About a week ago my '97 started idling rough after warming up. Starts up fine, idles fine when cold but after about 5 minutes of driving it'll start missing and stuttering. Much more noticeable at low idle or acceleration than at speed (it doesn't sputter at speed but I can feel it missing occasionally) A check at the garage showed a water pump leak onto, of course, the opti... Aha. Fortunately that was under an earlier warranty repair so I got that gratis. Water pump and opti replaced by a dealer garage.

But... that didn't resolve the problem. Dealer garage suggests it might need new spark plugs/wires. (Both my usual garage and the dealer garage independently determined that cylinder 7 was giving me grief).

No SES codes (yet) and it's not bad enough to have stalled out.

I'm inclined to agree with the plugs/wires but I don't understand why it seems to run okay when cold?

(I should point out that I've had the Ignition Control Module go out before and its exhibiting similar, but not the same symptoms and I'm wondering if it's not the same thing.)

 

I'd double check to see if the water pump isn't still leaking onto the opti.

 

You should do a search before posting this type of question, this has already been discussed exhaustively. Your car seems to be running worse once it enters closed loop, meaning once the o2 sensors (and modified sterms) enter the fuel correction loop.

Do a search for runs worse in closed loop or some other wording of that, and you should see several posts on the topic.

Plugs/wires could well be the source. O2 sensors/wiring could be the source. A bad fuel injector/wiring could be the source. An exhaust leak before the O2 sensors could be the source. A vacuum leak could be the source. These are all likely possibilities, because they wouldn't need to exhibit very noticable symptoms in open loop, and could mostly be constrained to one cyl, thereby making an o2's compensation for the whole bank counterproductive (hence running really bad in closed loop).

There are other possibilities, but most of them become less and less likely and would probably be more and more noticable (in open loop even) -- maf, ect, fuel pressure regulator, fuel pump, and as you mentioned, opti and icm. I'd say these are all unlikely suspects in your case, though they aren't out of the realm of possibilities either. But while their symptoms might be worse in closed loop, they would all likely also exhibit noticable symptoms in open loop.

Good luck, do a search next time, and let us know what you find out as you check these things.