[V8] MPG - oh no!!!

[Scott Phillips]

Jason, 

A leakdown 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 is 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, etc.

If all the cylinders have low compression but show minimal leakage, the
most likely cause is incorrect valve timing. The timing belt 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).

Here is a example of a leakdown tester:
http://www.speedunleashed.com/shop/customer/product.php?productid=20051&
partner=froogle

HTH..

Scott