2015 GLK350 21k miles, throttle lag/engine hesitation after 5-second WOT on freeway
 

I have a 2015 GLK350 with about 21k miles on it. After I took it on a long 500-mile trip, and had to give it a 5-second WOT on the freeway at one point, from a standstill, for very quick (and noisy! - engine really revved up) emergency acceleration, now I experience throttle lag/engine hesitation when starting to move from a standstill or when trying to quickly accelerate from a slower speed. Also, the fuel efficiency has decreased some: on the trips where I was previously able to get 29-30-31 mpg (YES!), now it only goes up to 25 mpg; and on those where 26-27 mpg was the norm, now I can only get to about 21-22 mpg.


Having already read some of the forums on here, I've tried to follow the procedure to reset my TCM a couple of times. Each time, the reset does help "wake up" the throttle quite a bit, yet only at first; during the very first trip, it seems to slowly "dumb down" again.


Also, during idle, the engine seems to sound a bit rougher than usual; and during the initial acceleration from standstill, it now sounds a bit like diesel (this is a gasoline engine taking 91 gas.)


The vehicle is fairly new and is still under warranty, so I really can't imagine something going wrong in the engine or the transmission simply from that 5-second emergency acceleration on the freeway. Could this simply be a TCM or ECM software issue? Thank you for sharing your opinions and advice on the above.