I have a 97 F150 with the 4.6. I have been having issues with backfiring and loss of power. I had p0174 and p0171 codes, but am unsure if they are related to my issue. I first cleaned the MAF, then replaced it. I also replaced the fuel filter. I replaced the catalytic converters, and found that one was completely hollow (bank 2 side, I assume it hollowed out due to the backfiring). This cleared up p0171 (bank 1). I found a few vacuum leaks and repaired them. After this there was no improvement. I replaced the EGR valve, plugs, and wires, cleaned the injectors and replaced the o-rings. The no 6 plug was fouled badly (bank 2). This brought back the power and ended the backfiring, but p0174 remained. After 2-3 weeks, the truck is backfiring again but there is no loss of power this time. The backfiring only occurs under acceleration, normally shortly after shifting into a higher gear. I am not sure where to look next, and do not want to just throw parts at it. Any help would be appreciated.

 

You may want to take a close look at the coil packs.

 

I appreciate the advice. I inspected and cleaned the coil packs and had no luck. They seemed pretty clean in the first place. I replaced the one that leads to the number 6 spark plug (I believe this is the cylinder with the issue) and the truck still backfires. I will try to test the resistance on both, but I am pretty sure they are both functioning properly. Can a bad O2 sensor cause the backfire?

 

The coil packs could have carbon tracks, could have cracks, or can be breaking down internally.

Have you done a compression test?

 

I haven't done a compression test, but I will get on that. Do you suspect a stuck valve?