I purchased a 99 F-150 4x4 a few weeks ago. It has a 5.4 and about 70k miles. Truck has been running great and I use it to commute to work 100 miles each day. Yesterday it was raining when I left home, and about 5 miles into my trip, the truck started missing and stuttering as I went up hills. When the truck downshifted, it ran smoother, but may have just been because of the increased rpm. I filled up with gas the night before, but had no problems on the 50 mile trip home. The check engine light did not come on, so I assume it was not plug wires. It had no problem accellerating, as long as it downshifted, so I assume it is not a fuel starvation problem. By the time I got to town, it would miss at idle at one stop sign, then idle fine at the next. At work, it quit raining and when I went to take it to a shop to have it looked at during lunch (4 hours later), it ran fine. The trip home was fine and the truck seemed to run fine, again with no rain. Just in case, I poured some fuel system cleaner in the gas before I drove home. The only thing I can think right now is that it probably relates to the moisture/humidity. Any thoughts?

 

Check for moisture in the spark plug holes. These trucks do not use spark plug wires. They have individual coils on the plugs and sometimes the boots leak water into the hole and short the coil.

You can replace the boot for a tight seal, but also try to find where water is getting into the engine bay.

Mike

 

Are you getting a knock type noise?

Try putting some higher octane gas in her.

 

Coilpack or water past the boot

 

I'm having similar problems with my 2001 F150 4.6 V8
it runs great for miles the all of a sudden acts like it dies for a second, rpm drops, but doesn't shut off. getting worse last night it cut off 2 times in my drive way, I park and it idles fine for 15 minutes. help!!!
PS I had Ford Change fuel filter & run diagnostics, NO CODES !!

 

A garden-variety mis-fire.

Probably a COP starting to go bad.

This engine has no plug wires.

Steve

 

I had the EXACT same problem with my mustang. It ended up being that some water was getting throught the filter (K&N relocated to inside the wheel well). This only happened when it was raining as well. Do you by chance have an open filter?

 

My number four spark plug cavity gets water in it and starts missing. I pull it out and dry it/clean it and it's ok then.
Still can't figure out how the water gets in it.