I have a 99 f150 w/ 4.2l. The truck was running great the other day. Went to lunch and it ran fine. Four hours later I start it up and by the time I walk to the truck (remote start) it was idling really funny almost like it had a very bad miss nearly to the point of where it would die out and then it would rev back up. This usually happens if I let it idle for about 5 mintues. Never could figure out the problem. The codes it always has read bank one and two are too lean so im guessings a vaccum leak I never could find though. Anyways, I drive off and give it gas and it sputters some for the next mile or 2 and it was reasonably cold out so I thought ok maybe it needed to warm up and sure enough it stopped doing it.

Now whenever I get on it and hit around 3-3.5k rpm it starts to sputter real bad and jerk around and wont accelerate. Once I let off the gas it is fine. On other occasions I can get on it and itll rev right up to 4.5k-5k and shift. Then when it shifts and goes back down to 3-3.5k range itll start to sputter real bad again like it wants to die out pretty much and stops picking up speed. I thought ok maybe its the transmission being that it only happens in a certian rpm range BUT I can obviously hear the engine spitting and sputtering actling like it wants to die out.

Does anyone have any idea as to what the problem could be? Im thinking wires/plugs or dirty MAF? That doesnt exactly make sense though as the plugs are bosch platnium +2's with only 10k miles on them, the wires are just some cheap xact ones with 15-20k miles on them. The 2 upstream O2 sensors have 10k miles on them as well. The airfilter was cleaned as soon as I got home after this started happening and the last time I cleaned the MAF was about 6k miles ago. The fuel filter has about 15k miles on it too.

 

That pretty much explains it right there.....

Steve

 

Also try cleaning your IAC valve that sits on top the throttle body. It's held down by two 10mm bolts. It problably has carbon build-up in it. Use throttle body cleaner to clean it with. Blast the heck out of it with the TB cleaner, and use a q-tip to get to the hard to reach places. Don't use WD-40 because it takes longer to evaporate, and can draw alot of gunk after you re-install it. And be sure to clean the little tension coil-spring really well. I usually soak it in TB cleaner, and immediatly blast it with about 50-60 psi of compressed air to blow the gunk out of the spring. Hope that helps.

Ditch those Bosch plugs. They are literally known not to work to well with Ford engines. Use OEM Motorcraft or Autolite Double Platinum plugs, and Motorcraft or Autolite wires for the best performance.

I've had Motorcraft "finewire" plugs and Autolite Professional Series wires on my truck for about 3-4k miles now with no problems.

 

Thanks for the help. Tomorrow I plan to do the plugs,wires,and clean the maf & iac. I will let you guys know how it goes.