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Truck stumble/stall on return to idle while rolling.


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2001 Ram 2500 6 speed. 240K miles. Intake, Exhaust, Pacbrake PXRB, Bosch RV275 injectors 20K miles old, FASS 95 at 14/15 PSI. Re-man vp44 of unknown millage or manufacture. MoparMan high idle switch. No tune or engine other mods.

Within the last 1000 miles I have noticed a new problem with my truck. With the exhaust brake active coming to a stop when I push in the clutch the engine will stall. Like the brake is backing up the engine too much. I don't think its actually the brake causing the problem though, just making it more apparent by stalling the engine. In my trouble shooting process I have found the symptoms depend on when the truck is rolling or not. It doesn't matter if the engine is warmed up or cold. I can be going 10mph or 60 mph and push in the clutch and let off the throttle. This problem is not dependent on clutch position. It is not dependent on if I have my foot on the brakes at all. The truck returns to idle normal if the truck is not moving. It only stumble if its moving.

Basically
 

  • Truck rolling, push in clutch with exhaust brake active = stalled engine, or extreme dip to 3/400 rpm and climb back to idle.
  • Truck rolling, push in clutch with exhaust brake off = engine returns to idle with about 100/200 RPM dip and climb back to normal idle. Noticeable dip.
  • Truck stopped, bring up to 1500/2000 rpm and let off with exhaust brake active = perfect return to idle, 0 to 50rpm dip and climb back to normal
  • Truck stopped, bring up to 1500/2000 rpm and let off with exhaust brake off = perfect return to idle, 0 to 50rpm dip and climb back to normal
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The fact that there is a noticeable dip with the exhaust brake off while rolling makes me think its not exhaust brake related but that there is something else messing with the fueling. 


What if any sensors should I look at that could be messing withe the fueling depending on vss. I don't have any codes or other problems. VSS works fine, no brake light on dash, cruise control/speedometer are fine.

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ECM is original as far as I can tell. Injectors I put in 20K miles ago. New rv275s, that doesn't mean they haven't gone bad though. It seems exactly like the anti stall isn't working correctly. Its just weird to me that it only happens when the truck is physically moving down the road regardless of speed.

 

I don't know what the Cummins ECM is looking at to determine this stuff or if it even takes into account VSS in its control. If it was an LS I could tell you all about the features in tuning that have to do with VSS and can make this happen.

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The ECM on these trucks us a software style governor. They are typically known as PIDs Proportional Integral Derivative controllers. The PIDs have coeffiecents or tuning parameters that are selected for smooth operation, usually some trial and error is involved. Tuning is often different based on engine and tranny types, they have to idle smooth and not buck or surge and compensate for all kinds of things. Problem is the PID can loose control so they are wrapped with more code that establishes fuel limits and response and even more code to try to make a malfunctioning or severely worn motor keep running. The motor keeps a small amount of fuel going while coasting downhill to prevent bucking or starving the fuel distributor of lube rather then going to zero (automatics may go to zero I'm not sure). Then when stopped at a stop sign it has to transition to smooth idle mode. There are default fuel values built in to the code that try to keep things happy but tuners and worn injectors plus other mods like exhaust brakes cause havoc for the PID and can cause rough running and stalling. It needs VSS to determine if your coasting or stopped to select the desired PID tuning parameters. In an Ideal world the PID could compensate for anything reasonable but that is often not the case. 

 

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