I have a 1999 S-80 2.9 with 150 K miles on it. last week the check engine light came on and the car was shaking slightly when sitting at an idle or at a stop light. Mechanic did a reset and changed out spark plugs, plugs 1- 2- 3 where whitish but plugs 4-5-6 were black. Driving last night gas milage at 60 mph was about 21 mpg where I think it should be 27 to 30 mpg.

Any suggestions on what I should be looking for to fix this?

 

Is the car still running rough? Do you still have a cel? What brand of spark plugs did your machanic use?

 

I had the same exact problem last summer.
Check engine light and the car shook while stopped but running.

I had someone scan it and it was cylindery misfire and i had to change out the ignition coil. It was a quick and cheap fix and it worked.

I would get car scanned, look up code and fix it.