Leak Down Test ?

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, 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).

Hey John, I did curts test at 100 psi which makes the numbers idiot proof on the loss. If you are blowing a regulated 100 psi and your loss guage is 90 psi then you have a 10% loss and so on. A leak down test is not going to tell you if you have a head lifting or a gasket leaking under boost. It is hard to compensate for dynamic compression ratio. so you are just lower psi test. If you had access to a oxygen tank and could get 150- 200 psi then you could get more info. It will show as blake said a burned valve or melted piston/rings and show it by noise in the area its exiting. If you rotate that motor with any psi in the cylinder it will rotate at a high rate and take your fingers off if you are using the clutch to do so. The compressed air is dangerous, top dead center is crutial.

Jim

Be prepared to spend 2 hrs doing this test, the valve overlap is so tight its very hard to get to TDC unless you had the engine apart. Took me forever, but the data is so worthwhile. I will do it every season so I can monitor how my boosted engine is doing over time.

Good pointer on the safety thing Jim. Jop i would take it some where and tell them exactly what all is happening. Though i do not know about your set up it may need more than just leak down testing. It does sound like a very good place to get the ball rolling. Better hurry it's coming fast.

Jon, definately do not spin the clutch with pressure in the cylinder, i know. We found TDC by using some welding rod cut to size and put it on top of the piston, spun the crank till we found TDC.

Jon, your gonna have to do something if you want to keep up next year.:beer;