No guesses please . I need hard knowledge.

An engine's compression is typically 150 psi but two cylinders are now showing 125 psi compression (three repeat readings) but with the leak down of 10% and another cylinder a compression of 135psi with a leak down of 8 % and all the other 150 psi compression tested cylinders 5% to 10 % leak down.

The question is: are good cylinder leak down test results of 10% or less plausible with same cylinder's comperession being 15 to 25 psi below typical (150 psi) for this engine?

Is there a chance that you had any fuel or water in the cylinders with the higher readings. If it is a carb engine, did you open and close the throttle a few times? Any liquid in the cylinders will raise the compression numbers quite a bit. Enough liquid can raise it by 50 psi or more. Hope this helps, Eddie.

Thanks Eddie.

No nothing in the cylinders for the readings mentioned. Each cylinder was repeated three or more times with minimal variation in the compression readings (while the other cylinders were also spining with the spark plugs out so anything there was blow out). The ~150 psi baseline for the engine is a good number.

in my opinion, the leakdown numbers are the be all and end all of the diagnosis... assuming it is done properly. i have seen leak tests done at extremely low pressures and that will mask a problem but if :

1) you are saying you put 100 psi into the cylinders and it holds 90 to 92

2) you can hear where its coming from... hissing breather or exhast or intake

then i would suspect you are ok... but having said that, a leak test is traditionally done at TDC. i suppose it is possible that you have bore damage below the the ring pack... like a wrist pin clip worked out and scored a bore badly... then a leak down would say everything is fine but a compression test would be low.

for me, the definitive answer to your question is " if the leakdown is 10 % or less, then that is ok with me. i start looking at things seriously at 12 % or higher.

Thanks stevesxm,

Bob Lloyd of Full Throttle Marine also suggested that poor ring sealing below TDC could be the cause of my lower compression but good Leak Down results. Yes to 100 psi air source but I did not track the source of the leakage ie intake / exhaust or crankcase though I definately heard leakage for all cylinders. That will be done next as well as a heavier oil in the low psi cylinders to see it that improves ring sealing. I tried a light oil (Merc Storage Seal oil) squirted into the low cylinders but that made no difference in the compression reading.

Bill

PS I have been taking the compression readings on a cold engine.

How long has the engine been sitting without running? This also makes a difference.

I rasn the heck out of the engine the day before I took the readings.

How many hrs on the engine?? which 2 cylinders were low?

You got me baffled,in past experience when I find a cylinder that has 10-15 lbs less compression then the others on a motor that cranks 150 psi normally the leakdown has been much higher on those cylinders. Do you have reference compression numbers (i/e those "bad" cylinders have bumped 150 in the past? Here's something thats waay out there,what if the motor was built with some rods that were reconned and some are shorter than the others and the cylinders have always been low but you never knew it and the ring seal is fine? Smitty