my friend has a high fuel use going on , did the code search, replaced oxy sensor, replaced fuel filter, was beyond bad , put seafoam and in tankcleaner hoping to clean system, will be pulling fuel tank to replace fuel pump in tank filter. every time i work on it it will run super ! up goes the mileage, great power,then next day fuel mileage goes to heck. also new deal happens once and awhile, trans seems to slip shift between like 3rd and 4th up shift, if you mess with the shifter then does not do it again for maybe a week or two, not sure what type of trans will be getting number off the case, for was thinking lock up converter could be going bad, also will be running a code check again to see if code 21 came back , for then might have broke wire ect, for after i had replace ox sensor was good. so any other thoughts,

 

Tough to tell. They made Jeep Cherokees for 17 years and they weren't all exactly the same. Could you elaborate? Year, engine, mileage. I'm assuming it has an auto (that would be an Aisin-Warner AW4) and you've got codes so I will further assume that it is an OBD-II era Jeep but that's just me showing off.

 

Thank you for getting back, it is 1993 cherokee two door, has a 4.0 engine, with fuel injectors, auto trans but the postal jeeps at time have different component than what is consider normal, that is why not for sure on trans, also have super hard time getting right parts, for the last stuff , fuel filters ect are what is called for on the Grand Cherokee, lol, Looking in my motor trans repair book it does not look like a aw4, But what gets me is it does good on fuel for a day then bam, gone. now trans acting up, will be checking on the wiring for ox2 sensor, but imay have to disconnect the computer wiring to let reset also, i have just read that sometimes by disconnecting battery may not clear codes, so i was also wondering if it still thinks ox2 is still bad ?? I was going to do a complete engine overhaul with all new part underhood, even injectors ect, but need to get 18 to 20 miles a gallon to make it pay ! oh mileage is 138.000 with a great maintenance record . thank you for your thoughts

 

Anything above 10 MPG is good.

Mileage is a very subjective and variable number, as no two vehicles are driven alike in the same environment by the same driver.

55MPH, level, no wind, light load, original equipment sized tires aired to maximum on sidewall, you MIGHT get 18-20 on a good day.