Dear Doctor: My 2004 Mercury Mountaineer has 85,000 trouble free miles, but recently after filling up the tank one night the engine ran fine for 25 miles then the next morning started misfiring and the “service engine” light came on. My mechanic installed new plugs and wires after reading PO304, cylinder #4 misfire & PO316 engine misfire on start up first 100 revolutions. I have driven 500 miles and the engine continues to misfire. The misfire can be felt at all speeds. I added Chevron Techron fuel injector cleaner. My mechanic has hit a wall. What do you advise? RPM
Dear RPM: Before anyone can successfully diagnose the problem, a lot of checking has to be done with a professional scan tool. The technician will monitor fuel trim and oxygen sensor voltage, as well as look at the spark plug color. A full engine performance test including an injector balance test will need to be performed. There is no magic wand to checking the source of a misfire. I can tell you that I have replaced a lot of fuel injectors in a variety of vehicles.