Leak Down test explained?
 

Can someone describe to me how the leak down test works for diagnosing valve guide wear? How exactly does it work.


thanks!!

Compressed air is introduced through the spark plug hole; twin gauges allow you to calculate the actual and percentage pressure drop in the cylinder (IIRC the first gauge measures input pressure at the tester, the baseline reading, and the 2nd gauge measures pressure in the cylinder). The pressure drop is the amount of leakdown. By listening carefully, the leakage can actually be heard through the breather or at the head. The location of the leakdown 'hiss' indicates ring wear (breather) or valve guide (head) wear. Leakdown tests are somewhat subjective depending on technique and equipment, but large or outlying leakdown values (when compared to other cylinders) can help pinpoint problems. Leakdown is a static test (engine is not turned over).

Best results are obtained by considering leakdown in conjunction with compression test results. A best practice (generally anyway) is to conduct both tests back to back. Otherwise, results of either test independently may be inconclusive. Compression tests can corroborate the results of a leakdown test. (If a cylinder leaks down at 11% versus 3% on all others, you can develop the expectation that compression test results on the same cylinder may also be poor due to bad sealing).

dtw, thanks for the reply. I don't get how a bad valve guide whould show up IF the valves themselves seal. Is it just by listening? If so, seems very subjective. I am debating with a shop the source of smoke at start up, contaminatng the plugs and emmissions.

Well if the leakdown value is low/within spec, you probably wouldn't take the test any further. But if the results show higher ranges of leakdown, then yes, there is the somewhat subjective test of listening for the source of the leak.

What color is the smoke at startup? Is it only at startup? Sometimes oil seeps past the rings and pools a bit in the combustion chamber. This then get blown out at startup. I've seen this phenomenon on 911s that had no issues with ring wear...but I've also seen much larger startup 'puffs' on cars with worn out rings.

Do a search in the archives, there is a five-page topic on this somewhere...

-Wayne

Re: Leak Down test explained?
 

I don't see how it could diagnose valve guide wear. When the valve is closed nothing is going to leak through a valve guide.
-Chris

Troof. I was thinking valves and saying guides.

Have you gone to:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/144650-leakdown-results.html

Best,
Grady

thanks for the replys. based on what I see the leak down is not useful in detecting valve guide wear, mostly rings or the valves themselves. I had understood it could also diagnose the guides, but seems the answer is no.

When guides and/or valve stems are worn, the valve rocks and sometimes does not seat squarely. So, potentially worn guides could be found this way. When valves rock and fail to seat properly, they retain, instead of transfer, their heat. They get too hot and burn. Then they do not seal. Intake valve hiss is heard in the intake. Exhaust valve hiss is heard in the exhaust. Ring blow-by hiss is heard from the crankcase, which is easy to hear if you remove the oil tank cap. Before making conclusions, fire the car up, drive it a bit and repeat. Carbon can get stuck under the exhaust valves. YOu can have a pretty good idea of what's going on in a combustion chamber before you remove the engine.