If its running rich / poorly maybe even smoking looking closely at the FPR is a good place to go based upon my recent experience with the 348. The FPR diaphragm developed a small hole in it. Fuel would push out the diaphragm and into the vacuum line. This caused the engine to progressively run richer and richer. When the problem just began I noticed it just wasn't running quite right. Finally the CEL came on and we began hunting for the problem. By the time we found the problem it was smoking really bad it was so rich. It eventually dumped so much raw fuel into the intake manifold that I had to change the oil / plugs and run the car quite a while before all the un-burnt fuel cleared up. Two months later it began running slightly off again. This time I changed the other FPR on the right side and problem went away. I did dissect / destroy both old FPR's and found both had holes in the diaphragm.
I don't know if that's your problem but this is what happened to my 348.