View Full Version : Leak down engine test

Dr. David Fleming
10-10-2010, 10:53 PM
Does anyone regularly use a "engine cylinder leak down tester"?

This tool is used by professional engine builders to qualify the condition of the engine without invasive inspection. I heard of it by reading Smokey Yunick's books and he discribed it as able to identify ring and valve problems before they ever surfaced. The racers dream.

Considering the estimated 400 hour engine life of most Mercruiser Big Block engines I think I could spot any engine performance issues long before they begin to get expensive. Would be nice to measure the engine in its healthy stage and then as the hours go up actually observe any changes.

I bought one from Eastwood a couple of years ago but haven't quite got the knack of using it - mostly because I need to have some kind of degreed engine vibration damper (flywheel). Unlike the old days I don't know if my stock 502 MAG MPI engine has any timing marks? I need to figure out how to mark the damper so that the cylinders could be brought to TDC for the test. Some kind of degreed tape that could be applied to the damper? Then it would just be a matter of spinning the engine over and applying the tester to the cylinders.

Anyone got any experience with this? Seems like a lot of professonal mechanics have the leak down tester but I never see or hear of them using it.

10-11-2010, 06:33 AM
The 502 has timing marks like any other motor. The leakdown test is pretty easy to do. Take you time and post the results.
What year is your boat?
If I remember correctly, Buizilla told me that a "weak point" on the 502 was the valve springs. Your leakdown test might show that it is time to replace them.


Carl C
10-11-2010, 07:47 AM
It's very easy. Disable the ignition. Hold a finger over the spark plug hole on the cyld to be tested. Have someone bump the engine with the key til you hear and feel the poof of compression. Insert a narrow screwdriver into the spark plug hole onto the top of the piston. Rotate the engine slightly back and forth by hand while watching the screwdriver reach it's highest point. I've done many leak down tests. Have all plugs out so it will rotate easier by hand. Put the air into the cyld. Check for sufficient pressure retention. If low, put your ear to the intake (throttle open), exhaust and crankcase (via oil fill) to find where the air is escaping.

10-11-2010, 10:06 AM
what carl said except for the screw driver thing.. I only do one cylinder at a time then put the plug back in and move to the next cylinder..keeping the plugs in the engine helps the engine stay at tdc in that cylinder,i also use a breaker bar and a socket to move the crank pulley around while the cylinder is pressurized to make sure both valves are closed..