2012 ISX 12 CM2250 - rough under throttle

[Alan50gt]

Greetings,

First time poster/new member. Long time lurker/reader.

Apologies in advance, this may get long winded.

My issues pertains to a 2012 ISX 12L CM2250 in a Peterbilt 579 790K miles (U.S. based, ALL AFTERTREATMENT INTACT AS MANDATED AND INSTALLED BY MANUFACTURER)

I have 50+ hrs into it so far with no definitive cause. Lots of data/testing. If I forget to list something that may help, I probably have the info, just maybe not immediately.

I was tasked with diagnosing a rough running issue with this truck. It happens ONLY when the truck has not been running for 30+ minutes. NO FAULTS, CEL, OR MIL.

Under throttle, via peddle or Cruise C. idle up, the engine will stumble, choke, shake, seemingly miss, and is believed to be heavily overfueling. (Turbo is spooling CONSTANTLY during this time)

From the beginning:

- I started with fuel. Boss thought it was injector(s). HP fuel testing revealed leaking FPRV off the jump. Replaced. Once that was addressed, it will build and hold pressure the best Ive seen across any of our fleet. (30K PSI for well over 10 minutes) Moving on from there pump leak, injector leak all checked well under acceptable values. Injector test passes. Cylinder balance shows no obvious deviation.

- From there I did several hours of monitoring and logging values. Noticed variance in the Commanded vs Actual Turbo position and surging turbo speeds. Got it to trigger a VGT actuator mechanical malfunction.

- On to the turbo. Pulled VGT actuator. Sector gear was binding, harsh scraping/noise from turbo. Actuator very noisy, loose bearing on the gear. Both were original. REPLACED BOTH. No fix to the issue, but turbo values now appropriate.

- Aftertreatment checked. DOC and DPF cleaned/baked last year after an EGR cooler failure. Checked again to be sure. Regens check out. Analyzer values all within spec. Differential values low. Builds to appropriate temps, runs about an hour. Filter restriction passes. Discussed my logs/values with our local Cummins rep, who didnt see any issues.

- checked EGR system. Pressure reference tubes/ports clear. Some soot on walls but no excessive buildup blockage. Differential values check out. Pressure values in spec in comparison with ambient pressures. IMAP sensor replaced.

BEHAVIOR: Truck will run fine post regen. (When its nice and warm) Let it cool down for an hour or 2, problem returns. In the time window this occurs, it wants to constantly go into SCR Warmup mode. Discovered this when monitoring to figure out why the turbo wanted to constantly spool up. If I slowly up the rpms I can sometimes get it to go into PTO mode (via cruise) or normal operation (via peddle) without stumbling, at which point it will run fine. Once I come back down in rpms, within about 5 seconds, SCR warmup will start again and turbo will spool back up. ** This will happen within 1 minute of running but also after 20-25 minutes of running with coolant temps at 180-200/SCR temps 400+. Some of this may be normal operation, but it seems particularly extended and repetitive. WHEN IT STUMBLES/RUNS ROUGH, A STEADY OUTPUT OF GRAYISH SMOKE OUT OF THE STACK. FILLS OUR 2 BAY GARAGE WITH A NEARLY UNBEARABLE/UNBREATHABLE EXHAUST CONCENTRATION. (which I interpret as overfueling or incomplete combustion)

This is the only ISX 12 CM2250 we have left in the fleet. The last one locked up a main bearing during an injector test about 2 years ago, so I have nothing to compare it to. Ive spent most of my time/experience on newer cummins and MX based engines.

The magic plug: Disabling the EGR system via EGR temp or EGR valve makes the problem go away 100%. Obviously disabling this changes/disables other systems, so it hasnt helped pinpoint the particular issue.

Thanks in advance for any thought and/or input. I will try to provide any additional information as needed. Probably forgot some things up front.