M998 6.2 roller coaster idle issue

[signsup]

Recently acquired M998, so no history to refer to and forum search does not reveal exactly this isse I am having. Smart Start system works and after engine start engine idles smooth. But, after a frew minutes the idle increases and then settles back down, increases then settle back down. Not racing, but a definate increase in rpm,s and then back to a "normal" idle speed. Idle is smooth, not rough as if some sensor is searching for a sweet spot. I'm thinking of taking a short video and posting a link, but just wondering if any usual suspects for me to check until that happens.

I'll hang up and listen. Off to the tire shop to try to get valve core replace in valve stem without having to dismount tire. Fingers crossed.

 

[Mogman]

Are you only running it at idle? what I mean does it run OK at half or full throttle?
That is called "hunting" and can be an issue with the IP but if you are only running it at idle it could be fuel starvation.
EDIT, or possibly a fuel return restriction.

 

[Mogman]

You might also try unplugging the cold advance wire (the wire at a 45 deg angle on the rear of the IP housing)
The cold advance works by restricting the fuel flow out of the housing (return) thus raising the pressure in the housing causing the timing to advance.

 

[signsup]

I have only been checking this at idle, usually while I am working on something else on the Humvee body. I will try some additional throttle and see what it does and also unplug the suggested lead. I have taken a video of the current situation and will post it on Utube with a link here.
Thanks for the leads.

 

[Mogman]

See if you have a good flow from the fuel filter drain, it does sound like fuel starvation, less likely but also possible a return problem.