Forum Discussion

maxwell11's avatar
maxwell11
Explorer
Aug 02, 2015

diesel truck, limp mode question

do our diesel trucks have a limp mode? I have a 2005 dodge diesel

we were rolling along fine the other day around a bypass in Louisville, KY.
I notice a 18 wheeler with its flasher lights blinking way in front of me. I start slowing down, but could not pass as there were vehicles in both lanes coming up behind us.

when I got up to the truck, vehicles gathering up behind us at high rates of speed and having to emergency slow,

I was just sure it was going to be pile up time, before this incident was over.

but, as the vehicles passed and I was able to get around the 18 wheeler (he was trying to get it off the road) the truck was going very slow, say 5 mph, but it was still moving and pulling, as if he was in limp mode.

so my question: if we have a problem with our diesel trucks, will they still pull in slow limp mode or do they just stop running?

if the traffic had not been so heavy and traveling so fast, there would not have been a problem,

I thought we had bought the farm in that one, but we made it.
Tow Vehicles
  • 6_6_Oilburner's avatar
    6_6_Oilburner
    Explorer
    Aug 03, 2015
    The Duramax's derated as a fail safe for a variety of reason starting with their release in 2001. I don't have alot of hands on experience diagnosing the common rail Cummins trucks, but I would imagine the 3rd gens had provisions to derate for different reasons.
  • RooDude09's avatar
    RooDude09
    Explorer
    Aug 03, 2015
    Don't think they did it that far back. I know fords pretty well and the first limp mode they did was '08 that will go into reduced power. The newer 6.7 have multiple modes of derate depending on what's wrong.
  • 45Ricochet's avatar
    45Ricochet
    Explorer
    Aug 03, 2015
    cmeade wrote:
    Read about new trucks with diesel emission fluid DEF will go into limp mode if reservoir gets extremely low.

    X2
    I have never heard of a pre 2006 engine going to "limp mode". They usually just leave you with little warning they are about to die on the side of the road. I'm sure the rig ran the DEF out and went to limp mode. I always had a extra fuel filter for our 2006 Ram, but never needed it while on the road.
  • maxwell11's avatar
    maxwell11
    Explorer
    Aug 02, 2015
    thanks guys,

    I will just continue to keep on top of my fuel filter change schedule and buy my fuel at the large truck centers along the road if possible,

    last thing we need is to be dead on the side of the road in no-where's-ville.

    man o man was it scary watching all those vehicles start piling up behind you and you have no where to go,

    am sure they all thought it was just another old fXXt, that did not know how to drive his truck or where he was going, that was holding them up from going 75+ mph.
  • boogie_4wheel's avatar
    boogie_4wheel
    Explorer
    Aug 02, 2015
    The are many things that can cause the '05 to go to limp mode.

    Engine:
    Overboost: If I remember correctly it will limit rail pressure to 20kpsi, and something else.
    If an injector harness shorts to ground, the computer will drop those 3 cylinders (the front 3 or the rear 3, whichever half of the engine had the problem).
    There are many more.

    Transmission:
    Over temperature, 265* or something, the torque converter will lock and default to 2nd gear.
    Loss of overdrive relay will get you stuck in 3rd, but you can swap out the heater mirror relay to prove the relay itself is at fault.
    Loss of speed sensor will default you to 2nd, since the truck doesn't know how fast it is going.


    A lot of the time, a key cycle will get you rolling again if you are in the middle of nowhere. But checking code(s) are ideal.
  • john_bet's avatar
    john_bet
    Explorer II
    Aug 02, 2015
    Modern electronic controlled vehicles can have any number of things that will limit their power real quick. Even semi's. Call it limp mode or just will not run properly.
  • ACZL's avatar
    ACZL
    Explorer
    Aug 02, 2015
    Semis will go into a "limp" mode and limit power thus speed if it detects a DEF related problem. Last winter, my F350 went into "limp" mode when it detected "low fuel pressure" ( basically gelled fuel) and it limited power and speed. So for the OP, doubt you have a "limP' mode on the '05. Anything from say '10-'11 and up----yes.

    BTW, our VW Passat diesel will also go into "limp" mode if it detects/runs out of DEF issue.
  • cmeade's avatar
    cmeade
    Explorer
    Aug 02, 2015
    Read about new trucks with diesel emission fluid DEF will go into limp mode if reservoir gets extremely low.
  • romore's avatar
    romore
    Explorer II
    Aug 02, 2015
    He wasn't in limp mode, something else was seriously wrong. In our vehicles the transmission should default to 2nd limiting speed to 40 mph. I had that happen in a late 90's Chev.

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