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.