2011 Peterbilt roll off SCR

Discussion in 'Heavy Duty Diesel Truck Mechanics Forum' started by springerj, Oct 13, 2021.

  1. springerj

    springerj Bobtail Member

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    May 13, 2014
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    I’ve got a 2011 Peterbilt Paccar with low SCR efficiency. Truck was requesting parked regen multiple times a day. Replaced DPF, DOC, BPV, DEF doser and EGR valve. Had SCR Cleaned. Replaced DEF fluid and filter as well as all temperature sensors for after treatment system.

    During regen my temperatures are….
    Before DOC 740
    Before DPF 1050
    After DPF 1030
    Before SCR 890
    After SCR 850

    DEF pressure 130
    DEF dosing rate 18
    Nox before cat 289
    Nox after cat 193
    SCR efficiency 30%

    Any ideas?
     
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  3. springerj

    springerj Bobtail Member

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    May 13, 2014
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    Only fault code the truck has is P3977 SCR catalyst - Efficiency degradation.
     
  4. Goodysnap

    Goodysnap Road Train Member

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    Replace the outlet NOX sensor

    Datalog or monitor and road test operating steady state on the highway with cruise set.

    Looking for SCR eff. to reach high 90's to 100%. Outlet Nox ppm should be single digits to near 0

    The outlet NOX is your key player on the DEF dosing rate and SCR eff numbers. It is most likely Biased high.

    Frequent regens.......

    I would like to see you regen History. Regens should be 25 hrs. If sooner suspect Biased or stuck in range DPF pressure sensor. Check freeze frame data for all stored codes, DPF pressure may show 900 plus PSI. Dead giveaway bad sensor. Check pressure tubes for blockage and proper orientation.

    Consider Engine managment cleaning as well, Boost sensor plugged?
     
    Last edited: Oct 13, 2021
    springerj and AModelCat Thank this.
  5. springerj

    springerj Bobtail Member

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    May 13, 2014
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    Nox sensors were replaced less than a year ago. DPF pressure is almost always at 0 unless it’s regening since the filters were replaced. DPF was cracked and DOC was separating from its casing.

    Is there anyway to test these Nox sensors?
     
  6. Goodysnap

    Goodysnap Road Train Member

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    Remove the sensor from the pipe start a regen to increse exhaust temps and monitor the sensor once it comes online. Out of the pipe it should also read near 0 ppm.

    Have you flow checked the DEF doser and measured its flow/volume rate to be in range? Decomp pipe is clean and diffuser grid in place?

    If the Nox sensors and DEF dosing is accurate. The only thing left is SCR itself. Dont replace many. Inspect SCR inlet for blocked cells or face plugging.
     
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  7. springerj

    springerj Bobtail Member

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    May 13, 2014
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    Have not done the flow test on the doser yet. I’ll check the Nox sensor and doser tomorrow when truck comes in off of route.

    Appreciate the help.
     
    Goodysnap Thanks this.
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