i have a 2001 durango with a 4.7l.. i bought it in 06 with 115k now i got 145k on it.. i used to get 15-16mpg avg, 18-20 hwy and was happy with that. now im lucky to get 12...what i have noticed is it seems under powered. slightest hill or little gas pedal its down shifting out of overdrive and it drives always like i have a trailer...it seems like somethiing is holding it back.. brakes are all good and free, new plugs, ohm checked coils, but it does have a lot of valvetrain noise at cold start up. runs good other wise....

 

Transmission filter and fluid change time especially if it's heating up or a CAT plugging but you should do the trans service (NO flushes!) and you can have a back pressure test done on you CAT before and after the CAT. Anything over 3lbs back pressure between the manifolds and the CAT go ahead and replace.

 

Could be a dozen things. Change the PCV and clean the IAT/TB. If still bad, replace TPS and MAP.

Engine slap is common on the 4.7l on startup. Damn small engines.

IndyD