Limp Mode Puzzle

[More Power]

A caller yesterday reported that a customer's truck wasn't setting any trouble codes and was not illuminating the SES, but it was behaving as though in limp. The engine would idle fine and would move the truck slowly. Pressing the accelerator wouldn't do a whole lot to engine speed. A check of the APP using a Snap-On scanner showed the APP was applying 0-100%.

The fuel filter was replaced, with no effect on the limp drive-ability. Plenty of fuel in the tank.

No codes, no SES, and in limp..... Even though there were no codes in memory or history, he cleared them anyway. No effect. I asked that he disconnect the batteries for at least 30 minutes. No word yet if that had any effect.

Any ideas?

Thanks,

Jim

[Mark Rinker]

Year of truck?
Was it recently washed under the hood or driven through deep water or in the rain?I would be suspicious of wet engine harness connector(s), if second question was 'YES' ... otherwise maybe a fried ECM?

[THEFERMANATOR]

Any tuner installed? Is it an 01?

[More Power]

It's an LB7, not sure of the year. Don't know about a tuner/performance products. A limp appearing with no codes or SES doesn't happen very often....

[ToddMeister]

If it has a tuned ECM some of the codes may be disabled or turned off, or limits changed so as not to trip a code...

[THEFERMANATOR]

I know 01 was famous for limp mode with no codes or check engine light, and sometimes you would get a limp with a check engine light and no codes. Tuned trucks had ALOT of problems if it was an 01 operating system in the ECM.

[More Power]

I sent a note (and link to this thread) to the guy in Canada, suggesting he take a look. He hasn't responded yet. But, I do appreciate everyone's thoughts here.

Jim

[Jakemax04]

i had the same problem except i was able to accelerate slowly up to speed but any excessive pedal push would cause limp mode. my problem was the fuel pressure relief valve. the valve was worn out and causing fuel to prematurely return to the tank causing a low fuel rail pressure code. You should check the codes while the accelerator is pressed if it throws a code under a load then returns to normal after you let off i would suspect a defective fprv.

[Mark Rinker]

JAKEMAX04: I have been 'throwing parts' at a similar low fuel rail problem, exaggerated by the 2006 ECM programming that sets a SES code/limp mode once actual and requested FRPs vary by 10% or more for about 3 seconds...

http://www.thedieselpageforums.com/tdpforum/showthread.php?t=37459

Replacement of the flexibe sections of supply side fuel lines, replacement of FPR (mounted on the IP), and addition of a lift pump have all failed to fix the problem entirely - although the addition of the lift pump may have helped lessen frequency to some degree.

My rationale to date for not replacing the FPRV has been the various requested/actual pressures at which the code is set...again, just the variance is needed, no correlation to certain threshold or rail pressure has been observed - in fact in 3rd or 4th gear/ 3000 RPM my rail can hold 24K# requested/actual pressures indefinately, while towing 15K#...shift to 5th, letting requested/actual drop to 16 or 17K#...BING there goes the limp.

What is the part cost and GM part#?

I may throw this one at the problem before diving into injectors and/or IP replacement...just to rule it out.

[Kennedy]

i had the same problem except i was able to accelerate slowly up to speed but any excessive pedal push would cause limp mode. my problem was the fuel pressure relief valve. the valve was worn out and causing fuel to prematurely return to the tank causing a low fuel rail pressure code. You should check the codes while the accelerator is pressed if it throws a code under a load then returns to normal after you let off i would suspect a defective fprv.

This is really easy to test for, especially with a Tech 2. Simply ramp the presssure up at an idle and watch to see that actual follows desired. If not then remove teh hose from teh end of the rail and plug it. Attach a new hose to the rail and run to a bottle or watch for a drip.

Since Mark can make psi some of the time it's not this valve.

FYI this relief valve gets beat up due to pressure cycling. Lift pumps generally prevent this from happening.

[Jakemax04]

mark, i got the fprv from hdiesel.com (huckstorfdiesel) the part number he provides is BOS1110010013. It cost 152 bucks which is a lot better than what the dealer wanted, 300 bucks.

[Jakemax04]

oh my first reply was directed towards "More Power" because i didnt get an SES light with my limp either although i did set a low rail pressure code.

[usaf_rigger]

I just bought a PPE race FPRV for $44.00 from xtremediesel.com. Fixed my problem of low fuel rail pressure under high demand..

[THEFERMANATOR]

I just bought a PPE race FPRV for $44.00 from xtremediesel.com. Fixed my problem of low fuel rail pressure under high demand..

LLY is a completely different animal from an LB7.