[NeighborsComplaint]

As others have said, there is no set duration for a test. It is simply the differential between the pressure supplied vs. the pressure maintained by the cylinder and is accomplished the moment the cylinder is pressurized.

Assuming you remove one plug at a time to test, 10% leak-down would be considered acceptable. 20% or higher indicates problems either in ring seal or valve contact.

Following a "dry" leak-down test, you can pour a small quantity of oil down the cylinder and retest "wet". The oil will improve the sealing of the rings. If the leak-down % improves, the rings are suspect.

Conversely, no improvement can indicate leakage past the valves. This is usually accompanied by the sound or air leakage within the intake and/or exhaust tract.

[Schurkey]

Originally Posted by NeighborsComplaint

As others have said, there is no set duration for a test. It is simply the differential between the pressure supplied vs. the pressure maintained by the cylinder and is accomplished the moment the cylinder is pressurized.

Yes.

It takes (much) longer to find WHERE the pressure is going, (Listen at the throttle body, listen at the exhaust pipe, look in the radiator for bubbles, wave your hand around the spark plug holes of the cylinders next to the one being tested) than to look at the gauge(s) to determine the AMOUNT of leakage.

Originally Posted by NeighborsComplaint

Assuming you remove one plug at a time to test, 10% leak-down would be considered acceptable. 20% or higher indicates problems either in ring seal or valve contact.

No.

1. Removing one plug at a time is poor practice, because you might miss leakage from one cylinder to it's neighbor(s). If you put air in one cylinder, and you get air out of the next cylinder's spark plug hole...you have a badly-damaged casting, or a popped head gasket.

2. As I've said before...you cannot pick a percentage number and say that it's "good" or it's "bad". The indicated results on the gauge depend on the ACTUAL LEAKAGE, AND the construction of the leakdown tester being used. There is no industry-standard, law, or regulation of automotive leakdown testers. "10%" indicated leakage on one leakdown tester may be caused by more leakage than an indicated 20% on another leakdown tester. You cannot compare test results from one leakdown tester to another, different leakdown tester.

You MUST "calibrate your expectations" of the leakdown tester in-use by one of two methods: Either test known-good cylinders of similar bore diameter to the engine you have questions about; or use a Master Orifice.

Originally Posted by NeighborsComplaint

Following a "dry" leak-down test, you can pour a small quantity of oil down the cylinder and retest "wet". The oil will improve the sealing of the rings. If the leak-down % improves, the rings are suspect.

Conversely, no improvement can indicate leakage past the valves. This is usually accompanied by the sound or air leakage within the intake and/or exhaust tract.

No.

"Wet" (oiled-cylinder) tests were said to be valid for cranking compression testing. I quit doing those decades ago, because I could not understand how the oil was supposed to climb uphill to seal the rings on engines with angled cylinder bores--V8s, Slant 6 or Slant 4, horizontally-opposed engines, etc.

"Wet" cranking compression testing may have been valid on upright engines--Inline 4 or 6, "Straight 8". But mostly, the compression numbers go up because adding incompressible oil to the cylinder raises the compression ratio slightly.

Adding oil to a cylinder leakdown test is useless in any case...the oil just blows down the ring gaps, or around leaky rings--and the rings ALWAYS leak. It's just a matter of whether they leak excessively.