Before i hear the responses of its a kleenex box on wheels; I know I have owned/do own numerous ones!

Okay, Ive got a 1999 Sport I-6, Auto Trans, 4WD. I am getting roughly 10-11.5 MPG city, and thats going easy on the skinny pedal. I am pretty well lost as to where my mileage is going. Heres whats been done:
Plugs (champion copper)
Wires (bosch)
Cap (accel)
Rotor (accel)
Throttle body removed and cleaned
IAT Cleaned
High Flow air filter (drop in)
Gatorback Belt
new idler pulley
new o2 sensors (NTK)
3 runs into a poor mans trans flush
tires are 235/75/15-filled to factory spec
784 injectors (rebuilt before they went in)

no check engine lights, runs well, drives well, ran diagnostics and everything is running the way that it should,

Just can't figure out why the mileage is so horrible, my 89 got better mileage when it was dumping gas!
Any thoughts are appreciated for sure!

 

Welp hows the MAP doing? Did it seal properly to the TB? Also hit your cat with an IR thermometer, maybe it's clogging up (hairball? Ha... Ha...)

Your 235s are throwing off things as well. Losing a bit of power and messing up the odometer

 

haven't messed with the cat at all yet, good idea though, ill try that this evening. See i thought about the tires, but then again my 89 had the same tires and it was better on gas. I know I know, the 89 is a totally different beast, I was planning on regearing the speedo anyway, but was planning on running 30's and would have to have a different gear so didn't want to waste money twice!

Map sensor made a good seal to the TB, double checked it before i put the TB back on.

 

I noticed the same problem a few years ago in the late fall and did all of the same tune up stuff and then realized The gas stations out where I live are using the 10% ethenol blend during the winter. late the next spring when the ethenol mix went away I gained a little over 2 MPG . When my jeep was stock I could safely go about 300-320 miles on a tank of gas, now with 31's and a 3" lift during the winter I can maybe go 220-230 miles. Not a fan of Ethenol, no sir I'm not.

 

Maybe it is the blended fuel, the only place to get non ethanol fuel is the co-op, but the non blended is 90 octane, no need to run that high octane in the 4.0, I get about 180-195 miles to the tank, I wish I was in the mid 200's!

 

Update on that one, cat tested good temp wise, 3 and some change before, 4 and some change after, new idea!

 

Is this a drastic change from before or is this a new-to-you Jeep?

 

Semi new to me, the 89 got decent mileage, and the 98 got decent mileage, just can't seem to figure this one out!

 

I noticed in your list you did not do a fuel filter... You might want to check that out 1st Hope that helps

 

Curious, how many miles are on your 99?

One thing to check is what kind of shape your exhaust manifold is in.

 

I would love to be able to change that but unfortunately on the 97+ it's in the tank as part of the entire assembly. Not serviceable

 

167xxx, inspected the manifold really well and it appears to have been replaced at some point, it's way too shiny to be original! Not a single crack that I could see anywhere, now I haven't done a smoke test yet but I'm pretty sure I'd be wasting time if I did, it's really in good shape.

 

My first thought was the winter blend fuel, as another poster mentioned. The coolant temp and intake air temp sensors come to mind, also. Do you have access to a scan tool, to see if their readings seem reasonable?

 

I agree that it might be time to get some test equipment on it and see if that yields any clues. Scan tool for starters and engine analyzer can be helpful too.

 

Got one, snap on ethos, eveuthing is looking good as far as diagnostics go, that's what's puzzling about it.