Leak Down test and oil use...again

Bernie Benz b.benz at charter.net
Mon May 17 23:41:05 EDT 2004

Derek, your 
Please explain the meaning of the percentage values for your leak down test
results for those of us who may hopefully will never have a need to know.

You're right, and I've always believed that a compression test with and
without the sealing capabilities of an oil adder may be as good as a leak
down test.

You may well have just a broken ring in one or more of the offensive
cylinders requiring replacement (low parts cost, but lots of R&R) if the
bore is not scored.


> From: "Derek Pulvino" <dbpulvino at hotmail.com>
> Hey Gang,
> I finally got around to doing a leak down last weekend...it looks like the
> engine might be getting tired.  These are the number I got:
> Cylinder 1 -  11%
> Cylinder 2 -  18%
> Cylinder 3 -  27%
> Cylinder 4 -  20%
> Cylinder 5 -  8%
> In doing the check, I also pinpointed the pistons as the source of leak.  No
> air seepage heard out of the intake or exhaust pipe, but did hear it from
> the crankcase breather hose.  I was also able eliminante the leak down by
> putting a tablespoon or two of oil w/in the cylinder.
> I thought of a way of using the compression test numbers.  The Bentley gives
> numbers for compresion.  For a "good" engine, compression check would be
> from 189 to 130 psi.  A worn engine would be 102 psi on the compresson
> check.  Based on that, I figured the lower good number is about 70% of the
> top good number, and the "bad" number is 54%.  If this were true, then the
> engine is still "good" but getting pretty low/close on one cylinder.  Direct
> interpretation to leak down perhaps?
> All in all though, looks like the test showed different wear between
> cylinders...perhaps due to the differential heat stored within the block?
> Perhaps this is the reason for oil use.  Ring blowby leading to pressurized
> oil pan, oil forced out breater hose and into intake path...
> Also time to check the compressor wheels on the turbo...may be another
> source of oil use, but looks to me like the engine is getting tired.
> Any other ideas/critiques?
> Derek P
>> yep, #1 is the easy one:)  again, I've not yet done a leakdown on a 5 cyl
>> Audi....4 cylinders are more "straight up"........starting with #1...then
>> 180 degrees on the crank pulley to the next cylinder in the firing
>> order....another 180 degrees to the 3rd cylinder int he firing order, etc.
>> ..........if you aren't on tdc for the cylinder you are working
>> with...you'll know real quick, the air pressure will move the piston back
>> down the cylinder....quick!!
>> there might be some info somewhere with a more precise approach to using
>> the
>> leakdown and what should be expected but my approach isn't so much a
>> comparison between cylinders, as with a compression test but more analyzing
>> each cylinder as I check it........deviation is certainly important but
>> personally don't necessarily approach it that way.........and not
>> necessarily an absolute number either........guess 100 would be
>> absolute:).......quite often when I use the leakdown it is because I had
>> done a compression check and found a low cylinder or cylinders so then did
>> a
>> leakdown to determine better where the problem area is......I would guess
>> that over 90% of the engines I have checked that were in good condition
>> checked out at less than 8% leakdown for all cylinders...and often having a
>> leakdown of no more than 5 or 6 percent.  The best one I remember, did a
>> leakdown on a VW aircooled engine I had built for mini stock circle track
>> racing...it had just 2% leakdown on 2 cylinders and 3% on the other
>> two....surprised me, usually the race engines seemed to have a little more
>> leakdown than stock...especially the aircooled bug engines............
>> the second part of that question...I have never seen a chart that converts
>> compression to leakdown and giving that some quick thought it may not be
>> that simple.......example, take a higher compression engine in good
>> condition will have higher compression readings...something with let's say
>> 10:0-1 compression will have a lot higher compression reading than
>> something
>> lower like...going back to my experience....the VW bug is low compression
>> engine...depending on the year, give or take in the 7:0-1 range....a
>> typical
>> low mileage bug engine in good shape will put out 120-130 psi while a high
>> compression engine will be up close to 200 psi.........yet both of those
>> engines, if in equal "tightness" condition, they could both have the same
>> leakdown, maybe 5% leakdown with both of these engines yet the one will
>> have
>> a much higher reading doing a compression test.............so a "tight"
>> engine with a low leakdown reading doesn't really relate directly to the
>> compression reading.........hope you followed that long explanation:)
>> ..........one engine, 190 psi & 5% leakdown...another engine 130 psi & 5%
>> leakdown
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