Engine compression tests can help to locate the source of a problem, and they can also provide a snapshot of the car's overall health. But what exactly does engine compression mean? Well, in order for the cylinders to properly house the combustion of gas and fuel, the piston needs to exert the correct amount of pressure on the gas mixture before the spark plug fires and ignites it. The pressure in the cylinder is compression.
Poor compression can result in a decline in fuel economy and can have an adverse effect on your car's emission levels. A loss of pressure may be caused by a damaged valve or a blown head gasket. When a cylinder's seal is leaking, it will lose pressure, which may cause the engine to run roughly, lose power or even misfire. The same leak can also increase the risk of oil leaking into the engine, which will be evidenced by blue smoke coming from the exhaust pipe.
So how do you figure out if there is a problem? A compression gauge can be used to measure the amount of pressure being exerted in a cylinder by the piston on the fuel-to-air mixture before it ignites. Depending on what kind of gauge you use, you will either need to hold it in place or screw it into the spark plug opening.
Before you can actually conduct the compression test, there are a number of parts that need to be disconnected. The metal connector of the distributor wire, which runs from the coil to the distributor cap, needs to be moved as far away from the spark plugs as possible. Newer cars that send electricity to the spark plugs electronically will not have this wire and you will instead need to disconnect the electrical connector at the engine control module.
The next step is to disconnect the fuel injection system so gasoline is not being sprayed into the cylinders. This could lead to fuel leaking out of the spark plug holes because the spark plugs need to be disconnected and removed as well. Make sure to label the wires, plugs and boots before you disconnect them so you can be sure to put them back in the proper order.
Whether you attach a remote starter to the car battery or you have a friend sitting in the driver's seat, you will want to turn on the ignition so the engine cranks a few times. While this is going on, check the compression gauge and record the readout and which cylinder it correlates to. Reset the gauge and repeat this process for each cylinder. Once you're done, compare all of the numbers. The proper pressure readings can vary from engine to engine, so consult your owner's manual to find out what works for your car.
If one number varies wildly from the others, the cylinder that does not match up may be the source of the problem. When all the numbers are within about 15 percent of one another, there could be an overarching issue, such as worn or maladjusted valves that are allowing pressure to escape.
When the test is complete, you will have a better idea of what is causing the engine problems. Don't forget to replace the spark plugs and anything else that was removed for the sake of the test.