06 engine in 05 truck
 

I have installed an 06 engine in my 05 3500 dodge dually and my fuel mileage has dropped to appx 10 mpg when towing. Before I could get 18 loaded. Do I have to get the computer reprogrammed? What else could be going on?

Any CEL lights or soft codes? Hand calculated mpg?

No lights
 

Yes computer says 11.7 but by hand it's closer to 10. No lights are on

No lights
 

Yes computer says 11.7 but by hand it's closer to 10. No lights are on

Are you using the 06 or 05 ECU?

05
 

The 05 computer

That might be your issue right there.

might be or is does anyone know for sure?

That's your issue, found answer below in a cumminsforum thread...in a nutshell

The 2005 ecm runs the truck using the SCI interface, while 2006 up uses a MUCH more complicated CAN bus system.

carcomputersystem posted on here on 4.18- send them a PM and get an 06 computer. The best thing is you can get EFI tuning with your 06 ecm so you can increase your mileage over your stock 05 #s!

I was pretty sure it was your issue, but wasn't positive. Likes like he found your answer.

Thanks
 

I'll give it a try

Hi, Billy. Not sure if you found the answer you were looking for yet. If you give us a ring at 888-875-2958 during normal business hours we'll be glad to answer your questions.

So the harness is the same on an 05 and an 06? Just the ECM is different??
What about the dash instruments? Anything else "CAN" system operates?
Motors are the same internally bad mileage could be hard part related. Bad injector or melted piston or both! these motors do a great job of compensating for a bad hole!