Truck is a 1998 F150 4.6 2wd. 160,000 or so miles. New iridium plugs and coil packs less than 6 months ago. These are the 4 outlet coil packs, one on each side, NOT one per cylinder.

The problem a non rhythmic miss, that is not throwing any codes?? It idles smoothly, but every 2-8 seconds there is a single BUMP in the idle. No set pattern. It's pretty noticable, so not just a slight difference of rpm like all engines do at idle. It's also noticable in the acceleration sometimes, but not always. It should throw some kind of code if it's misfiring, shouldn't it?? I get a pretty good discount on parts working at a parts store, so I was about to start throwing parts at it to see if it can be resolved. The problem is what to try first? I do a Seafoam treatment every couple of months, and drove it on a trip 5 hours both ways a few weeks ago without a single issue. Anyone have a clue what this is?

So far I was thinking wires, fuel filter, and maybe an IAC? They wires have only been replaced once, so they're due I guess. Third set of plugs, and less than 10k on the plugs and coils.

 

what brand plugs? and coil packs? what brand wires? did you put dielectric grease inside the wire boots? you could spray the wires down with soapy water(be careful not to get it down into the spark plug bores) and if you see any arcing then you'll need to replace them, or you could look at them at night and sometimes you'll see them arcing. sometimes they will throw a misfire code and sometimes they wont. sometimes you have to run the p*** out of it to throw a code. when working with the ignition system use motorcraft parts only.

 

Denso Iridiums do work well in the Tritons, but you need to use the correct heat range for your level of tune.

 

If you had an IAC problem then you would get a CEL. Your RPM's would be way outta wack too. It would not idle at all, and if it did i would either be way too low or way too high.

 

I have a 2003 XLT. JUst had the same problem. Rough idle under load and problem was intermittent. Figured coil pack issue but received no code. I did seafoam treatment as well. I replaced the fuel filter and the problem has gone away. If there is no trouble code I would suggest starting with the least costly fix and work your way up. Replacing the fuel filter may benefit you. Good luck.

 

Forget the fuel filter, replaced it and truck ran fine for a day then started actin up again.

 

You may have a ton of crap in your tank and the new filter is already clogged.

 

changed out the coils and plugs. Thanks for the info, got a set of 8 coils for 64.00. Work fine..