I have 68,000 miles on it and have had it at the dealer at least 6 times in the last year for this problem. It runs like a top for a period of time (it varies) then it may run horrible for short periods of time. It is more noticeable pulling under a load through the mountains and hills. The dealer continues to plug it into the computer and it doesn't show anything. I have a loud knocking sound when accelerating heavy. It will last only a few seconds. I told the dealer that I was going to plug a chip into it to try to improve the horsepower problems. So I did and the truck never acknowledged the chip being plugged into it. So I took it off and took it back to the dealer now the dealer is saying I voided the warranty on the truck, because in conversation I told him I put the chip on the truck. Is there anybody that can give some input on this ?

Probably an injector or two, and you need to find another dealer.

I have 68,000 miles on it and have had it at the dealer at least 6 times in the last year for this problem. It runs like a top for a period of time (it varies) then it may run horrible for short periods of time. It is more noticeable pulling under a load through the mountains and hills. The dealer continues to plug it into the computer and it doesn't show anything. I have a loud knocking sound when accelerating heavy. It will last only a few seconds. I told the dealer that I was going to plug a chip into it to try to improve the horsepower problems. So I did and the truck never acknowledged the chip being plugged into it. So I took it off and took it back to the dealer now the dealer is saying I voided the warranty on the truck, because in conversation I told him I put the chip on the truck. Is there anybody that can give some input on this ?

Take it to another dealer and call gm to open a customer service ticket. Do not take any crap from any dealer.

Open a ticket with gm, they will call the dealer.
Call the better business bureau.
Call the manager and president of the dealership.
Raise hell to anyone that will listen.
Call the national highway safety board.
This process worked for me.
:exactly:

Did you ever say you did put it on or just were going to? If you never told him you did tell him you didnt and to prove otherwise. If you told them you did they still have to prove what ever you did caused the problem. Sounds like you have an incompetent tech/dealer and now they are trying to get outa fixing it. You may need to try another dealer or talking to gm.

How long have you had the truck and has this been happening?

Edit: I would probably use the phrase get Censored ed to the dealer but a bit of diplomacy at first is probably the best method ;)

Yes, I put the chip on it. The dealer knew it to. I have owned the truck since 29,000 miles and it started giving me problems around 40,000 miles. I have had it at two local dealers so far.

I will state the obvious for anyone else reading:

DO NOT GO TO THE DEALER WITH A TUNER IN YOUR TRUCK. DO NOT VOLUNTEER INFO ABOUT WHAT TUNES YOU HAVE RUN.

Injectors have nothing to do with the tune, yet the dealer can give you grief if you tell them (or they spot) engine modifications.

I noticed all you said was "chip". Since the Dmaxes do not use chips, was it an uploader or a harness box?

It was a harness box bullydog (dominator)

Had a simmilar problem, one of my intercooler pipes rubbed a hole in itself against one of my power steering components. Seemed normal under daily driving but not the power for pulling. Dealer said there was a tsb and it is covered under the emissions warranty 0$ deductable, good thing its a 300+$ pipe.