Leak Down test and oil use...again

Wed May 19 00:32:32 EDT 2004

Derek,

I still don't understand your explanation of the percentage figures for your
leak down test because I don't understand what test flow restriction is in
between the measured input and output pressures, but if adding oil to the
cylinder increased the compression pressure, you do have a ring problem, not
(only) a valve problem.

Bernie

> From: "Derek Pulvino" <dbpulvino at hotmail.com>
> Date: Tue, 18 May 2004 09:36:14 -0700
> To: b.benz at charter.net
> Cc: 200q20v at audifans.com
> Subject: Re: Leak Down test and oil use...again
> 
> Bernie,
> 
> 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
> assessment.
> 
> 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.
>> 
>> Bernie
>> 
>> 
>> 
>>> 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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