First you will need to remove all Spark Plugs from each cylinder in order to perform a compression test. Next, you will need to insert a compression gauge one cylinder at a time. Crank the engine over for 3 -5 seconds and read the gauge. It is a good idea to write the number down. All cylinders should be within 20 psi of each other. If 2 cylinders that are next to each other are low, this can be a sign of a blown head gasket.