2001 4.0 XLT 4x4 Auto with 85k miles on it.

I've redone plugs and wires and used OEM Motorcraft stuff. It idled many times better with the OEM stuff than the previous owners whatever brand stuff he had on it. Fuel filter has been replaced also. I have a brand new fuel pump assembly with all the fixin's to install soon. The truck idles nearly perfectly but once in a bit it kind of seems to have a little stumble at idle. The truck kind of moves a little. It's very subtle. Otherwise idles smooth at consistent RPM. The tiny stumble doesn't seem to be present when accelerating as it's buttery smooth and powerful when accelerating.

Any ideas before I start randomly replaced IAC and other things.

Regards,

Brian

 

Welcome to the forum


Put a can of Seafoam or similar injector cleaner in the gas tank, I do this once a year on my fuel injected vehicles, does smooth out the idle on all of them.

Dirty injector tips always happen, no way around it.
At idle very little fuel is injected, dirty tips "drip" that fuel instead of spraying it, so you get a poor air mix now and then causing a stumble at idle.
At higher RPMs 100% spray comes back.

Also clean MAF sensor every year or two, easy to do, and does help with smooth running at all RPMs

IAC Valve issues can cause idle RPMs to drift, but your description reads more like slight misfires than a change in idle RPMs, RPM change shouldn't "vibrate" vehicle, misfire would
But cleaning IAC Valve every 5 years or so is good to do

 

I hadn't even thought about that but it makes a ton of sense for a 17 year old vehicle. Thanks a lot for your time and response. I'll clean the MAF and run some injector cleaner through it.

Best Regards,

Brian