Idle creep with AC on

[Mathes308]

Searching for this problem and can't find any results.
Truck runs great, idles about 760-780 rpms in park.
Few months ago I was waiting for the wife to grocery shop and had the baby asleep in the truck so I left it run with the AC on and me in the drivers seat.
Reading away I kept thinking I was feeling the truck shudder a bit, thought it was me.
Started staring at the tach, it takes about 2-3 minutes but the rpms slowly increase about 5 rpms, then drops suddenly back to a steady 780 or wherever it's at, then begins the slow climb again.
Doesn't do this with the AC off, idles rock steady, no problem when in gear.
Tonight noticed while standing behind the truck, idle, AC on, I could hear what sounded like it missing one fire, here it was slowly ramping up about 5 rpms and then quickly dropped. Seems like maybe takes a minute or two.

Any ideas?

Otherwise not a hiccup or skip, super smooth, text book exhaust temps.
Common?
Thanks.

 

[cerberusiam]

The AC pump is dragging the engine down forcing it to ramp fuel to maintain idle speed.

 

[Mathes308]

Shouldn't the ECM be able to stabilize that?
I was afraid it was a side effect of having the smarty jr set to full timing, 70hp, and stock TM.?
If it's just the AC, I can live with it.
I'll give the old girl this, the AC is stout, very cold, very quickly.

 

[cerberusiam]

If it idles between 760 and 780 how can you se 5 rpm's? As the pressure rises with the AC pump on it will start a drag on the engine that it will try to compensate for. If you have a tuner it may over compensate, like CC surge with certain tunes.

 

[Mathes308]

Giving my best guess, meaning it's not a huge climb, the needle creeps up and you can hear the very slight speed increase, then a "brup" as if you were pressing on the throttle and suddenly let off.
But not just one when the AC is first turned on, it will keep doing this cycle the whole time while idling and the AC clutch is engaged.
At first I thought it was the AC clutch cycling and giving the rpm changes but it's not, when the defroster is on and the AC cycles it's a huge noticeable difference, you can hear the clutch slam on and off.

It could just be the paranoid engineer in me and I'm new to the HPCR, genius putting a super delicate system on a reliable workhorse. Like a pacemaker in a Clydesdale.
I'll set the smarty back to stock tomorrow and see if it still does it.

I was afraid it was maybe a tell tale sign of a worn injector starting or other nightmare.
Have the family convinced she's reliable enough to drag the camper across the country, don't want er to let me down

 

[Tuesdak]

Sounds like classic idle surge from a bad FPA. Check the desired and actual rail pressures. Looking for FPA (on the CP3) going bad or high pressure leak. (Injectors, relief leaking, etc.)

 

[Mathes308]

Have to see if my local cummins dealer can check the rail pressure easy.
I'm not aware of any way with what I have. Smarty jr, otherwise stock.
202k on it, wouldn't be a shock.
This thing even has the factory lift pump on the block.
Will be a fass system when the family budget loosens

 

[Bob4x4]

A Cummins dealer will not have the software to see your truck unless they spent the money to buy it from FCA

 

[Mathes308]

Whoops, replace dealer with repair garage.