Leak down test

Just out of curiosity, does anyone have some reference % number for a leak down test done on a 996 cold engine?

Not on 996 specifically but aircooled 911s Cold will not give very accurate/reliable readings. On a healthy warm engine 2-5% is good. On a cold engine it could go 25% or more and still be a good engine if all cyl are within ~5% of each other. I have had cold, air-cooled, engines read 20-30% leak down with 10% variations that would drop to 5-10% with 5% variations warm. That said generally if they are all showing ~25% with little variation one could infer that the engine is at least balanced with fairly even wear. If there is one outlier like 50% somebody is sick.

Is this the engine that you just did all of the work on? Scroll down to the bold type?? http://autos.yahoo.com/maintain/repa...ques057_1.html A leak down or "cylinder leakage" test is similar to a compression test in that it tells you how well your engine's cylinders are sealing. But instead of measuring pressure, it measures pressure loss. A leak down test requires the removal of all the spark plugs. The crankshaft is then turned so that each piston is at top dead center (both valves closed) when each cylinder is tested. Most people start with cylinder number one and follow the engine's firing order. A threaded coupling attached to a leakage gauge is screwed into a spark plug hole. Compressed air (80 to 90 psi) is then fed into the cylinder. An engine in great condition should generally show only 5 to 10% leakage. An engine that's still in pretty good condition may show up to 20% leakage. But more than 30% leakage indicates trouble. The neat thing about a leakage test (as opposed to a compression test) is that it's faster and easier to figure out where the pressure is going. If you hear air coming out of the tailpipe, it indicates a leaky exhaust valve. Air coming out of the throttle body or carburetor would point to a leaky intake valve. Air coming out of the breather vent or PCV valve fitting would tell you the rings and/or cylinders are worn. A leakage test can also be used in conjunction with a compression test to diagnose other kinds of problems. A cylinder that has poor compression, but minimal leakage, usually has a valvetrain problem such as a worn cam lobe, broken valve spring, collapsed lifter, bent push rod, etc. If all the cylinders have low compression, but show minimal leakage, the most likely cause is incorrect valve timing. The timing belt or chain may be off a notch or two. If compression is good and leakage is minimal, but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery (bad injector) or ignition problem (fouled spark plug or bad plug wire).

Frank, I did a leak down on one cylinder just for curiosity, the car runs strong actually better than before after the major service. On a cold engine I got 18% leak, the leak came out from the bottom end. I was not worried, assumed that probably due to a cold engine. I just wanted to compare the numbers with others. But I guess the number is meaningless without comparing it to other cylinders. Thanks