I did the high idle mod on my LLY a while back and recently it has begun to surge every once in a when the high idle is on. I don't remember it doing this in the past. It fluctuates a few hundred rpms at most. Why is it doing this?

I noticed the surging on my LB7 awhile ago with the high idle on.

Was your A/C on?

Yeah. And the last time I really used it was in the winter, which of course I would not have had the A/C on....and it was not surging then.

could be the high idle mod messing with the electronic gains of the engine. The way the software is setup you could be requiring just enough fuel that your in a transition zone on the maps, and your bouncing between two points.

Well when I noticed it the A/C was on, truck was sitting in the parking lot. I think its only done it once. But lately the A/C is on, and no surging. :confuzeld

hard to say, when the A/C is on in any vehicle it does weird things with the ECM.