2002 A4 Turbo 1.8L

I have had this car for about three years, and last month it started having issues with acceleration (shakes if you press on the gas), idling really rough, and CEL flashing.

I have the Ross Tech VCDS, did a scan and found faults on 3 of 4 cylinders, with the 4th being intermittent. I replaced all four ignition coils and spark plugs, which fixed the problem. The car was purring! Left for a road trip, over 1000km, and the week after I come back, the car started having the same problems.

Scanned the car again, and it came up with a fault in cylinder 3. I thought maybe the ignition coil was a dud, so I switch coils 2 & 3, and scanned the car. Sure enough the fault moved to cylinder 2. I replaced the coil again, and again the car was running smooth.

After another 150km... same problem... same cylinder.

This time I replaced the coil AND the plug in case it was a bad plug, and made it back home (20km) without problems. The next morning however, the car started having a fit again.

Any ideas on what could be causing only coil #3 to fail?

 

I think you have a wire harness issue, you will have to change all four connectors and partial wire, my mechanic told me that this type of problem happens sometimes when another wire from another connector causes a misfire jump between wires in the harness on the cylinder and ''burns'' out repeatedly the same coil.

Did you keep one of your old coil, try it back ???

 

yeah I would agree with qbert some ignition systems use a shielded ground wich I think is the case on our cars hense the ground wire on the valve cover. you have a couple options here you can get a harness probs be expensive and a bugger to change our you can get a closer look at your ignition harness if it was me I would do this. the shielding looks like a metal sheath on the wire harness and it probably has a exposed wire that is wraped inside of this, and is connected to your valve cover. if your handy with some wire and a heat gun or lighter for all that matter take a look at this and re rap this shield I think #3 is the closest to the ground on the valve cover so you really shouldn't have to go far. you can use any conductor like tin foil or auto body tape. that's what I would try chances are you have a shield leak. I hope this helps good luck

 

I should have added this earlier moisture will make the leak worse as well if it was particularly damp or rainy lately.
good luck