My 2002 XE-SC has been running rough forever at idle. Intermittent check engine light that is on more than it's off. Mechanic says the code is a misfire but doesn't tell them which cylinder. I have gotten horrible mileage (12) for a long time but dealer told me it was not uncommon with SC engine. I have had a full tune up (not 1/2 tune up dealers do) and up to date on maintenance. Dealer and my other shop seem unable to solve the problem.

I'm driving my X down the Baja in June and if the X dies on the way, I probably will too so this is something I'm going to have to figure out before then. Any ideas on cause or things to try?