misfire cyl 1 &4 replaced coils---prob came back next day, need advice
 

Saturday car ran rough and CEL came on. Pulled codes and got 16685 (cyl 1 missfire), 16688 (cyl 4 missfire) and 16684 (multiple cyl missfire).

Picked up 2 ignition coils and put them in. All seemed well and Sunday, drove about 120 miles OK. Stopped briefly and upon restart problem reappeared.

Same codes generated on RossTech. Ran car with vag com hooked up and looked at measuring blocks for missfires. If a few seconds, hundreds of missfires on cyl 4, and a handful on cyl 1. All other cylinders show zero missfires.

Since it was only 2 cylinders on both banks, I thought coils, and not ign. control modules. Now wondering if both control modules or something else. Trying to diagnose an avoid throwing parts at it.

Did you look at the plugs? Are the replacement coils brand new? Try swapping the plug and coil from #4 to another cylinder and see if the problem moves.

Misfires can be caused by many things (fuel injector, head gasket, etc...), the coils are just the most common problem.

Plugs looked fine at #1 and #4 and only have about 15k on them. I didn't move the coils around the first time, but should have. Replacement coils were brand new and when they apparently fixed the problem, I concluded that the coils were the problem.

I'm wondering about injectors. But why would a new coil temporarily fix the problem?

Perhaps the ignition control module for bank 2 where #4 is, but I always thought that a bad control module would cause misses across the whole bank.

swapped ICM wires and problem moved
 

Looks like it's the Bank 1 Ignition control module.

The ignition modules on the early engines are always suspect. The bolt in coils rarely go bad. Only the spark plug ends need to be replaced.

Looks like an easy fix.

Yup, just installed the new one and all is well. Now I know that the ICM's have separate circuits for each of the 3 cyl that they trigger.

If I had studied the Bentley circuit diagrams first and tried the ICM wire swap, I could have saved the time and cost of 2 new ignition coils. So the bin of spares grows!