Leakdown test

[Sören]

Does the piston position matter when I do a leak down test on my -68 472 engine?

[Cadillac Jim]

In any leakdown test, the cylinder under test must be at TDC compression (firing). This is because both valves are open at TDC exhaust/intake, and the intake valve is still open at BDC intake, and the exhaust valve is still open at BDC exhaust. If the piston is not at TDC or BTC and the valves are closed, the compressed air will turn the crankshaft.

You also need to hold the crankshaft during the test, possibly with a breaker bar on one of the torque converter bolts or a special tool for this that engages the starter ring gear on the flywheel and holds against the bell housing.

[Sören]

If I add 50 psi pressure, what should I expect in loss?

[Cadillac Jim]

From Wikipedia, "Leakage is given in wholly arbitrary percentages but these “percentages” do not relate to any actual quantity or real dimension. The meaning of the readings is only relative to other tests done with the same tester design. Leak-down readings of up to 20% are usually acceptable." Reference:

http://en.wikipedia.org/wiki/Leak-down_tester

A variable that is not mentioned in the quote is the orifice to the test cylinder or the time interval allowed for leakdown. You should follow the instructions that come with the leakdown tester that you are using.

Most leakdown tests use 100 psi so that the percentages can be read directly in psi. For example, at 50 psi test pressure, 20% leakdown would result in a reading at the cylinder of 40 psi, while with 100 psi test pressure, 20% leakdown results in 80 psi at the cylinder.

A big advantage of leakdown testing is that you can use your ears to tell where the leakage is. If an intake valve is leaking you can hear a loud hiss through the throttle body. If an exhaust valve is leaking you can hear the hiss out the exhaust. If the head gasket is leaking, you will be able to see bubbles in the radiator cap if you watch long enough with pressure in the cylinder. You may be able to hear a hiss in the crankcase vent if the rings are leaking.