I know this is been covered, and I've done searches on here but i'm still clueless. 120k on a 02 screw 4x4 5.4. Stock other than gotts mod. plugs were done at about 100k (motorcraft) just did brakes and the parking brake is not draging, Changed rear diff oil, changed oil (motorcraft 5w20 synthetic blend and ford filter) new air filter, cleaned throttle body, tranny flushed at 110k. Had it scanned no codes. passed emission test with flying colors. The last 4 tanks have been from 4 different gas stations. ranging from 9.45 to 10.69 mpg. I was getting 12 to 13.5 around town. This started before the last oil, air, diff change, and brake job. I've used the same brand and wieght oil and filter for 10k. Runs great, shifts great driving habits havn't changed. Any thoughts?

 

Several things could be winter gas, also clean the Mass Air Flow Sensor and its probaly getting ready for some upper O2 Sensors, Also has the Fuel Filter been changed recently?

 

probably they changed your gas mixture ethonal is likely to blame. Be nice if they posted that on the pumps..

 

Maf gets cleaned regulary, Fuel filter was done 6k mi. ago. Ran a can of fuel treatment at that time too.

 

With 120,000 + i would definetly get a couple O2 sensors, after 100,000 they start to show signs of wear, good luck

 

Should I replace all 4 or just the 2 uppers for now?

 

Just the 2 uppers, sounds like you have done all the maintance to the truck that is needed so i would do the o2's and go from there. No guarantee that it will "fix" it but it sure wont hurt it. Like mentioned before it could be winter gas or ethanol. I bet you will notice a increase with new o2's. Let us know what you find..

 

It will be awhile untill I get it done. I'll report back here with results. Thanks for the advice!

 

Any results yet?

 

well, summer gas is here, in town is 11 to 12mpg. highway 15.5 to 17.5 mpg depending on traffic. haven't done the o2's yet. I used to have a k&n drop in filter, but trashed it because i don't trust the oil. I guess I'm happy with the milage since there are no leaks and doesn't burn oil. Love the mileage of diesels but not worth the maintenance cost.