The code was P2188.

Recapping:
Installed new plugs in 2004 S60 turbo. Runs perfect most of the time but the idle can dip following by mild engine surge. When this happens can hear the idle is rough. Engine stalled once. The surge is enough to make wife uncomfortable when it happens in reverse. Don't notice anything when cruising, if this also happens then it is too subtle to tell.

After a few days it set the above code.

Trying to narrow down the cause I put the old plugs back. The new ones were gapped right on .028 but I wanted to rule out a bad plug. Ok - ruled it out. Probably still there with the old plugs (FWIW they are gapped at .034 .034 .034. 031 .031. I didn't mess with this because I wanted to put it back like it was). Problem is infrequent enough that I didn't experience it during 5 minute test drive with multiple stops/starts but it happened the next day when wife was driving.

I cleared the code, will see if/when it comes back. I don't expect clearing the code to make it run better.

Very discouraging. Such a simple task.

Thanks for your help,

Terry

I'll work on getting the code tonight (PC software installation issues...).

In the meantime I put the old plugs back in. The new ones were right on .028 gap but I thought getting it back to its original state would be a useful diagnostic (plus it's my wife's car and she's tired of the problem).

She says it still does the occasional stumble. Probably no change. It did not do it when I did a test drive around the block but that's the way it has always been - generally runs fine, sometimes acts up for a few seconds. FWIW she says it tends to do it in reverse. I don't know if that is really true or if it is more unnerving to have the car "surge" when backing up (engine stumbles, idle drops, apparently computer detects this and bumps up the engine speed a bit).

One thing I should mention: I put anti-sieze on the plug threads. I wondered why Volvo doesn't do this but now that I have a problem thought I better make sure it is ok to use the stuff. If it is not ok to use it any suggestions for getting it out of the plug holes?

I guess I'll start looking for nicked wires around the coils. I can't think of what else to do (yes, I know, read the code).

Thanks for much for the help.