I have a 2008 F-150 FX4 with 90,000 miles. Been trying to figure out a misfiring issue since the beginning of the year.

At first, the truck would do the usual plug and coil issue of shuddering around 40-45 mph under light acceleration.

I replaced the plugs and coils twice. First time used motorcraft plugs and an offbrand coil pack. Problem was worse, so I returned the coil pack and went with motorcraft coils.

Computer was still showing misfire on a few cylinders so we replaced the plugs again. That seemed to help.

About a week later, started to make a loud pop under heavy acceleration from a rolling start of about 10-20 mph. Does it pretty much every time. Sounds like a backfire, and loses power for a second before it revs back up and accelerates normally.

Truck also occasionally loses power at around 50-60 mph and the check engine light flashes. Then it will go away and drive normal. Checking the codes after the check engine light flashes doesn't show any issues. We've reset it a few times and tried to check.

I have a K&N intake on it and flowmaster exhaust. Those have been on the truck since 2012 and didn't have these issues until end of 2016.

I also tried cleaning the mass air sensor, no effect.

NO IDEA WHAT'S GOING ON.

 

If you are using a K&N intake without a programmer with custom tunes, you are running too lean. If you still have the stock intake, put it back on, hopefully it's not too late. Running excessively lean can burn valves.

 

Thanks for the reply. Not saying you're wrong, but why would it wait 4.5 years to start giving me problems? Also, wouldn't the computer start throwing codes for running lean? I haven't heard anything about this intake running the motor lean. I thought the point was that the MAF sensor would adjust accordingly?

Thanks.

 

Now that I looked up the intake problem specifically, apparently it can cause the 3V motors to run lean. I'm just wondering why it would take so long to give me problems and has never thrown a code.

 

Not lean enough to throw a code, but possibly lean enough to slowly deteriorate the valves. The K&N filter is also known for passing more fine dust into the intake than a standard paper filter.

If you have Motorcraft plugs and coils, the ignition system *should* be fine unless one or more of them are defective. I've heard of new plugs cracking. How many coils have you replaced?

You may want to do a compression test.

 

I replaced all 8 plugs and coils, twice.

I took it to my mechanic today and suggested it might be running lean. He looked at it and said it certainly looks like detonation. threw it on the dyno, sure enough, running too lean.

He said there's no damage. He also said Ford programs these trucks to run way lean already for fuel economy. He's going to retune the computer, and I should be good to go. Hopefully this solves it.

Thanks for your help.