In response to the low compression on engines and evaluating the overall
condition of an engine.
The most comprehensive battery of tests to evaluate the condition of an
engine consists of a compression and a leakdown test.
Everyone knows what a compression test is, and it is a necessary test for
starting the diagnostic process of evaluating engine condition. If it proves
to be normal or consistent with previous tests, then no further evaluation is
needed. However, when you have suspect compression readings, the next step in
diagnosing the problem is the leak down test. The test utilizes a special
gauge, sometimes two, depending on the brand of tester used, to determine the
percentage of leakage in each cylinder. Acceptable leakage for street
applications is 10% or less. This 10% or less is attributable to ring end
gap. This means you will never see 0% leak down, unless you leave the
schraeder valve in the adapter hose. (Those of you who have done this test
will appreciate that humor.) Any cylinder that read low on compression will
show high leak down. A leak down test is performed by setting the cylinder to
be tested on Top Dead Center on the compression stroke. An adapter like the
one used on some compression gauges is threaded into the spark plug hole. The
leak down tester is connected to the adapter. The leak down tester is also
connected on the other end to an air compressor. This pressurizes the
cylinder. The gauge displays the difference in percentage of the air escaping
the cylinder. The key to the leak down test is to listen to where the air is
escaping. You would listen at the exhaust pipe for exhaust valve leakage, the
air box for intake valve leakage, and the oil filler hole for ring/piston
leakage. This test has to be repeated once per cylinder until all have been
tested. The usefulness of this test is that it pinpoints why you have low
compression and where you are going in your teardown process. Of course, if
you get leakage past the valves, you would first want to ensure it is not
merely an adjustment (tight) that is causing the problem. However, bear in
mind, if you have tight valves, adjustment may or may not eliminate the
problem. It is advisable to readjust and retest. Tight valves tend to burn
because they do not dissipate their heat to the valve seat due to reduced
seat pressure and contact surface. The head of the valve conducts its heat to
the cylinder head by way of the valve seat. Any reduction of that contact
will result in higher valve head temperatures. If it gets too high, the edges
of the valve will start burning or eroding away, which produces leakage and
loss of compression. Repair at this point is at least replacing the burned
valves and possibly requiring the valve seats in the head to be re-cut.
Whenever you get a low compression reading, you should also get a leak down
reading to help make the diagnosis complete.
