After replaced new Alternator and Belt, the MPG dropped from 12-13mpg to 8-9mpg. The alternator and belt were replaced at a Ford dealer with genuine Ford parts. It has been 3 month, driving on the same speed on the same flat highway with same load, both ScanGauge and at each fill, all showed the significant mpg drop. There is no error code showing on my ScanGauge, water temperature is normal (fan kick in at 213F), trans temperature around 195, gear shifting is ok and get to overdrive gear, 2000rpm at speed of 57mph. Tire pressure inflated at the 66psi at front, 77psi at back when cold.

I don't understand how a placement of alternator and belt would result of the mpg drop. Anyone has any experience, ideal or suggestion? I did not go back ask the Ford Dealer, I don't think they would believe me. This is Ford Econoline E450 Powerstroke 6.0 with no tuner.

Thank you in advance!

 

Same fuel? Same vendor?
Fuel pressure regulator leaks/bleeds were pretty common that go somewhat unnoticed. As well as the secondary fuel filter cap to bleed. Fuel module, lines etc.Has anyone looked it over for any possible leaks? Alt and belt wouldn’t make a significant change.

 

Thanks for the suggestion. I will check the fuel system. I may change all two fuel filters and motor oil and see if make any different. I don't believe drop mpg is directly related to alternator or belt, just wonder if something else bumped or loosed during the replacement process.
All the 3 refills were from same diesel station (2 of 3 even from same pump)

 

Join the club. My 1987 E-250 302 v 8 gets 8 mpg around town and a whole 11 mpg on the freeway as long as I don't go over 65 or so.

 

For what it might be worth,
I have found the older gas engines to be quite reliable, in commercial vehicles but depending on amount of load carried , mileage could be decent or just not that great.
Also , learned the value of sticking with OEM parts because it did make a big difference in performance and MILEAGE. ex; Motorcraft Spark Plugs.

 

This is getting even weird. After about 5 100 miles trips, when I was driving, the ScanGauge MPG all the sudden jumped from 7-8 mpg to 13-14mpg (not because relax the gas peddle or down slop). Since I did anything since the notice of the mpg drop, what would be the cause of the significant mpg drop and how it came back to normal by itself? Is it possible that, after they replace the alternator at the dealer, they reset the computer, and the computer has to learn the proper mixture after so many miles to optimize the proper mixture? I don't know if there such things in 2006 ford.