poor mpg no codes

2009 volvo with Cummings isx 480k miles. The truck was getting 5 mph. Noticed two exhaust leaks and fixed them. After fixing it the next 2000 mi It got 7 mph. Next trip its avg 5.3. Truck runs good. No power problems. No codes. Question should I change the fuel regulators? Any other suggestions? Ive read the isx threads and seems like most the issues with poor mpg are connected with other problems. All injectors test good. Turbo test good. Cou ld I be gett ing air in the fuel only periodically? Every once in a while while ideling the truck will flutter just a little but not often. Any thoughts would be appriciated.