Engine/Transmission (1994 - 1998) Idle speed drops after long highway run?

[FordCummins]

96 12V, specs in the sig. After a long run on the highway (2+ hrs) the idle will decrease by several hundred RPM. Idle is set around 800 w/o the AC on, a little low I know, but after a long highway run it will drop to 500RPM or so. With the AC on, it will stall the truck at idle. I am going to raise the cold idle to 850-900 with the AC on and see what happens then, but I was wondering if a 3-400 RPM drop in idle is normal with a warm motor? When I say warm I mean normal operating temp, the engine temp never gets over 180F, even on the hottest days, with the AC on, towing a load. Thanks for the help.

 

[Missouri Mule]

Ilde for manual trans = 780 rpm with trans in neutral and ac on. I think mine is higher than that.

Problem could be lift pump or overflow valve.

 

[DieselDoghouse]

My bet would be overflow valve which when heated up, dumps at a lower pressure - just a guess IMHO.

 

[Vaughn MacKenzie]

My '98 does the same thing, only happens in summer. Stalls easy too. I doubt it's an OF valve thing but I don't have any other ideas.

 

[champspa]

just a suggestion but check to make sure the idle screw is actually touching the pump arm . After adjusting the gov. springs the idle started doing what yours is doing and I found that the lever wasn't touching the adjustment .

 

[FordCummins]

I have noticed my fuel pressure dropping a little bit lately, although it was way high before, (40psi damn near all the time, unless it was under a WOT run) Now its down around 30psi cruising, and almost to 0 at WOT. I will check the idle screw, and maybe set the idle a bit high (850ish) to see what happens. Thanks for the help so far, I will post an update after I check the idle screw.