Eddie Christensen

Hello again Rennlisters,

I have read many times in various inputs about "Leak down test". What can it tell you and how is it done, please?

Eddie C.

Bill Gregory

Eddie,

The leak down test is done by raising the piston in the cylinder being tested, injecting air through the spark plug hole to create a pressurized environment, and measuring air pressure reduction, or leakage, over a short period of time. Leaks can, for example, take place past the piston rings or valves/valve guides. The leak down test is a different test than a compression test.

Gary J. Spinella

It's my understanding you want to perf the test on a engine around normal operating temp. This should allow a more accurate reading.

frankd

A Leakdown test is, first of all, the best investment you can make in addition to a full PPI when buying a pre-owned 911. A leakdown measures each cylinder's ability to hold pressure. Each cylinder is set to TDC and pressurized to 100 PSI. A second gauge measures the amount of pressure lost within each cylinder. There are differing opinions however 3 to 5% leakage is just fine,even up to 10%. What this is telling you essentially is the integrity the engine cylinder heads and if a rebuild is needed which may cost 1 to 3K depending on what is rebuilt. The technician may also be able to identify if the leakage, if present at a significant level, is coming from the exhaust or intake side of the cylinder. A motor may sound and appear to run just fine but a leakdown will tell you what 's really going on inside the motor.