How about cylinder leakage, that is going to be your real determining factor. You can have compression differences that would not seem abnormal, but still have a degree of leakdown that can cause a misfire. If you are unfamiliar with leakdown testing, it involves a leakdown tester. It has two gauges, one side attached to compressed air, and when applied to a sealed cylinder(valves closed), the second gauge measures a percentage of loss. From that percentage of loss, if deemed excessive, then you would listen for air leakage through places like the exhaust(exhaust valve), the throttle body(intake valve), crankcase(ring/piston), cooling system(headgasket/crack), etc. If for instance you had a worn cam, you may potentially see higher than normal compression readings as the valve(s) would not be open as long as they should. This would apply more to worn exhaust lobe, as a worn intake lobe will not allow enough air to enter the cylinder in the first place. A vacuum measurement from the manifold when at idle may also show a sign. A good sealing engine should have steady vacuum gauge reading 18-20" H2o. Significant fluctuation or pulsing of the needle indicates a problem.
Friday, August 5th, 2011 AT 3:05 PM