Possible failure of ECU- anyone experience this?

[Chung]

A boost leak on a mass air equipped car like the RS6 can result in the condition you're describing. Do you smell fuel at all either while you're cranking it or as it's running before it dies?

If that is true would an error from the MAF reading also cause this? Combining yours and LIRS6's suggestions it could be the MAF wiring (though this should be easy to check with VAGCOM)? I had to replace my MAF wiring but the failure was just no boost.

And since we are grasping as straws are you sure it is not that your engine is too small and you need to see if a larger more powerful engine will solve the problem?

[hahnmgh63]

Did your shop ever Vagcom the car for codes? Any OBDII code reader should be sufficient since it is an engine/drivetrain related code were looking for? Without running codes its a hit and miss shot in the dark. With that said, what about the DME relay/fuel pump?

[papadoc]

Yes, VAGCOM was run, and that is the mystery- no codes thrown. My shop feels this is a failure in the ECU. Spoke with them today, and they acknowledge that the wiring harness could be bad, but the cost to do that as a guess is high- very time consuming and expensive to boot. When they test power to the leads to the ECU, there is power, but then they don't get the response from the ECU. That was their initial thought and initially sent the whole thing back to APR, thinking that their chip was bad, and they installed a new one on the ECU. That did not change the result, and before OK'ing the purchase of a new ECU, I had them try replacing the crank position sensor. So next step, new ECU. The joys of a car out of warranty...

[10SecS4]

Something here isn't adding up. If the ECU wasn't responding then they wouldn't be able to connect with the VAG-COM to check for codes. If you turn the key to the right but don't try to start it, are the CEL and EPC lights illuminated? If so, I really doubt it's the ECU. Typically when there's a botched solder EPROM solder job or the ECU is fried, you won't even be able to connect with the VAG-COM. If you have the APR chip, have you tried switching programs and then starting the car? I'd try the stock program, race gas program etc and see if there's any change.

[papadoc]

When the key is turned to the right, the CEL and EPC lights come on but only very briefly and do not stay on as they should. Yet the car will start, but only run briefly. I thought that if it was the ECU that it wouldn't start at all which is why I had them try the crank position sensor first. APR had installed new chip and it is still in stock mode as far as I know, and the problem persisted. They also noted when the first had the car, before sending the whole ECU back to APR, that when they started the car and shook the ECU, the car stopped- which is why their first thought was that the APR chip or solder job on it went bad. When APR had the ECU, their tech said they "noted some loose leads and some that looked questionable", but they had no way to simulate running a car with the ECU in place. Tech support at Audi NA also suggested that the problem could be in the ECU.
And not completely true that no codes were thrown, as I look back over my emails. Only one that was relevant:

Inst cluster - 3 faults
Key signal too low
Contact ignition switch short to ground
Engine control module no communication

So that is the next route. Really appreciate the input from all.

[MaxRS6]

...With that said, what about the DME relay/fuel pump?

+1
and fuel filter

My less than .03 worth...

[SYNCRORS2]

Going back through the history of my RS6 Avant, it has had the engine ECU replaced.

There isn't any info on what the issue was, just that it needed to be replaced.

MG

[LIRS6]

Any resolution to the problem?

[papadoc]

Any resolution to the problem?

Yes, finally, and just today! Tried a number of the suggestions here without success, and finally ordered a new ECU which had to come from Germany. Was installed and brought to the local dealer to undo the immobilizer code, and the car is running again. Was in fact something bad in the ECU. Now shipping it back to APR to reflash and hopefully next week I'll finally be back on the road. It will be nice to get out of the Yukon XL, but I have to say people get out of your way when you merge lanes in that thing...

[lswing]

Great to hear!...you can get out of the way and ahead now. I often wonder what other drivers think when you start merging at 50, then are at 90 in 3 seconds...

[LIRS6]

Great news! A happy camper again