[s-cars] 94 S4 hesitated

[Robert Myers]

Several possibilities, Sam.  Oh, BTW, welcome to the s-cars group.

First, that digital boost gauge is (or was, anyway) notoriously 
inakurit.  ;-)  That's basically why we lost them on this side of the 
pond.  In a "normal"  (unmodified) s-car you should see a maximum sustained 
long pull boost of about 1.7 bar on that gauge or about 8 or 9 psig of 
boost.  It will peak higher than that but then will fall back to about that 
level as rpms build.  I sounds like your boost level is significantly 
reduced from where it should be (assuming your gauge is reasonably 
accurate.  This could well be the result of some rather massive boost leaks 
as someone else has suggested.  Likely suspect is the Michelin Man hose or 
one of the other hoses in that circuit.

The on/off/on/off... behavior could have several sources.

1.      If this sequencing is very rapid what you are likely experiencing 
is a significant engine misfire which may be a result of a failing ignition 
coil for one of the five cylinders.  It is more difficult to fire a plug at 
high mixture pressures than at comparatively low pressure.  Higher boost 
means higher cylinder pressure which could be enough to resistance to 
firing to permit a weak coil to not fire its plug.   The solution is to 
identify the one causing the miss and to swap it with a new coil.  As a 
test, if you can have temporary access to a whole coil pack for a few 
minutes from another s-car, swap an entire coil pack assembly and see if 
the miss goes away.  Failing that, pull the plug for injector #1 and go for 
a test drive.  Get as much boost as possible.  If #1 is the cylinder with 
the miss the miss you get from pulling the injector connector will not get 
worse as boost  builds.  If it does get worse then go on to #2, etc., until 
you find which coil is not firing under boost.  You are looking for a 
situation for a 5 cylinder engine, hitting on only 4 (bad) suddenly starts 
hitting on only 3 (worse).  The bad coil is the ones that doesn't get 
worse.  Alternative procedure #2, obtain a cheap (<US$10) variable gap 
spark tester and see if you can detect a weak spark for one cylinder.

2.      If the on/off sequence is much slower than the repeated misfiring 
of one cylinder then you may be experiencing overboost (unlikely since you 
are getting boost information saying "low boost").  Boost builds, exceeds 
the set upper limit and ECU turns the engine off until boost falls.  Then 
engine comes back on until boost gets too high again.  The process will 
feel rather violent.  This could result from a torn or split wastegate 
diaphragm.  If so, this is fairly easy to replace.

3.      If the decrease in power feels more like some little guy is sitting 
inside the works and he flips a switch causing the power output of the 
engine to drop but not to quit then you may be experiencing knock sensor 
problems caused by poor fuel.  The ECU detects knocking (pinging) and 
instantly dials back spark advance .  After a bit it tries again.  Timing 
advances again and boost builds some more and then more knocking and we 
repeat the process.  This is not nearly such a violent behavior as the 
overboost response.  It's very noticeable and annoying but it doesn't feel 
like you've just run into a  brick wall.  This is most commonly caused by 
too low octane fuel.  These engines require good fuel.  At least 92, 
preferably 93, rated fuel (by US specs [R+M/2]).  That corresponds to 
something like 98 octane as I suspect it is measured in your part of the 
world.  As a test try running higher rated fuel or add some octane booster 
to your fuel to observe the effect.  The solution if this is the 
problem?  Use better fuel.

There are likely several other possibilities, Sam, but these should cover a 
lot of the likely faults.

Someone will likely suggest a failed PSO (power stage output).  IME, a 
failed PSO results in a full-time miss, even at idle, not just a miss under 
boost.  May I suggest that the PSO is a POS but I don't think that this 
particular POS is causing your present problem.

Good luck and keep us informed.

At 01:14 AM 6/11/2005, Sam Clarkson wrote:

>Hi
>94 S4 6 speed
>I was driving hard(ish) up a hill the other day when suddenly a massive 
>hesitation occurred, like someone had turned the key off, then on, then 
>off, then on. This has repeated itself since from about 1.2 bar up on the 
>digital boost/vac readout. Mid throttle driving is fine. All other gauges 
>etc read OK.
>I drove it again today, initially it did the same thing, then the boost 
>readout went flat at 1.0 bar and the hesitation appears gone. The boost 
>reads 1.0 bar no matter what speed/throttle position, whether full boot up 
>hill or full overrun downhill or at idle. Power seems normal, everything 
>drives smoothly.
>All other readouts are as normal (fuel economy etc readouts)
>What gives gurus?
>Sam Clarkson
>Edge to Edge
>P.O. Box 27
>National Park 2653
>Ph 0800 800 SKI (754)
>http://www.edgetoedge.co.nz
>
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Bob
Http://Chips-Ur-S.com