Originally Posted by Smoove1010
Thank you Mr. Zimmerman for passing that along. Would you care to weigh in on the compression test procedure, i.e., plugs in or out?
I've always done compression with all plugs removed, but I really don't do compression checks on 911s.
For the reason you cite, it seems like it would be best to perform the leakdown first, followed by the compression test.
Yes, definitely do the leakdown test first. If you know the rate of oil consumption you don't need a compression check, because the leakdown tests everything but valve guides. When you have compressed air entering a combustion chamber, and the second gauge on your tool indicates more than 10% loss, listen for air noise at the exhaust (ex. valve problem), the intake (intake valve problem), or the crankcase (piston/cylinder/ring problem).
If your car uses a quart of oil, or more, in 300 miles, you need a valve job. If your car uses a quart per 1,000 miles, all is OK. If your car uses a quart every 3,000 miles you have a rare 911! Because the valves are closed during leakdown, the valve guides are not exposed to compressed air, therefore aren't tested.
If you must do a compression check, use a mirror to check how much crud is at the base of each spark plug. If it's not too dirty, pull the plugs. Use full throttle to test (ignition DISABLED!), have a helper crank the engine while you watch the gauge. You want to use about 8 engine "cycles" per cylinder; you can hear and count the cycles. You want to watch how quickly the gauge builds pressure, which will indicate overall health. If each cylinder builds to about 160 pounds, but one cyl takes four more cycles to get there, a problem might exist.
One more thing, if you suspect broken head studs, when the leakdown tester is pressurized, lay under the car and listen at the joining point of the cylinder and head. You will usually hear air escaping if the problem is in an advanced stage.
As I recall, the engine has to be at operating temp for both tests to be accurate. I'll have to work fast and wear long sleeves!
NO! Not operating temp. Cool enough to work comfortably in the engine compartment. The difference in leakdown readings between hot and cold engines is so minimal that it won't matter, mainly because your combustion chambers are being evaluated against each other. It doesn't matter if you get 4lb and 5lb readings on a hot engine, and 5lb and 6lb readings on the same engine when it's cool. The engine, in either case, is sound.
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