[s-cars] What constitutes a leak?

Wed Aug 17 14:22:09 EDT 2005

Tony,
Your expectations for a "leak proof" system are unrealistic.  You are expecting to pressurize the intake tract, and have zero leaks... is what it sounds like from your description.  When you put a plastic bag over the exhaust pipe, you were on the right track, but drawing the wrong conclusion.

When you pressurize the intake, where does that air go?  Well, first through the turbo compressor housing, then the plumbing (BPV, and IC), and finally the intake manifold.  At that point, the pressure goes down each intake runner.  If that particular cylinder's intake valve is open, the cylinder will fill.  BUT, if it isn't open, that cylinder will NOT fill... or fill very slowly.

Suffice it to say that air under pressure WILL (not yelling, just declaring the certainty on this fact) go into some cylinders.  Likewise, some cylinders WILL have exhaust valves open, or partially open.  As a result, you WILL certainly create a path for the air used for pressurizing the intake to eventually escape through the exhaust system.  This is not a fault, but simply a reality of testing in this fashion.  You could be super anal and remove both the intake and exhaust camshafts, thereby closing all valves, and reducing (not eliminating) all possible paths for air to "leak" past the cylinder head.  OR, you could remove your downpipe, and seal the output flange on the turbine housing.  Then you would  be able to expect zero, or very nearly zero pressure loss.  Bottom line... if the space shuttle were powered by a reciprocating engine, the test results would be the same... it would "leak"... or more accurately, simply have a path for pressure to escape the system.

Your questions....
Do you have a leak?  My answer... If you can't hear it, you don't have a leak subsantial enough to cause mixture issues.  Further, if you can't hear it and trace it to a point between your turbo compressor outlet and the point at which the intake manifold bolts up to the cylinder head, you do not have an issue.  Your issue is elsewhere.  I would give you the same answer to your first 3 questions.

Another behavior you can expect from even minor leaks... the engine will typically die out when you lift throttle and allow it to return to idle.  Is it doing this?

I'm assuming you have a means to measure mixture (wideband?).  If you do, I would suggest that you pull your ECU and install an unmodded box that is "known good."  You will need to swap the stock injectors back in as well.  See how it behaves.  This will effectively eliminate the software and injectors as a possible source of your woes.

If there is no improvement... I would move on to the fuel side.  Check fuel pressure to insure that the FPR is working properly.  To much pressure = rich mixture.

IMHO, you have adequatley tested the intake system, and it's time to move on to other possibilities.
Best of luck,
Dave

-----Original Message-----
From: Tony Guttmann <T.Guttmann at ms.unimelb.edu.au>
To: s-car-list at audifans.com
Sent: Wed, 17 Aug 2005 22:46:40 +1000
Subject: [s-cars] What constitutes a leak?

Inflated system to 20 psi. As I watch pressure gauge, it slowly drops, over a minute
or so to 4-5 psi. But surely, I think, some leakage through the turbo must 
occur?

I check by tying plastic bags around exhaust pipe outlet, and sure enough,
they slowly inflate as the pressure drops. So it slowly leaks out through
the (new) RS2 turbo.

My question, ladies and gentlemen, is do I have a leak? My mechanic, who was
doing all the work, claims that (a) expecting a turbo to be leak proof is
unrealistic (he put it more graphically), and that (b) such a slow leakage
would not significantly affect air/fuel ratios.

I told him that my impression, from the collective wisdom of this august
group, is that under the conditions I have described, 20 psi should hold.
Bullshit claims he, it's not the @#$%^ space shuttle! I shall ask the guru-hood
I reply. So here's the question:

What constitutes leak-free? I've already spent more than 12 months of my time
and my tuner's time (by remote control--we are 20,000km apart) trying to get
a decent chip set for my installation, thus far without success. Am I, or
more importantly (on this occasion) is my car leak free under the above
conditions?

And yes I have replaced the OX sensor, the ISV is leak free (100%), the
plugs and plug boots are new, and the whole car has only done 55,000 miles
(new cam belt, RS2 6-speed gearbox, Koni sport shocks, blah, blah, blah). 

I eagerly await the collective wisdom. My thanks to anyone who deigns to 
respond.

Tony Guttmann
'94 UrS4, '94 80 2.6 quattro (the daughter's)
'96 328i convertible (the wife's)
2004 Kona single speed and Giant OCR Compact (the son's) + 1 dead Volvo 244

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