COMPRESSION TEST: Now I'll show you how to perform a compression test. A compression test is the procedure taken to measure the pressure in each individual cylinder thereby allowing you to get a feel for how much wear the motor has. Let's take a minute and demonstrate how to do a compression test. What we'll need to do first is remove the spark plugs. Again, remove the spark plug lead, place your wrench over the spark plug and remove the spark plug itself. Once the spark plug is removed, we can insert then compression gauge into the spark plug hole and screw it in as though it were a spark plug. After inserting the compression gauge, you'll need to locate the wire that connects the coil to the distributor cap. Disconnect this wire where it connects either to the distributor cap or coil and ground it to the frame or engine. Some cards incorporate the coil inside the distributor cap and the low tension circuit will need to be disconnected. Refer to your repair manual for the proper procedure for your car. Once this is done, we need to go in the car and turn the car over as though we were trying to start it for a good two to three seconds. Once this is done, we'll take a look at the gauge and notice the pounds per square inch reading. In this case, we have approximately 110 pounds. Now, it's always good to have high compression but the most important thing is that all the cylinders in the car are within 10 percent of one another in average compression readings. So what we would want on this car are all four readings for each cylinder to be within 10 pounds of what we have here. Should a gauge reading from one of the cylinders deviate substantially from the others, you probably have one piston rings, valves or burnt valve in that cylinder. Take the car to your mechanic for further diagnosis to prevent further engine damage.
