[s-cars] Baffled over boost

[TMULLANE at snet.net]

Bruce,

This sounds like a boost leak.  Did you visually 
inspect the ends of the michelin man hose?  Sometimes 
it can blow off on one end, yet it is so stiff that 
is forced against the end of the xover pipe or IC 
enough to seal it up to 15 psi or so.  Get under 
there and yank on it a bit.

Tom


Date: Thu, 26 May 2005 09:56:09 -0600
From: "Mock, Bruce J (Bruce)" <bmock at avaya.com>
Subject: [s-cars] Baffled over boost
To: <s-car-list at audifans.com>
Message-ID:
<21FFE0795C0F654FAD783094A9AE1DFC07AFEB5D at cof110avexu4
.global.avaya.com>

Content-Type: text/plain; charset="iso-8859-1"

Hi Guys:

This list is truly priceless. I've been on this list 
almost since its hu
mble beginning and have soaked-up immeasurable bits 
of wisdom to the poin
t where I feel like NO matter what goes wrong with 
my '93 S4, (and at 17
5,000mi it really has not been much at all! ) I can 
tackle the problem. 
I know most all of us would agree about the great 
worth of this virtual 
community. Thanks guys.

This little problem has me completely stumped:

Again: '93 S4 w/ 175,000mi stock expect for MTM stage 
1 chip and RS2 Ex.
manifold.

Car runs fine until boost builds past ~15psig (I have 
an Autometer gauge 
on the A-pillar) Normally boost would continue to 
build - depending upon
what gear I am in - to ~20-21psig. But now, at 
approx. 15psig, instead 
of the boost building - the car goes Drrrrrrrrrrrr. 
(technical term, I k
now) Seriously, It sounds like a toned-down, quieted-
down version of wha
t a Diesel big-rig sounds like when they throw on the 
Jake-brake" Drrrrr
rrrrr. If I keep my foot to the floor this behavior 
continues. Drrrrrrr.
If I pull my foot off the gas it stops and the car 
runs fine - BELOW th
at magic threshold. Behaves this way in 2nd, 3rd, 4th 
and 5th gears. B
ehavior is very consistent -whether the motor is hot 
or cold. Again car 
runs, starts etc. just fine, no hint of anything 
wrong... until you put 
your foot down hard and let the boost build up - no 
boost beyond ~15psig.
Drrrrrrrr.

This has been going on for quite some time now. I 
can't pinpoint when it
actually started. I went to to get on the highway one 
day - got on the 
gas hard- and this happened. It'd been like this ever 
since. So... At le
ast it's not an intermittent problem!


Here's what has been done to troubleshoot so far:

Tested for boot leaks by pressurizing the intake 
tract - everything is t
ight. As I increase pressure up past ~12psi the oil 
cap seal starts hiss
ing slightly. (I have Samcos and and a high-dollar 
MTM Michelin-man hose)
I also inspected boost lines near the moisture trap, 
etc.

Multiple high-quality tanks of gas have been run 
through - so I don't bel
ieve that it is "bad gas" Also, the fuel pump was 
recently changed due t
o a 'no start' problem.

Waste Gate Freq. Valve changed out for a known-good 
unit. No change in b
ehavior.

Near-new plugs changed-out for brand new ones. No 
change in behavior.

MAF cleaned/inspected. No change.

Swapped-out the waste gate spring with a known good 
unit. No change.

Disconnected the exhaust at the down pipe thinking 
that the CAT(s) was po
ssibly clogged and restricting exhaust flow. Man was 
THAT a loud test dr
ive :) No change.

Throttle Pos Sensor tested per Bentley (Resistance, 
etc. checks) No cha
nge.

Wholesale swap-out of all 5 coil packs/spark plug 
boots/wiring - by this 
I mean I removed and swapped the entire aluminum 
cover section (the one t
hat says "20V" and houses the coil-packs/boots, etc.) 
that sits on top of
the valve cover with a unit from another car. No 
change.

Swapped ECU with known good (stock chip) unit from 
another car. No chang
e.


With all the recent discussion re knock sensors, I 
will check to see if t
hey both are tight. The rear knock sensor looks to be 
a real bear to get
to. Any suggestions on how to get to this guy?


Well... there you have it. I feel like I know my car 
pretty well thanks 
to this group. But this one has me beat!

Appreciate any thoughts on where to go from here from 
the collective wisd
om pool!!!

Bruce