Strangest driveabilty concern ever !!

[cbriggs]

I had an 04 6.0 in today for the weirdest shit ever. Customers concern was: after driving for 1/2 hour or so truck coughs, sputters, smokes badly, wont rev over 2500 rpm and backfires. So i drive truck around for an hour or so, never misses a beat. Has several egr codes, but owner says he has egr blocked off so ignore egr codes.( owner is a hd mechanic at the commercial transport shop in town ) Only thing i noted on test drive was a mildly sticking vgt, and that it will easily bark out the intake when let off the throttle on hard accel (a bit worse than normal). So i called the guy,( i kind of know him, hes a pretty good c-t mechanic) to see if he was interpreting turbo bark as a "backfire". he says no , it backfires out the exhaust, definatly, if it did it , i would know. After phone call i thought i would try it once more, well, it did it. Even in neutral outside, floorboard the accellerator , rpm maxes out at 2500, bounces up and down from 2000 to 2500 like a gas engine on the rev limiter, smoke badly ( i thought fire dept may show up),bark out he intake (turbo bark) and backfire, yes , backfire out the exhaust, shoots smoke and suit 20 feet out the taipipe.( every 3 to 5 second if held on the floor).Smoke is very heavy white unburnt fuel smoke. I made several recordings of event, tryed taking command of everything i could think of to stabilize, as all sensors and outputs go crazy when its happening. I checked everthing, fuel quality and pressure good,no exhaust gas in fuel rails, customer has removed and cleaned tank, replaced pump and filters. No codes except egr codes p0401,one i forgot & p0488. I tried a known good test ficm. Nothing i did made any diffence. Then as i looked at codes again i thought, hmmmm, p0488 is egr tp, not valve related. I monitored pids for egr tp, and there it was. Egr tp was flatlined at .3v, command would creepup to 70% at around 2500 rpm, trying to create egr flow,which was not possible as cooler is blocked, egr tp was not increasing so pcm kept closing it and choking the engine. I removed the throttle plate from the shaft and tried it again, presto, concern resolved. Lesson i learned, never overlook and trouble code. I never thought a diesel would backfire, like that, and from such a stupid little thing causing it. Customer was extatic, as he nearly ordered a $1600 pcm trying to fix it himself.