2001 LB7 fuel system/limp mode problem SOLVED

[Kennedy]

Since doing Mark's I have done several others with great success in all but one. This particular truck is a repo and is new to the owner. It has other issues that need to be checked out. Thankfully it's nearby in Southern WI.

I've also fielded numerous calls and spent a lot of time explaining the situation. My hope is to get some further documentation worked up so I can refer to testimonials, etc and spend less time explaining the same thing over and over again. I hope to work up some details soon.

[T Hugo 22]

I had a similar problem with my '01 Duramax. It started going into limp mode pulling my 32' 5th wheel. That started Memorial weekend on my way back from an up north camping trip. I took it to a dealer and they diagnosed a P0093 code. After replacing the CP3 pump and fprv, it was still going into limp mode. It got to the point that it would go into limp mode just pulling up a slight hill, empty, not pulling a trailer. We checked the pickup screen in the fuel tank, and replaced a couple of fuel lines. It would still go into limp mode. My mechanic was stumped. I came across this thread and read the post by Kennedy. I e-mailed Kennedy. He suggested his ECM tune. He's only about an hour and a half from me so I drove up to have it re-tuned. On the way up, on a flat road, I pull out to pass a semi. I get right up beside him doing about 65, and it trips into limp mode. There I am, can't get enough speed to get by him and have to slow down and tuck back in behind him. I got the re-tune on 8/12. I could tell an immediate difference. More snap, very smooth. I've pulled my ATV all over the hills area south of Tomah, I've hauled a load of ties, and I've pulled my 5ver. It's running awesome, and it's pulling awesome. After I get a few more trips on it, I'll update with the results. But right now it's running better than it ever has. I'm glad I got my truck back. I couldn't stand not being able to get out and go, when I wanted to. Thanks Kennedy.

[More Power]

Thanks for posting your problem and apparent cure!

For those not well versed in what the diagnostic trouble code P0093 is, the book says "Fuel System Large Leak Detected (2001 model year)". The description doesn't tell the whole story or indicate what else may be causing the problem.

In short, this code indicates that the "commanded" and "actual" fuel rail pressures are outside a window of acceptability (the computer really has no way of knowing whether a leak exists). The ECM commands a particular rail pressure, depending on load, accelerator pedal position and other inputs, then commands the Fuel Pressure Regulator (FPR) and high-pressure pump to deliver that rail pressure. Ordinarily, the system will respond and generate those pressures.

There are a variety of causes of a P0093 trouble code, it could be that the injectors are worn and their internal clearances are allowing too much fuel to leak past the internal components of the injector(s). The FPR and high-pressure pump can't generate enough pressure/flow to compensate.

This code could also be the result of a faulty rail pressure relief valve, which leaks fuel pressure back into the fuel return line faster than it can be generated. The fuel pressure sensor could be at fault, as could the FPR or the high-pressure pump itself.

There is an injector fuel return test outlined in the service manual - using a set of four graduated cylinders, which are used to measure injector fuel return flow for one bank of injectors at a time. I doubt many dealer techs do this, but the test is available for those who want to confirm whether the injectors are the problem.

The window of acceptable pressures could also be so narrow that some trucks have a hard time staying within that narrow window of acceptable rail pressures. The 2001 trucks were the first to get a high-pressure fuel injection system like this. Variability in trucks, engines, fuel systems produce variability in setting codes.

We know that within the first year of production, GM widened the vehicle speed window when using cruise control because of driver complaints. I suspect there may be other areas of vehicle programming that needed some fine tuning as well. While we haven't heard the recommendation lately, during the first few years of Duramax production, GM recommended updating the ECM programming whenever the vehicle was returned to the dealer for service. I've had my 2001 ECM programming updated a couple of times. I wonder if maverick2's truck was recently updated by the dealership for this trouble code?

Always change your truck's fuel filter when a P0093 or any other rail pressure codes appear. Check your fuel tank cap for excessive vacuum. A faulty pressure/vacuum relief valve in the cap could add to the possibility of this trouble code. These diesel caps should vent at 2-psi positive pressure or 1" of vacuum.

Jim

[Kennedy]

bump for reference

[jde0330]

Anyone know where to buy a fuel pressure relief valve? I've been searching the web and can't find one. I'd rather not shim the old one unless I have to.

[More Power]

Anyone know where to buy a fuel pressure relief valve? I've been searching the web and can't find one. I'd rather not shim the old one unless I have to.

Have you checked the relief valve for leakage? Kennedy has another recent post here that outlines how to perform the simple test for leakage.

Jim

[Kennedy]

The LB7's do not typically leak or weaken. It's the LLY-LBZ-LMM that can be weakened IF they are beat up on by pressure cycling.

It's P0093 season and I am doing many 2001 LB7 ECM's with the same fix mentioned at the start of this thread.

Also seeing the same issue starting with LBZ and LMM P0087 codes.