Poor fuel Mileage after at Dyno Tune at Atlantic Cat.

Hi Guys, I was just wondering if anybody else has had experiance with poor fuel mileage after a trip to the dyno or if its the fact that the dealer maybe screwed the ecm on my truck.
My truck did return 5.75 us gal mpg for the longest time then after a trip to Atlantic Cat in Moncton NB the truck now is awful on fuel in fact has forced me to park it up. With a total new mpg of just 4.2 us mpg.
The dealer originally in November ran it on the dyno looking for non related issues(a little stumble around 1100rpm) but found nothing, he then adjusted the burn rate on all injectors, flashed the ecm with latest software and then told me that if we put in the heavy haul software the truck will be a better drive and better on fuel. The truck after the dyno session is in their defence a better drive as it pulls like a train but at a cost of almost 2.0 mpg and has boost pressure of almost 60psi. I of course told them about it and they promptly said they would take out the heavy haul software and send me out again to re-check the mpg's. Well after another trip to Cat to remove haul program no difference was noticed! They now have decided that it's got to be Winter fuel and that it's normal for a Canadian truck to be down to 4.4mpg in the winter. I have been doing this work for 7yrs and have never notice winter fuel taking unto 2.0mpg's off your truck and just believe they just don't want to admit fault in case of a compensation claim for lost revenue.
If anybody can point me in the direction of a Cat specialist in Ontario or Quebec that can diaginose a truck instead of plug in and hope the computer tells them whats wrong, I would much appreciate it.
My truck is a 2007 pre emission Peterbilt 379 with c15 acert twin turbo,18 speed, 3.36 rear , 24.5 rubber . I haul light loads in a dry van throughout eastern seaboard.
Need to get this truck back on track asap.

Thanks Guys.

 

Thanks for the reply and pics of CAC, it's definitely worth getting that looked at. The truck before the trip to Dyno would boost around 40psi and rise to that at a steady rate. It now is quick to boost to between 55psi and 60 psi and holds there well. I find the truck produced its best power around 1200-1500 before the trip to Dyno and now seems to be around 1400-1600 rpm at 55-60psi. No complaints of power just mpg's

Supernova.

 

I think an ECM tune is the way to go also. It does seem too much of a coincidence and my fuel reports tell the story to the exact day of the dyno run but the dealer thinks I am nuts to go back to old software and I should be singing happy tunes for 4.4mpg this time of year.
What are you thoughts?

 

I will get the CAC checked but I don't think that is the problem but at this point ready to look at anything.
Thanks for the input.

As soon as I have this figured out, I will post the result on the forum.

Supernova