xr4tic

I was on my way back from ordering parts to fix my other car (which broke yesterday) and when I came to a stop, the idle stumbled, and then the car became real rough. Damn.

I limped it home on 3 cylinders and hooked up the VAG-COM. Just as I thought, Misfire on #2. This happened about 10,000 miles ago, but would clear up after driving a little ways. This time it's here to stay.

I swapped coil from #1 to #2 to see if maybe it was the problem. No change, still #2 misfiring.

I pulled all the plugs, and all were white/grey, except for #2 which was brown.
I did a compression test on all cylinders:
178, 180, 178, 175 (1, 2, 3, 4) so that's fine.

I dropped in a used spark plug I had, and the problems continued, it eventually turned brown.

The plug does not smell like gas, and is not wet.
So I'm thinking maybe the injector isn't firing, or (God forbid) the valve isn't opening.

So then I disconnected all the coils except #2, put plug into the coil, grounded it, and turned it over to check for spark. Nothing. Tried the other connectors with that coil, and I get spark. So coil is good, plug is good. Connector isn't firing the coil. I tried it before and after clearing the codes, with the same results. No codes were set afterwards.

Can the ECU detect when a coil isn't firing, and then disable that cylinder's injector so it doesn't waste gas? It doesn't make sense that the plug stays dry. Or maybe even disable that coil for whatever reason (although I would think that would set a code)

I just checked the all the coil connections, it has ground, power, and a good connection to the ignition module (aka power output stage), and the ignition module has good connections to the ECU.

I can't test the ignition module or ECU since I don't have a LED tester that the manual says to use. It tried the wiggle test on the ignition module wires with no success.

Any ideas or suggestions?

dingster

One of the cylinders weren't firing and it turned out to be the ignition amplifier on top of the airbox. Email him for more info.

Good luck.

TOMT

Check the power stage on air box. And also check for short wire from power stage to #2 coil wiring with a ohm meter.

xr4tic

I checked all the wires for shorts - no problem. I dropped my module into another A4 - it ran on only 3 cylinders as well (misfiring on #2)

Audi dealer wants $290 for a new one. I'll find out what Clair Parts wants on monday.

I bet I could make one of these things for less than that. It's probably just a bunch of FETS or something.

Mike O.

I had the same thing. The heat sink (ignition amplifier) that plugs into the top of the airbox is the culprit. 5 minutes to change it and $315 later and your car will run like it is new.

I would really not recommend that you continue to drive on 3 cylinders. You could damage a main bearing becuase of the imbalance that creates.

Hope it helps,
Mike O.

Mike O.

FETS is all it is (fu*k Audi for charging $315 for this thing)

Bollinger

Imagine getting that specific an answer anywhere else. There are few mechanics whose experience level comes anywhere near the aggregate of the people on this group.

Cool Craig

any thoughts on why ONLY Cyl#2 ?????

xr4tic

It has 4 inputs (and ground), and 4 outputs.

So basically, 4 stages, one for each cylinder, and they run separately from each other. Just one burned out, so the other 3 are good.

I think the way it works is, the coil has +12V applied to it for the primary side. You will see this at the module.
When the ECU sends the signal to the module, the module will then ground the coil wire to the cylinder designated by the signal. If the ECU did the direct grounding, then it would have to source (actually sink, or was that drain?) all the current, but the module takes the load for it.
When the coil gets grounded, the primary charge transfers to the secondary side (It's basically an AC transformer - stepping up the voltage, but you need a change in voltage, ie A/C signal, for it to work)
When the ground is removed, then the secondary side transfers the voltage to the spark plug, creating spark.

xr4tic

It would have been sent to page 3 in a matter of hours because vs. and poll threads were repeated for the sixth time.

OK, a little exaggeration - Maybe.