I am getting ready to tackle the plug change on my 99 5.4 4x4. It doesn't idle like it should. The start up and die went away after I got it back from the body shop and the body guy said it never acted up on him?

At idle to just touching the gas letting it pull away it seems a little rough to me by feel. Also if I rev the engine from idle in park to about 2500 it revs fine but when you let off of it the engine seems to sputter then idle back down. So I think I will change the plugs and see how this helps. I have some bosch plat 4's I bought a while back because I had such good luck with them in a Jeep and my Mustang I race. I hear such bad things about them in the 5.4 I think I will just let them set, it has been too long to return them.

So what plug should I buy to replace the stock plugs with?

Should I expect any difference after the change, or should I change the IAC at the same time?

I have owned the truck for almost 40k miles now and have always gotten the same gas milage around 11-12 in town and 13-14 on the highway. It has 265/70/16's on it and came factory 255's so I don't know how far that throughs my calculators numbers off when dividing out my gallons and my trip odometer. I am hoping this plug change brings me up to 13 in town and 15-16 on the highway.

Thanks for any help,

Rod

 

Rod I dont think you can beat Motorcraft plugs like the factory installs.
I would also check for vacuum leaks and try cleaning the IAC.
You'll find lots of info on doing both of those if you do a search here.