2012 ISX 12 CM2250 - rough under throttle

[Alan50gt]

(03-31-2022 )tree98 Wrote:  So when the rail pressure was at 29k did you shut the engine down and see how fast it would leak off? What you want is less than a 4000 psi drop in 1 minute.
If it falls off quicker than that, you start capping injectors and running the test again. If the leakdown rate improves, you know the injector you capped off is bad.
Maybe you have an injector or 2 hanging open? You say they were replaced 200k ago, do you know if they were replaced with factory cummins or some ebay junk?
Is the engine making oil?

I forgot to mention, you need to open a separate data monitor window in insite to watch the rail pressure.

HP fuel testing was completed with the truck up to temp. Once the FPRV was replaced she holds well. 30k for quite a long time (10 min range) or until restart. They are genuine Cummins RX/PX reman injectors. THEY ARE AT AN INJECTOR SHOP AS WE SPEAK. So far there is a delivery issue at some specific period with 1 of them (some tech talk I dont understand, and I didnt speak to them directly), other 5 checked out. Were going to get that one new, and theyll run a baseline against the other 5.

I had capped injectors individually and in pairs when cold, No improvement in running condition. Did not repeat leak back testing when cold because it will not run well enough. Original leak back for all injectors was well below threshold @1 oz total across all 6 (4 oz allowed)

Not convinced the single injector is the issue but we'll see

[Alan50gt]

(04-01-2022 )Mikanic Wrote:  Grayish smoke.. makes me think of exhaust valves staying open maybe... engine brake actuator bypassing excessive oil pressure in rocker shaft when deactivated? Loose or stuck exhaust brake plunger when cold. I dont know if cummins have a procedure to monitor oil pressure in rockershaft, I know volvo does. You did a top set but maybe it would be good to take apart the engine brake rockers aswell.

Also if turbo is acting up, monitor exhaust gas pressure, is it stable when turbo is stable? Is it acting up before the turbo? I seen bad exhaust gas pressure connector make the value unstable but still valid and the turbo was acting up weird without setting faults for hours of operation before setting a fault.

Jake is off at the moment. I will go through checking what I can via quickserve. This is a different style than Im familiar with. Ive rebuilt the more condensed version on a newer ISX15, this one is like a '"spider" design. If theres any particular thing I should look for give me a shout.

Thanks

[Mikanic]

I have never dealt much with ISX 12, but I went and got a peak on quickserve and the jakes seemed very different to the bigger engines. Scroll to group 20 in the service manual, there is some info there that could help.

I went back and read your post again. If the smoke as bad as you say I feel like a bad injector would not be your issue, but if it was a cut out test would of probably help you isolate this. I personally feel like if you had a leaky injector and all the timing and compression was equal you would not stumble much or have a rough running engine.

I think you may be in the right direction with unpluging the EGR valve. You said you though it was the turbo at one point because of VGT position. Monitor heavily and graph it so you can look at it after your road test, your sensors that control turbo behavior. Like I said before the exhaust gas pressure sensor I feel like is often overlooked, check your ports and tubing on it. EGP should behave similar to MAP, but always higher. If your EGP sensors reading is off and there is excess backpressure, you could still have incomplete combustion creating lots of smoke and probably bad enough to create a stumble.

Something else I would try is running it straight pipe with DPF unpluged for a bit, until it faults up. Take a good look at the exhaust gas. Is it smoking at start up? Does the smoke get worst at higher rpm? Unplug EGR temp sensor, then is it still smoking? Does the smoke eventually clear up? Do your cut out test and see if it changes more on some cylinders?

[Rawze]

Something else that can be done for checking smoke at startup is to slide the exhaust manifold away from the head a bit and crank it up cold.. see if it is only one or 2 cylinders smoking (getting wet), etc. or if it is all of them. Unless it is oil getting into the intake due to turbo slobber, Most of the time it is only one of them due to valve guide seals or leaking injector when cold.

I have had to do that several time in the past with people to find smoking issues like that.

[JimT]

not familiar with the isx12, but I have a 2012 pete with the isx15 and I know that these aftertreatment systems are notorious for bad NOx sensors. Sometimes supplying valid but wrong values, most notably the 2nd sensor at the end of the exhaust, after the SCR can. The other common issue is DEF buildup inside the decomp tube, the so called DEF/SCR volcano. Either issue could cause the system to go into SCR warmup mode. You could also have clogging in the SCR can if soot or other particulate somehow got passed the DPF. If you haven't yet I would check out the NOx sensors, SCR can, decomp tube and DEF doser valve.

The only times I ever had any smoke come out my exhaust pipe with an intact aftertreatment system was:

1) when the DPF/DOC were no good and the injected fuel was not catalyzed properly, coated the DPF, caused enough restriction to trigger a regen and slowly smoldered as the system attempted to active regen (parked regen usually worked properly though). This was grayish with a heavy fuel smell.

2) when my EGR valve died. The spring inside the valve broke. I could push it open with my finger using little effort and it wouldn't spring closed. That resulted in black smoke when under a load/high boost.