LB7: Cali LB7 - No Code Limp Mode

Ian@MPE

New member
Jul 14, 2012
3
0
0
Hi,


I am a performance and combustion engineer for Cummins but work on Duramaxes as a side business. My current issue is that I have an 03 LB7 with EGR (Cali version). I live in Indiana so this is the only cali LB7 I have seen. I tuned this truck with EFI Live (I do my own tuning) and it ran fine for 3-4 years.


Recently, it has gone into limp mode (engine speed is limited to 2200 rpm). It stops at 2200 rpm and bounces there as if it is hitting a rev limiter so pretty sure it is in LIMP HOME MODE. However there are NO fault codes from the ECM or TCM to explain the limp mode. And it seems to go into limp mode as soon as the truck is started (ie. can reset codes or even disconnect batteries, then start the truck up and rev it up and it is still locked to 2200 rpm).


Here is what I have done with no success of figuring out why it is in limp mode:
1. Loaded Stock Cal
2. Only code is C0550 (EBCM Performance) - no ABS light so this code doesn't seem to be effecting anything, and certainly shouldn't cause limp mode.
3. The Balance Rates on the injectors are a little out of spec so it may need injectors, HOWEVER the actual and desired rail pressure is spot on with NO deviation which as far as I know is what trips limp mode due to very bad injectors due to the injectors returning too much fuel therefore the CP3 cannot keep up.
4. Commanding the CP3 to 1600 bar at idle checks out OK
5. Swapped mass airflow sensor with a known good one, no change.
6. Tried bumping all 5 LIMP HOME MODE tables out of the way via EFI Live tune, no change. So this limp mode is governed by some Limp Home table that is not set up in EFI Live.


So what is strange about this limp is that it is 'engaged' as soon as the truck is keyed on. It doesn't require driving at all to 'engage' it.


Baro pressure, MAP, Mass Airflow, fuel pressure, engine speed, intake temp, fuel temp, oil pressure, etc all appear to be reading 'good' values via EFI Live.


So the only thing I am wondering is if something related to the LB7 EGR system could cause this? I read something about a 2nd Baro sensor and I cannot read values from this sensor via EFI Live. Again, I am getting no codes but could the LB7 EGR system be capable of tripping limp mode?
It seems like GM did not set up the diagnostics correctly which is why I am wondering if it is something unique to the Cali LB7 (low volume sales).


Any help / thoughts would be greatly appreciated.
 

chevyburnout1

Fixing it till it breaks
Aug 25, 2008
2,368
1
38
Berthoud, CO
Sounds like the PTO switch is engaged. Look under the dash for any aftermarket switch wired in. Or change the PTO based fueling in EFI Live to 100mm3 and see if that changes anything.
 

Ian@MPE

New member
Jul 14, 2012
3
0
0
Well, to close this one out. Turns out there was a hidden Valet switch that nobody knew about. I picked up on it when I was swapping ECMs from another LB7 to test the bad ECM theory. When doing this I saw a couple extra wires going into the ECM harness, traced them through the firewall, and sure enough found a valet switch that must have gotten tripped by the owner erroneously as he did not know about the switch nor did I.


So I am feeling a little dumber and my pride is hurt a little, but mystery solved.