[s-cars] 94 S4 hesitated

[Sam Clarkson]

Feels like scenario 2, but the way it drives does not feel like 
overboost, and what gives now that the boost readout stays flat but the 
hesitation is gone? I'll have a poke around today sometime.
On 11/06/2005, at 11:43 PM, Robert Myers wrote:

>
>  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
>
Sam Clarkson
Edge to Edge
P.O. Box 27
National Park 2653
Ph 0800 800 SKI (754)
http://www.edgetoedge.co.nz