"Surging"/Stalling Idle when Coasting in Neutral (Manual Trans)

[DThomas70]

I am adding the link in from CF. Nothing against CF or the guys helping there, just feel that the knowledge of Mopar Man himself and the guys here is incredible as well as the resources provided by the site here. 

 

https://www.cumminsforum.com/threads/fluctuating-idle.2528169/#post-29295619

 

I have  99, 5 Speed, 4WD, CCLB, Motor was rebuilt at 206k, currently at 215k, New FASS Titanium Signature 165, New RV275's, VP is a different story, will mention below. No tunes. 

 

Motor ran good in Montana and California throughout the winter. Came to California throughout the virus and since started finding problems. Keep in mind the temp differences between these places, seemed to follow the warm weather for the most part, but potentially unrelated to temps. 

 

The primary issue I am having is the motor when coasting in neutral or the clutch pressed in/ off the throttle, will lope between 500-1000 RPM's. Usually it just kinda lops through, but recently has progressed to stalling when coasting. Once I reach 5MPH it will self correct and idle flawlessly.

 

It has started easy on this VP44. Initially with my rebuild I had a rebuild from thoroughbred which magically started to hard start on me three months out of warranty. I am lucky enough to have had a spare and swapped it in and no more hard start. Still have the funky coasting idle. One thing to note is the idle and hard start problem appeared together. 

 

I have 0 codes. At one point I had the low voltage MAP sensor, swapped that for free cause I could. Code is gone, nothing else showing. You can see in the CF forum my scan tool Live Data readout. One error by my scanner software is it doesn't decipher the APPS voltage correctly. I have probed that and confirmed I have the solid 5 volts there like I should. 

 

Cleaned my grounds. Checked continuity of APPS to ECM. Baffled as to where to go next. 

 

My problem seems too much of an "electrical" issue to me, however I am eyeing a new tank of diesel. What are the mechanics behind bad diesel being potentially causing this issue? I have had the same tank of fuel for the past 1000 miles (I have a bed tank plumbed into my main tank- 2500 Mile Range). Thank you all in advance for the help. 

Edited May 23, 2020 by DThomas70

[Mopar1973Man]

Normal. Mine does it all the time. Lopes about 100 to 200 RPM till stopped then it idles nearly exact at 800 RPM. (+/- 2 to 3 RPM)

 

Only suggestion to lighten is check valve lash.

 

I'm running stock SO VP44, and 7 x 0.010 @ 320 Bar. Since I'm in Arizona and its 105 to 108F it lopes pretty good here. Get back to cooler country it stops.

[DThomas70]

That sounds about right. I did see it for the first time in the heat of Nevada. Funny that you mention checking valve lash. About 4ish months ago I was fighting a fueling issue/turbo issue, and purposely set them 1/1000 tighter than spec. Sound like enough to affect the issue? 

[Mopar1973Man]

Double check it might be tight, just set up and check with a feeler gauge again.