B5 V6 "unilateral" misfiring

[Huw Powell]

Chris Newbold wrote:
> The patient: 2001 S4 with the 2.7 biturbo V6; I was deliberately vague
> in the subject about the car in question because I don't think it's a
> unique S4 issue...
> 
> The problem: occasional rough idle; stored misfire codes as read by
> VAG-COM. Using VAG-COM to read the cylinder bank-specific misfire
> codes with the engine idling reveals that cylinders 4, 5, 6 (bank 2,
> US driver's side) are racking up large numbers of misfires--in excess
> of the factory specification (Bentley says values should be < 15 per
> cylinder). Cylinders 1,2, 3 (bank 1) are virtually misfire-free.
> 
> Stuff I've tried:
> 
> 1) All new spark plugs
> 
> 2) Rearranging coil packs
> 
> 3) Swapping ignition output-stage amplifiers
> 
> 4) Ran though Bentley diagnostics for the MAF and oxygen sensors;
> everything checked out within factory specifications. The only strange
> observation is that the mixture for bank 2 where the misfires are is
> being leaned-out by the O2 sensors right to the limit of the factory
> specification--both the "additive" value (for idle) and the
> "multiplicative" value (for full-RPM range operation) are around
> -3.5%. It isn't clear to me if this is symptom (misfiring => unburned
> fuel => need to lean out) or if this is part of the cause.
> 
> I'm not sure what to try next; clearly something is not right here,
> but there isn't any smoking gun. It's really odd that it's just one
> side of the motor... Any ideas would be much appreciated. Thanks.

After reading everything you have tried on the spark side (and imagining 
that you have verified all-good electrical connections), it sure sounds 
like a fuel issue.  Are the two sides managed separately?  Could 4-5-6 
be running too rich?  So rich the system can't get them back to stoich?

-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/