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Frequent limp mode


My 2002 U320 M-11 450 goes into limp mode after 50+ miles, Insite shows no active or inactive codes thrown, no check engine light ever, turbo pressure slowly goes down to <2 psi =at fulo pedal. Normal coolant and oil temps always.  Engine idles well and accelerates easily in neutral during this limp mode. Believe no loss of CAC pressure. No squeal or noises from turbo and no white or black smoke ever.  Logically the Allison 6 speed transmission hunts with lack of turbo presure.  Oil temp and coolant temps are quite normal through all of this as read on my Road Relay 4.
During these limp modes it still idles well and in neutral it accelerates easily to 2000 rpm with normal pedal response. Believe no loss of CAC pressure. No squeal or noises from turbo and no white or black smoke ever.
After sitting for about an hour, it comes out of this limp mode when something changes and it is back on the road for another 50+ miles when something fails again or maybe tells the ECM to derate again. Guessing heat related, out of range sensor or fuel starvation.

What tests should I do? Any ideas?? Hoping Brett Wolfe and others will respond,

Jim
308-440-5154

Jim Frerichs
2002 U320 42'

Re: Frequent limp mode

Reply #1 –
Quote from: Jim Frerichs –
Guessing heat related, out of range sensor or fuel starvation.
My knee-jerk guess is number 3 on your list.  Did you see the old thread below?

Loss of power

1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Frequent limp mode

Reply #2 –
Low fuel pressure and supply to injection pump.

Lift pump going bad.

Accelerating in neutral OK is a symptom of low fuel pressure/supply from above. Probably filters.

Pierce
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)

Re: Frequent limp mode

Reply #3 –
Even though I'm guessing "fuel starvation", I'm wondering how smart the Cummins engine management ECM is.  Jim says no codes and no check engine light.  If the throttle pedal position sensor "sees" the driver calling for full throttle, yet the fuel pressure/volume being delivered to the injection pump is inadequate to support that demand, would not the ECM recognize this as a "problem" and throw some kind of code?

After doing a quick Google search for diesel engine "limp mode" and "derating" I could not find any instance where there wasn't some kind of warning light or code notifying the operator of a problem.  When the ECM derates the engine, it almost always tells you why it took that action.

Very mysterious.

1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Frequent limp mode

Reply #4 –
No smoke, no codes, no high heat for oil or coolant, Aqua Hot and Generator run very well (not a fuel problem for them), turbo and CAC pipes were OK at a Cummins dealer, not low on coolant, new primary and secondary filters changed just before limp mode problems started.
I will change and cut open the secondary fuel filter out of desperation.
Jim Frerichs
2002 U320 42'

Re: Frequent limp mode

Reply #5 –
If filters were changed just before this started, check carefully that the correct gaskets are in place.

Air leaking in (filter on the vacuum side of the lift pump) can explain your symptoms.

Also, look at the fuel line going into the primary filter for cracks that would allow air into the fuel system.  While there could be cracks other places in the line, that is a common on and very easy to visually verify.
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Frequent limp mode

Reply #6 –
Quote from: Jim Frerichs –
My 2002 U320 M-11 450 goes into limp mode after 50+ miles...
I thought the 2002 U320 came with the ISM-450?  Is that not correct?  Whatever - at least the 2002 model doesn't have EGR, DPF or DEF injection, so that rules out several potential "derate" situations.  :thumbsup:


1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Frequent limp mode

Reply #7 –

Quote from: Jim Frerichs –
new primary and secondary filters changed just before limp mode problems started.

This is what's telling me what your issue is. Fuel related for sure and almost a certainty the fuel filters
John Hobbs
1995 U300 SE
M11 / Retarder
Private toilet
1 of 1
Wichita, Ks

Re: Frequent limp mode

Reply #8 –
You didn't by chance unknowingly leave an old gasket on when your replaced the filters did you?
1987 Grand Villa ORED
2001 U320 4010

Not all that wander are lost... but I often am.

Re: Frequent limp mode

Reply #9 –
Brett,

Very good ideas...thank you.
JIm

2002 320
Jim Frerichs
2002 U320 42'

Re: Frequent limp mode

Reply #10 –
Quote from: Jim Frerichs –
Aqua Hot and Generator run very well (not a fuel problem for them)
I will change and cut open the secondary fuel filter out of desperation.
Generator and aqua hot have there own fuel supply and return. Just shows fuel quality. If filters(both) are good i would suspect air in fuel issues. No codes on scanner? There is no fuel lift pump on a U320. Mine will one day have one but comes without.
Scott

Re: Frequent limp mode

Reply #11 –
Is safe mode the same as limp mode.  I know when the starting battery isn't charged enough there is a safe mode that starts, that protects the transmission; the engine runs and I think it will go in first gear but that is all.  The transmission needs a certain amount of voltage to run.  Is the alternator charging your battery?
Rick & MJ Berry
1998 U320 40'
Ohio

Re: Frequent limp mode

Reply #12 –
Quote from: Jim Frerichs –
, new primary and secondary filters changed just before limp mode problems started.
I still think you have induced a air leak. I had one primary filter on the engine causing issues. I too cut it open but was never able to understand why it keep leaking fuel when shut down and air I'm sure while operating. Water drain feature was source of the leak. Was also not factory filter.