[s-cars] rough idle

[Brett Dikeman]

On Thu, Sep 10, 2009 at 11:13 AM, Martin Baggenstos
<martin.baggenstos at gmail.com> wrote:

>  Hopefully someone will be able to diagnose what is going on with the idle
> on my car which is a 1995 S6 sedan with RS2 mods and MRC software.. Whenever
> the car is at idle, according to my air fuel meter, the car runs very lean
> and if I turn the AC off it runs leaner and the cars idle is rough.

Have you checked for leaks past the throttle body, which is where you
would get unmetered air at idle?  Just a week ago someone with a
200q20v had a leak between the throttle and intake manifold, though
that was due to him opening that area up.

> I have checked my ignition system and it is fine.

Misfires in one cylinder can lean out the other cylinders as the
computer sees a richer mixture, but...I don't think that is your
problem.

Have you pulled the plugs to see if mixture problems vary
cylinder-to-cylinder?  You may want to clean them, reinstall, run for
a bit, and then pull them again.  In case you don't know how to 'read'
a plug:

http://www.google.com/search?q=reading+spark+plugs

> The only recent changes with the car are that I put a high
> flow in-tank fuel pump from 034 motorsports in the car. I also have the high
> flow delphi injections which go with my MRC software but even though the
> pump is high flow, it should not affect the fuel delivery from the injectors
> because they will only deliver what the software tells them to, correct.

Injector fuel delivery is dependent on timing and fuel pressure.  Have
you checked fuel pressure to make sure it is within spec?

Did you touch ANYTHING in the engine compartment in the course of
installing the new pump?  It sounds like the injectors have been in
for a while, but can you confirm?

Any sign of leaks near the injectors? (ie oily/dirty residue near a
joint.) If the whole engine/area is dirty, clean up the area around
the injectors and drive some more, see what 'returns'.

> Also could this be something to do with my oxygen sensor, which I have not
> changed in a while.

O2 sensors typically "fail" by getting clogged up and their response
time slows.  If the sensor has more than 50k on it, replace it,
however, as that is about their expected lifetime in the 20vt's.

> My idle control valve was replaced 1 year ago, but could
> it be a problem with that now.

Idle stabilization valves only deal with metered air, bypassing the
throttle plate.  They don't cause mixture problems, unless they let in
metered air (or their connections do.)

> I do not seem to have any other problems with
> driving or boost besides at idle. I have also run vag-com diagnostics
> without any warnings or codes.

If the check engine light comes on, there should be codes.

Do you have the Bentley?  Do the various adaptation and sensor data
blocks make sense?  I don't know much about what VAG-COM can
display/pull from the AAN computer, but I'm reasonably certain it is
at least as much as the 3B computer (which provides a 00 block of a
dozen or so values, including idle, 02, and MAF adaptation.)  The
Bentley features an extensive amount of troubleshooting info for the
200q20v based on those dozen-ish values, describing what they should
be, what it means if they're not in a specific range, etc.  The caveat
is that you MUST follow the prerequisites.

If the car really does run fine under boost, I think that may rule out
the MAF sensor.  Fuel pressure regulator failure also seems unlikely
for the same reasons (and the fact that you're running lean at idle,
not rich, which is more common from FPR failure.)

Brett