Have changed plugs dist cap wires & still get erratic misfire that initially shows code 0303 ?? Starts fine hot & cold & develops erratic on off misfire at mid range revs (1800 & up).. Could this be fuel injector? Tried liquid cleaners which seemed to help for a while. Mileage around 135000 km & no significant other symptoms.
What to chec/ replace next??

 

Are you also saying you're running rich? Are you getting an o2 sensor code?

My 3.9 used to misfire/backfire like a mother and my problem was a bad o2 sensor. Worked flawlessly till I blew it up

 

Did you get cylinder misfire codes or just O2 was misfire erratic? Thx for help

 

The running rich showed up after by a few days

 

Only misfire Codes. Try swapping your passenger side upstream sensor to ur driver side. If the misfire changes from cylinder 3 to 2,4 or 6 Its your sensor.

 

Question I got code 1294 popn up I cant find a vacum leak but I get the backfire an misfire on mid shifts I changed plugs cap rotor an o2 by the header on a 2.5 do you think my coil is going out its 97 an I dont remember changing it