark plugs to prevent dirt and debris from falling into the cylinders after removing the plugs.3. Remove the spark plugs using a ratchet, ratchet extension and spark plug socket.4. Block thethrottle opento make sure the engine has enough air coming in during this test.5. Disconnect the ignition coil wire from the distributor and ground it using a jumper wire. Connect one end of the jumper wire to the metal clip at the end of the wire and the other end of the jumper wire to a metal bracket on your engine. This will prevent sparks from jumping off the unplugged spark plug wires.6. If your car is equipped with an electronic fuel injection (EFI) system, remove the fuel pump fuse as well to disable the fuel system.7. Install the compression gauge in the spark plug hole closest to the front of the engine.Check engine compression test1. Ask a helper to crank the engine 5 times so the needle on the compression gauge moves 5 times with each compression.2. Record the compression reading and movement pattern of the needle on the tool on a notepad. For example, note whether the needle registered low pressure on the first stroke and then built up on subsequent strokes; maybe the needle remained low throughout the compression strokes, indicating low compression for that cylinder.3. Repeat this procedure for each of the engine cylinders, recording your results on the notepad as you did with the first cylinder.4. If any of your cylinders registered low compression, repeat the procedure on eachcylinder. This time, though, apply about three squirts of clean engine oil to the cylinder prior to the test. Then repeat the test and record your results.5. Compare your results to the specifications given in the service manual for your particular vehicle. Depending on the compression amount and needle pattern movement, the results will indicate the mechanical condition of your engine.An engine compression test will help you locate mechanical problems without tearing apart the engine when performed properly. You can then follow this test with more specific troubleshooting procedures, if necessary, to confirm your results. This will help you make a better engine-repair decision, help you save money and time.
