05 GMC 2500HD Duramax cough

[fishfay]

Hey guys, I am new to this site. I purchased an 05 gmc with 25000 miles. The truck has some kind of performance chip. There is a dial and a switch under the dash. When the switch is off the performance is stock(i think). When the switch is on the accelleration is great except it coughs or stutters just before it switches gears. Along with the cough is a bit of black smoke. When the switch is off the truck will still stutter but only when the pedal is all the way down. And that happens just before 5th gear.
The tire size is 285/65/18.

Any help would be appreciated.

[Bob O]

First, take it to the dealer and have it reflashed. The "cough" is an EGR issue. You can unplug the EGR if you want to try that. Also, there are blocker plates made for the EGR. You will get codes if you don't have any way to fool the ECM into thinking the EGR is working while the plate is installed or with EGR unplugged. Your best bet is to get the latest GM reflash and see how it helps. GM knows of the issue. Tell your dealer what it is doing. They shold know to reflash it. I believe the new reflash # is 15904640.