Leak Down test and oil use...again

[Derek Pulvino]

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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