I just changed my fuel filter and Allison filter, and when I started the truck, the normal idle was fine. I put it up to the high high idle (1800 or so), and the idle would drop about 100rpms on what seemed to be a steady pattern. The Low High idle (1200, I think) was normal and smooth. Any ideas why the 1800 RPM idle was fluctuating like that? I can't recall it doing it in the past.

did you have the hvac system on?

did you have the hvac system on?At first, yes, but I shut it off thinking it was the A/C cycling, but it didn't make a difference.

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Any ideas?