What does "% leak down" really mean?

[Wayne 962]

Zeke - the test is typically performed at TDC because that way the compressed air won't try to push the cylinder inwards, and the valves are completely closed as well. I suppose you could run it at a spot other than TDC if you locked the engine carefully.

Warren is right - there is no gold standard. I explain this in this tech article here:

http://www.pelicanparts.com/techarticles/mult_Engine_Rebuild/mult_engine_rebuild-1.htm

-Wayne

[Grady Clay]

Denis,

What we are now discussing is the PHILOSOPHY of a PPI. There are many approaches to a PPI depending on what the purchaser wants. Almost all are valid approaches, given reasonable knowledge of the issues.

I would rather pay more and have better knowledge of what I am buying than risk unpleasant surprises later. I would prefer to spend little money on the initial tests and then progressively more as there were no initial problems found. Over the 23+ years, I tried to communicate this logic to my customers. Many times a simple no-charge look-over would send the car packing.


There are two questions the buyer needs to answer:
Do I want to buy this car at all or just walk?
If I buy it, what should I expect for initial repairs and long term?
Part of the decision is also, given the condition, is this a reasonable deal?

I think Wayne is right on – you shouldn’t make these important decisions based on a single test (or even a small sample.) A competent mechanic doing a cranking compression test and finding a wide variation would repeat the test several times. Knowing the CO and HC adds. Finding no ignition problems also helps.

A cylinder leak test is not much additional expense when doing a cranking compression test. With the right tools and an experienced mechanic, it might add 20 minutes to an already 30-40 minute job. In fact the data from one can confirm or refute the other.

You posted “could a car that fails a LD test post good HC #s?” My experience is yes it can. I have seen many 911s with a burned exhaust valve (detected by a cylinder leak test) that still were in spec for HC.

I think Pelican Kurt V is a good example. As I recall his engine compression tests didn’t indicate much of a problem (he didn’t do leak), the CO and HC were within spec, the 911 ran great but he looked up the exhaust port during an exhaust change and decided to investigate. Kurt, correct me if I mis-stated.


IMHO there is no substitute for knowledge.

Best,
Grady

[ewave]

Here's a picture of my actual tester.



Chris, I'm sure I had it hooked up the right way. I reviewed my leak down notes, and a couple cylinders had 1% leakage, according the delta between the two gages. Thanks again for all the information!

[ChrisBennet]

Hi Paul,
I have the exact same gage. I get some very low leakdowns (1-2%) with mine also.
-Chris

[RoninLB]

Originally posted by ChrisBennet
Hi Paul,
I have the exact same gage. I get some very low leakdowns (1-2%) with mine also.
-Chris

ditto