03 f150 4.6 215000 miles. Replaced coil number 4 and 8 less than a year ago. Number 4 is misfiring already. A friend of mine had same model truck got 400k miles out of it and said one coil on his would go bad about every year. Whats the causes for this. Ive got set of coils coming, looking into buying all new wires and going to put set of plugs in it. Anything specific to check or replace?

 

Miss fire is not always the coil........ Daughter had a similar problem.. I installed 3-4 different coils in #3 position... Finally looked at real time data and saw fuel trim was odd....... Installed NEW INJECTORS and no problem since.

 

Generally speaking, you should replace related components when replacing coils. That is, if it's a coil on plug, you should install a new spark plug too. If it has individual coils with a short plug wire (or if it's one coil for 2 or more cylinders), a new plug and wire should be installed for the respective cylinder. The exception would be if you just installed new plugs and wires (or it's been done in the last +/- 5,000 miles).

If you're preparing to put new coils, wires and plugs in it then you're probably going to be okay for a while.

Have you seen any DTC's other than misfire codes? A rich or lean code from the upstream O2 sensor could indicate other problems, such as a vacuum leak or sticky injector.

If you haven't done much in terms of fuel system maintenance lately, I'd recommend changing the fuel filter (if it's inline), unhooking the battery and touching the cables together to clear the fuel trim data (unless you have access to an advanced scan tool), dumping a can of seafoam in the tank (make sure it's somewhere between 1/4 and 3/4 full) and going for a 10 mile drive on the highway with the cruise control set. This will give the seafoam time to circulate through the injectors (do all this after you install new coils/plugs/wires).

When you turn around, pull off on the shoulder and shut the truck off for about 30 seconds, then start it up and drive back home, again with the cruise control set. This gives the computer a chance to relearn information but also triggers monitors that can set DTC's if there's a problem.

Check for DTC's when you get back, even if you don't see a check engine light - one thing I've always liked about Ford is the ability to run the KOEO and KOER diagnostic tests because they override all the criteria the PCM would normally use to diagnose malfunctions. If you get any codes, diagnose them.