Question: What is a "leak down" test?
Answer: 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).
