[Biturbos4] B5 V6 "unilateral" misfiring

Chris Newbold chris at newbold.org
Mon Apr 14 08:59:19 PDT 2008

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; installed about 3000 miles back; much smoother
over-all power delivery, but still misfiring.

2) Rearranging coil packs; seemed unlikely for all three on one side
to go bad at once, but I swapped one or two from side-to-side;
misfires remained in bank 2 with bank 1 remaining clean.

3) Swapping ignition output-stage amplifiers; these are the
semiconductor widgets mounted to a heat sink on the airbox; there's
one for each cylinder bank. Swapping did not change the observed
behavior: bank 2 continued to misfire.

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.

2001.5 S4 Avant 6spd
2004 allroad 2.7T

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