[200q20v] Re: Help needed with a 1992 S4 Avant

[Brett Dikeman]

At 3:39 PM +0100 2/7/02, Phil Payne wrote:
>  > One of my friend's cars is showing signs of limp home mode or no
>boost...I
>>  suspect the WG FV or the diaphragm to be torn...
>>  We'll try to pull the codes this evening but the problem is that the
>"Check
>>  engine" doesn't seem to light (the oil pressure warning light
>doesn't light
>>  either and the Autocheck seems to have been somewhat disabled but
>it's not
>>  the fuse)...how are we supposed to pull the codes if the check
>engine light
>>  doesn't work? Does the LED procedure for a 200q20v work (SJM doesn't
>>  mention anything like this on his web site)
>
>The UrS4 uses two plugs inside the fusebox.  The procedure is the
>same.

Technically there are three fuses; ECU, memory for ECU, and a third
fuse for things like the O2 heater circuit and a handful of valves.
Only two are for the ECU directly; which is why they're both under a
little red cover marked "MOTOR"

   It will not cause code reading to fail, because the ignition must
be on in order to read them during which the ECU gets power from  its
main fuse; there will, however, not be any codes -if- the memory fuse
is missing and you shut the car off for more than a few seconds; you
will get a happy "4444", "all's well!" from the ECU(well, there will
probably be a complaint about the crank speed sensor since the engine
is stopped.)

I suspect that in this case there are missing bulbs in addition to
the problems, but that there is no code stored. Audi had a lovely
habit of not installing check engine bulbs unless required to do so;
note that Audi only started sporting a -specific- check engine light
around the time California required it.  Only California got the bulb
here in the states.  You two would know better than I if Europe, for
the most part, did, but it strikes me that if most of the US never
got the bulb, then neither did Europe.

It is also quite possible that there simply -isn't- a code to be
read.  The early ECUs were still pretty "stupid"; my car though a
blown wgfv the ECU though was a "bad throttle position sensor."  I
correctly diagnosed it; the PO's dealer did not, and simply replaced
the $$$ position sensor, then never bothered to see if new codes
popped up(they did, and the car would only hit 1.2-1.3 bar.)  His
loss; my gain.  $300 position sensor for $60.

Usually when someone messes around in that area of the engine bay,
they've either bumped, unplugged, or disconnected one of three things.

a)MAF sensor- car will idle etc fine but not much more getting going
in 1st and small hills will give you problems :)  Obvious enough to
check and this will generate a code, instantly.  Unlikely to be the
problem due to the severity of malfunction it would cause.

b)wastegate frequency valve- car will get about 1.2-1.3 bar, and
that's it; more if the owner has been messing with the wastegate
spring.  The 20vt never goes into a "no boost" limp home mode; it can
only limit boost to 1.2-1.3 bar.  Will not always generate a code,
and -might- generate a phantom code as described above.  Can be
checked via the output sequence test and via measuring the resistance
of the valve; I -think- 300 Ohms is standard(check the Bentley to be
sure.)

c)turbo bypass valve- if it is broken or the hose to it is
leaking(this is more common on the 200q20v, not the S4) then the car
will stumble or stall when coming to a stop at stop signs, traffic
lights, end of off ramps, etc., and will not get full boost many
times, particularly if chipped  This is the most "dangerous" of the
three, since it puts additional stress on the turbo; it is -very-
dangerous if the ECU is chipped since you run a -very- good chance of
overspinning the turbo.  If the engine is not chipped, than the stock
valve is more than suitable and will last quite a long time(60k or
more miles; t-belts last less.)

Thankfully the S-cars don't have such a moronic vacuum hose routing
as on the 3B; I saw eS2 conversion #2 (3B engine used) where the line
had been replaced with copper tubing the entire length of the run
behind the heat shield; very smart solution to the problem.  As for
the valve, thanks to the A4 and S4tt guys, there are now dozens of
$150(and up) alternatives to the Bosch valves($60-70) but I have
heard of problems with some(don't remember which, sorry.)  There is a
Porsche 996 turbo bypass valve that is rumored to be more durable
than the extremely common Bosch valve(used in everything from
early-eighties saabs to our Audis etc.)  It is somewhat of a shame
that Bosch still hasn't fixed the problem.  I imagine that they will,
shortly, given the number of S4tts in particular having problems with
them and overspinning their turbos; same with the 225hp TT.
Audi/Bosch's own damn fault; they knew the valve was busting in the
200q20vs and S4/S6's over time.

   If there are no codes stored and the car still gets 1.2-1.3 bar,
then you probably have an overheat signal being sent to the ECU; the
quickest check, if it is above 40 degrees F, is to hit the A/C button
and see if the A/C fires up.  If not(remember it must be above 40
degrees), replace the multifunction temp sensor, it is sending a
false overheat.

Brett
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