New O2's and I'm still getting a slight buildup of black soot on the rear bumper and if I go several days without WOT, a gust of smoke comes out the exhaust when I do get around to it. I wonder if the richness is building up in the cat then being pushed out at WOT?

Car runs very smooth and strong otherwise.

I do notice it runs better on cold days but I figure that's normal.

Nothing else seems bad... any ideas on what to check?

Car has 103k miles and gets mediocre mileage I reckon you could say... about 200/tank city.

 

Maybe your catalytic broke apart and isnt functioning. That would explain some. Dont forget, these cars are rich from the factory in WOT state.

 

do you have an exhaust leak? If air is getting to your o2's it'll make your car run rich.

 

I honestly don't think so. No ticking or other noise to indicate a leak.