Leak Down test and oil use...again

Derek Pulvino dbpulvino at hotmail.com
Tue May 18 12:36:17 EDT 2004


A leak down test uses two pressure meters and has two hoses/connections.  
One connection goes to a compressor the other hose screws into the spark 
plug hole.  The gauges then correspond to pressure reading on the input 
(commpressor) and output (spark plug) side of the pressure regulator.  The 
percentages I'd given are (input/output)x100...hence a measure of the 
engines ability to hold pressure in each individual cylinder.  The other 
advantage is being able to listen to the exhaust, intake, and crankcase 
breather for where the air is leaking.  I heard air coming from the 
crankcase breathe system, and just decided to put oil in to double check my 

Given this info, it also seems to give me a prime mover for the oil coming 
out my distributor and the excessive oil contents "stored" in the Michelin 
man hose...ie excessive blowby=overpressure...etc.

I think that provides enough background.  Also, what do you think on the 
idea of using Bentley's compression numbers in the correlary I'd described?  
It makes sense from my end, but another person I've been corresponding with 
on this who uses these leakdowns regular/semi-regular basis  generally goes 
with >10%= rebuild time.

Maybe I'll borrow a compression tester now to see if there are indeed 
numerical parallels "in the real world."

Derek P

>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 
> > 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 
> > the crankcase breather hose.  I was also able eliminante the leak down 
> > putting a tablespoon or two of oil w/in the cylinder.
> >
> > I thought of a way of using the compression test numbers.  The Bentley 
> > 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 
> > top good number, and the "bad" number is 54%.  If this were true, then 
> > engine is still "good" but getting pretty low/close on one cylinder.  
> > 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 
> >
> > Perhaps this is the reason for oil use.  Ring blowby leading to 
> > 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 
> >> Audi....4 cylinders are more "straight up"........starting with 
> >> 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, 
> >> ..........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 
> >> down the cylinder....quick!!
> >>
> >> there might be some info somewhere with a more precise approach to 
> >> 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 
> >> 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 
> >> done a compression check and found a low cylinder or cylinders so then 
> >> a
> >> leakdown to determine better where the problem area is......I would 
> >> 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 
> >> leakdown on a VW aircooled engine I had built for mini stock circle 
> >> 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 
> >> leakdown than stock...especially the aircooled bug engines............
> >> the second part of that question...I have never seen a chart that 
> >> compression to leakdown and giving that some quick thought it may not 
> >> that simple.......example, take a higher compression engine in good
> >> condition will have higher compression readings...something with let's 
> >> 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 
> >> 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 
> >> compression engine will be up close to 200 psi.........yet both of 
> >> engines, if in equal "tightness" condition, they could both have the 
> >> 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 
> >> compression reading.........hope you followed that long explanation:)
> >>
> >> ..........one engine, 190 psi & 5% leakdown...another engine 130 psi & 
> >> leakdown
> >>
> >
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