Compression Testing Activity CYLINDER COMPRESSION TEST The most common method of measuring compression is with a specially designed pressure gauge. A fitting attached to the gauge is put in place of the spark plug of the cylinder being tested (all spark plugs removed). Standard Compression Test For a dry compression test, remove all the spark plugs and insert the compression gauge adapter into the number 1 spark plug hole. Disable the ignition/fuel system by removing the Ignition relay. Be sure the throttle blade is fully open. If the throttle blade is not completely open, incoming air will be restricted and cause lower compression readings. Crank the engine at least five full cycles or until the cylinder has reached its peak compression pressure. Note: A cycle is two full revolutions of the crankshaft. Repeat the test steps for all additional cylinders. If one or more cylinders have abnormally low compression readings, perform a wet compression test to verify the results. If the same cylinder or cylinders confirm an abnormally low reading on the second compression test, a problem may exist in that cylinder. A leakdown test will determine where the pressure leak is. For a wet compression test with a standard compression gauge, add 28.4 ml (1 oz.) of 30-weight oil to the cylinder and perform the compression test again. Adding oil to the cylinder may temporarily seal worn rings to the cylinder walls. If compression readings increase, the piston rings are probably worn. If the compression readings did not change, the valves may not be sealing. 1. Remove all the spark plugs. Remove the Ignition relay. Conduct a dry compression test on 4 cylinders. Record the cylinder number and compression readings below. Cylinder # _____
Dry reading _______
Cylinder # _____
Dry reading _______
Cylinder # _____
Dry reading _______
Cylinder # _____
Dry reading _______
What is the condition of the four cylinders that were tested? _________________________________________________________________________________
8
