I am in need of some experienced advice.

1987 S4.

Occasionally at idle, the engine will develop a mild "shudder". In conjunction, I hear a "snapping" sound on the radio.
When the sound disappears, so does the rough running. It is not a plug wire sound, it does not change with engine speed.
At driving speed, the shudder is gone, but the sound remains.
Then, the sound will disappear, and the engine idles fine. There does not seem to be a "logic" to these occurances.

Ignition coils ?? Ignition module ??

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Sounds like interference from the ignition system or fuel system. My guess is on the fuse panel because this is the only part of the car where they may be close enough to interfere with each other.

The suppression relay may have gone bad (try unplugging it) or the ezk and/or LH relay could be having issues. The ezk/LH relays are just 53 relays IIRC so you can try swapping it with the horn relay to see if anything changes.

 

when were the spark plug wires and the coil wires replaced? what brand did you use?
Please post a picture of your engine bay so we can discern the condition of your engine

 

And.. how old is the battery, put a volt meter on it and see it it is voltage related, weak voltage will cause it, especially at idle.

Dave K

 

Thank you for the replies:

- the battery shows 12.5 volts disconnected, 13.7 volts at idle.
- the plug and coil wires are less than 1 year old, if it was a plug wire, the sound would change with RPM
- it is not a relay clicking, the sound comes over the radio

This scenario only happens occasionally, other than that, it runs fine

 

When did you last go around checking/cleaning your ground connections?

 

Remove both coil wires and check for corrosion
make sure the wire has not been touching on any part of its run if so you could have a shorted wire
fix the wires so they are free