MaxBruno

(Posted at SCAudi, too)

Hello all,

Last fall I installed a PES G2 S/C in my 98.5 A4Q. The car has been runing very well since installation, with no problems to report -- until now. I don't drive it often, so it probably has no more than 2000 miles since S/C installation. (I don't know that S/C has anything to do with problem I'm seeing; just giving you the full picture.)

The other day I was accellerating at a moderate rate (engine was still cold) and when I got to around 3500-4000 RPMs the engine faltered/stuttered/stammered badly. It has been doing so intermittently ever since, usually around that same rev range. Problem occurs regardless of engine temp.
VAG-COM says:
16684 - Random/Multiple Cylinder Misfire Detected
16690 - Cylinder 6 Misfire Detected
16688 - Cylinder 4 Misfire Detected
16689 - Cylinder 5 Misfire Detected

So, it seems that an entire bank of cylinders is losing ignition on an intermittent basis, apparently in response to a given RPM range and/or torque load on the engine. Given the abruptness with which the engine loses power during these episodes, I am guessing an electical fault in the ignition or FI wiring (vibration loosening connection at harness plug, or wire chafing resulting in worn insulation, or break in wire where engine harness flexes as it passes to chassis, or some flakey component?). But I am certainly open to other theories. Hopefully someone can direct me to a single point of failure that would shut down the entire 4-5-6 bank. I have full set of service manuals, but thought one of you gurus might spare me the search for the "needle in a haystack."

Thanks,
Max

4Rings::G2 boosted::

so if one cylinder is misfiring badly enough it's quite possible that it shows up at a misfire on the neighboring cylinders as well.

It's probably not the coilpacks, as each of those are linked to two cylinders, one on each side, so a bad coilpack would show up as misfires on both banks. Same thing for anything else in the ignition system. First check the spark plugs and the spark plug wires on the driver's side. I would guess one of your plug wires is probably bad.

If that doesn't work, check for leaks in the intake manifold gasket, the supercharger-manifold gasket or even the throttle body gasket. A leak in any of these can really screw up the mixture on one side of the engine.

MaxBruno

Thanks, 4Rings. Interesting. I didn't realize that knock sensor was employed to detect misfires, too; I thought that was done through analysis of fluctuations in angular velocity of crank, as measured by crank position sensor...? I'll see if I can find fault with the items you listed.

4Rings::G2 boosted::

by listening for misfires and checking at which degree of the crank they occour the ECU tries to determine which cylinder is misfiring, however in cases when it's not just a single misfire but more of a continuous missing it often assigns the misfires to the wrong cylinders.