Help with Diagnosis?

I have an '01 Chev pickup with Duramax and Allison Automatic transmission with 145,000 miles on it. In a long pull up a grade, the "Service Engine Soon" light comes on and the engine will go into "limp mode" preventing me from exceeding 1800 rpm. All the gauges read normal before, during, and after, and engine performance is normal (right up until it goes into the limp routine). Once this occurs, I'll pull over, shut the engine off, wait for a minute or so, re-start and it resumes operation as normal (until I hit the next long pull). The first time it occurred, went to my mechanic the day after it had happened and there were no "active" codes, but it had "P0093A" and "U1000" stored. I was unable to re-create the problem with a hard run up a 7 mile long grade, but couldn't make it happen. I had approx 5000 miles on a new fuel filter when it occured, mechanic couldn't find anything suspect and said injector operation appeared perfect via his scan tool, so we changed the fuel filter as an easy precaution and decided to continue on and see what happens. Ran it again this past weekend with loaded horse trailer in tow, and same routine occurred 4 times in a 200 mile round trip. Each time, engine would go into limp mode approx midway up the grade. In each case, I wasn't working the engine overly hard, as I could accelerate during the climb at any time. I played around with it a bit on the way back from the trip, and could NOT get it to occur w/o going up a grade. I'd try pulling hard from a dead stop all the way thru the gears to see if I could trigger the event with short bursts of peak torque or peak Hp operation, but it runs like a champ. Only seems to do it 2 or 3 minutes into a long, steady pull. Will get it back into my mechanic this week for another look, but thought I'd check here for ideas. Have noticed no change in performance in past 6 months. Cold starts, hot starts, and fuel economy are great, and no smoke under any conditions. Thoughts?

Hey, I, too, have an 01. Although I now have a new engine in mine the old engine did the exact same as yours (only when pulling a trailer). I had started the famous surge at idel about a year earlier. I changed the Fuel Pressure Regulator when this happened (easy to do in driveway) and the limp problem went away. Of course a couple of years after that the injectors went, then the engine, but those are different stories in them selves. :) Hope this helps and good luck!!

Maverick2,
You didn't say how heavy your horse trailer was. So, I will ask the obvious question, how heavy is the loaded trailer? If you are extremely heavy the truck will go into limp mode to save the transmission. Larger tires will also affect your gear ratio and could contribute to a limp mode under heavy loads.
Good Luck
John

Trailer is running between 6000 and 8000# loaded (depends on number of horses) and tires are 265x75-R16.

P0093 - Fuel System Large Leak Detected (2001 model year). Always change your truck's fuel filter when this or any rail pressure codes appear.

There's a specific fuel-rail pressure test that examines the pressure at 2000-RPM and again at 3000-RPM. The difference between actual & desired fuel-rail pressure must not exceed 20-MPa, as measured with a Tech II scan tool.

If the difference between the actual & desired rail-pressure exceeds 20-MPa, the GM service test procedure winds its way through the various possibilities for a low rail pressure, such as a high fuel return rate(s) due to faulty injectors, a leaking fuel-rail pressure relief valve and concludes with either an FRP (Fuel Rail Pressure) regulator or high-pressure injection pump replacement. The test set used to measure injector fuel return rates utilizes a series of graduated cylinders that allow for a precise measurement of fuel return flow for each bank of injectors.

Jim

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P0093 - Fuel System Large Leak Detected (2001 model year). Always change your truck's fuel filter when this or any rail pressure codes appear.

There's a specific fuel-rail pressure test that examines the pressure at 2000-RPM and again at 3000-RPM. The difference between actual & desired fuel-rail pressure must not exceed 20-MPa, as measured with a Tech II scan tool.

If the difference between the actual & desired rail-pressure exceeds 20-MPa, the GM service test procedure winds its way through the various possibilities for a low rail pressure, such as a high fuel return rate(s) due to faulty injectors, a leaking fuel-rail pressure relief valve and concludes with either an FRP (Fuel Rail Pressure) regulator or high-pressure injection pump replacement. The test set used to measure injector fuel return rates utilizes a series of graduated cylinders that allow for a precise measurement of fuel return flow for each bank of injectors.

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Based on the symptoms noted in the first post (specifically when the error is triggered and when it is not) any educated guesses on what the likely culprit is?

Classic 2001 Phantom limp mode. Some set the code and some do not. With a fresh fuel filter it could be worn fuel system components, OR the overly sensitive OE programming just being too particular.

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Classic 2001 Phantom limp mode.

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Assuming everything else checks out when I take it in for a look, how does one properly eradicate the "Phantom"?

Let's start with the air box. Does it still duct into the fender?

What if any other mods have been done?

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Let's start with the air box. Does it still duct into the fender?

What if any other mods have been done?

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Pickup is 100% stock, including air box, chip, ...

The air box should be updated (simply rip out the snorkels and open the side wide open as should the programming.

This subject has come up several times here but I haven't seen anybody get it resolved here yet. My guess is that most trade off the truck and let it be someone elses problem. I'm 99% confident that I can, provided nothing is worn out in the fuel system. I've done this several times already for customers, just none here that I can recall.

Probably the simplest way to do this is go to my web site and drop me an email. I have a good supply of core ECM's here and will extend an offer of a free trial provided you have the ability to test and reproduce conditions that cause this limp with both the original and tuned ECM's.

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This subject has come up several times here but I haven't seen anybody get it resolved here yet. My guess is that most trade off the truck and let it be someone elses problem. I'm 99% confident that I can, provided nothing is worn out in the fuel system. I've done this several times already for customers, just none here that I can recall.

Probably the simplest way to do this is go to my web site and drop me an email. I have a good supply of core ECM's here and will extend an offer of a free trial provided you have the ability to test and reproduce conditions that cause this limp with both the original and tuned ECM's.

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Thanks for the feedback and offer. Will check out the fuel system for mechanical problems, and if all looks good there, will be in contact about the ECM. I'm pretty sure I can get it to go into "limp mode" intentionally now, and will probably do so again for my mechanic.

Be sure to get an idea how much the fuel system diagnosis is before giving approval. This can be several hundred dollars depending on the dealer/tech, his proficiency, and his conscience.