A worn piston ring will have several effects on the engine. The compression within the cylinder containing the worn ring will diminish significantly, causing a rough running engine, poor fuel combustion, increased pressure in the crankcase due to combustion bypassing the rings into the case, fouled spark plugs, increased oil consumption with smoke at the tailpipe, fouled oxygen and mass air flow sensors and a fouled catalytic converter.
Pull all the spark plug wires off the spark plugs. Mark them if it appears that it will be difficult to determine their original placement later. Thread the compression tester hose into the number 1 cylinder then connect the tester to the hose.
Depress the gas pedal slightly to open the throttle body and crank the engine over for three seconds. Turn the key off. Check and record the compression reading. Perform the same procedure until all the cylinders have been tested.
Check the results of the compression test on all cylinders. All the cylinders should be within 5 percent of each other. Anything lower than 5 percent indicates a mechanical problem. Target the cylinder or cylinders that are showing unusually low readings for the next test.
Install the compression gauge hose onto the affected cylinder. Connect the cylinder leak-down gauge to this hose. Connect the air line to the other fitting on the gauge. Turn the dial to regulate the pressure on the input gauge to 90 pounds per square inch.
Observe the bleed-down reading on the opposite gauge. The numbers on the gauge are red, yellow and green. As long as the needle is in the green, there is very little leak-down. Should the needle indication be in the yellow or red, a problem definitely exists. This particular test will pinpoint the worn part, whether it is a worn ring or a burnt valve. While pumping air into the cylinder through the leak-down gauge, listen for a noise coming from the throttle body, which indicates a bad intake valve. Remove the oil filler cap on the valve cover and listen for pressure coming out of the valve cover, indicating a worn ring. Listen to the exhaust tail pipe for the sound of air escaping, indicating a burnt exhaust valve.
Rebuild the engine if the test indicated a worn piston ring because, unfortunately, there is no quick fix for this type of problem. If the problem was isolated to the valves, the cylinder heads can be removed and taken to a machine shop for a valve job.
