I'm attempting to help my brother fix his 2006 5.9L Cummins; the engine is stock other than exhaust, intake, and an EFI tune.

No idea what tuner, though; it has a complete built ATS Trans and a FASS 150(or 100)

It started surging this summer; while towing a ski boat to the lake, it would surge so bad it was almost impossible to drive.

Mostly would do it while driving at an incline (any gear, any speed)

We did the usual, cleaned all ground and battery terminals, and checked ECM plugs for corrosion and non.

This has always been an AZ truck, so rust and corrosion are not likely.

We then drove it for a get-a hundred miles (empty) with no issues, and the truck ran OK and had full power.

Next weekend hooked up the trailer and 40min into tow, same thing.

This time it threw app sensor codes.

We pulled over and noticed lift pump FASS wasn't running.

Towed the truck and trailer home.

We then replaced the FASS pump and then replaced the apps, cleared codes, did the pedal apps relearn, and all was good.

I hooked up the trailer, and the same darn thing surge was so unbearable we drove back at 30mph for 5 hours.

I thought maybe the torque converter lockup like the 2nd Gens.

We then locked the converter with a manual switch and monitored the converter; in lockup, it would surge, but the converter stayed locked (almost felt like no fuel but would build a significant boost); while surging, it would eventually pop out of lockup.

We are pretty stumped on what to check; the truck has no codes, we checked the app's voltage, and it's good; we checked apps to ECM wire, and it has .02 ohms.

I called the dealer and had a long talk with the service advisor, and he said he would have to flash it to stock to start diag.

This can't be done cause someone stole the EFI live.

He said he might have to replace ECU if they can't flash it.

It only does it pulling a trailer on an incline.

It only has 140k on the truck and has BBI injectors replaced last summer.

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