I have a 2004 F150 new body style Fx4 5.4, 214,000miles that backfires when I drop the hammer and get on it. It doesn't do it every time but when it does do it you can hear popping noise,(from under the hood)sometimes multiple times rapidly. (Like popcorn popping)
I took it to my dealership and they said that they were "leaning" toward the cadiletic converters being bad. I asked how much does that cost and they said $1600 with labor. I asked what if they are wrong I'll be out $1600 since they aren't sure. They also noted cylinder 5 had misfired 84 times in last 10 times I've started and ran my truck. They said that wasn't too bad believe it or not, they said other 7 cylinders were good. The plugs in my truck are 3 years old with 40k miles on them. I went home changed out #5 plug and coil pack and that didn't help.( I used Ford Motorcraft parts) I can't get it to backfire when I am in park, it only does it when I'm driving and engine is under a load so to speak. Any ideas would be appreciated! Thanks

 

Backfire is into the intake manifold.
Guessing on a Cat gets expensive.
When you open the throttle quickly, extra fuel must be injected to the cylinders or a lean condition can occur.
Back fire can result.
That function is the first thing to check out.
It should show up as a good change in short term fuel trim shifts for both banks.
Seem no shop is willing or able to trouble shoot very deep for these issues.
Good luck.

 

After 4 weeks I figured out my issue. Bad gasoline has been my problem. I stopped purchasing cheap 84 octane at my local Kroger gasstation and started buying gasoline at Shell. I bought the 94 octaine a few fill ups and it's been running great and no backfires when I drop the hammer. What's odd is Ford said my injectors were good according to their diagnostic data. Of course they can't conduct diagnostics while it was under a load. I definitely believe in Shell gasoline being better than most. Hopefully it holds up.

 

Your truck should not require 94 octane. In fact it is designed to run on 87 octane if I recall correctly. Before you declare victory, I'd run a few tanks of 87 through it to see if the problem really did go away.

 

I have been running the 84 octane fuel as I always have... only ran higher octane for a few fill ups, and my truck has not backfired since. I have been using some fuel treatment (injector cleaner) bout once a month to keep my truck running like it should. Definitely think there is a difference out there when it comes to gasoline stations who add or don't add the recommended amounts of cleaning agents to their fuel after doing some checking. Thus far I've gotten good results.

 

84 is too low unless you are at a high altitude.