There are about a bajillion threads on this topic...
Some will say they get much better, others (like me) will say it's pretty common.
Search away or wait for others to chime in
My 98 4 door auto with 3" lift, 31" Duratracs, and a heavy bumper hanging off the front of it was getting 9-10 after I corrected for the tire size. That was with a K&N panel, Magnaflow converter, and Flowmaster 40. I did a basic tuneup (plugs/wires/cap/rotor) and that got me up to 12-13. I seafoamed the intake, and the gas tank, and that got me up to a consistent 15.
My exhaust manifold was cracked, so I installed an APN header and the much-debated 99+ horseshoe intake. I average about 17-18 now. My best has been 18.9 mpg. My driving habits did not change. I'm not going to say which of those two parts gave me the bump, just that I changed both at the same time and it gave me a 3-4mpg increase. Those numbers are hard to believe, and if I wasn't the one getting them, I'd call shenanigans. But that's what I get currently (about 90% highway)
My bone-stock 91 XJ 4 door auto got almost 20 running all highway. But it was a little lighter than my 98.
I have a 95 2 wheel drive and I get about 16 around town. Mine is all stock. Changed tps, iac, spark plugs, dist cap, rotor, radiator, oil and filter, tranny fluid and filter, and a bunch of otherstuff to get better gas mileage. I pulled the injectors also and cleaned them and put new o rings on. It added about 2-3 mpg for me.
You may want to check the catalytic converters and the O2 sensors. If the cats are plugged and/or the O2's are faulty the computer is messing up the air/fuel mix.
There are alot of different possibilities, maybe it's just one of your O2's. It could also be that all of your coil packs are not firing. In which case you're blowing all that unburned gas out the tailpipe.
I have a '95 OBDl model, so I'm the furthest from having the same setup you do. But let's look at the similarities.
Do you have a dragging feeling or does it coast smoothly? Brakes, bearings, grinding noises coming from the axles?
I agree with bluejunior.. downstream 02. My 97 xj got 17 to 21 mpg from the day it was new. Around 170k suddenly dropped to 14. Tune up (plugs, cap & rotor, wires) didn't change mpg. Changed downstream 02 and back to normal. No codes
BTW bluejunior- If I hold your beer it's gone beer!! But I'd give you another.
adam721 Noobie noob of the planet noobitron here, but, did you reset the ECU after changing the injectors? Some leaks can be masked by evaporation. I had a mystery coolant leak once, never left a drop anywhere. Small leak between my intake and head (Chevy 305 no worries here.) Point is the injectors could do something similar. Seafoam, change the plugs, grease all the driveline components with fittings, check fluid levels in the diffs, t/c, transmission, investigate any funny fluids and tire pressures. Do you have an exhaust leak? A shorter exhaust, (see leak,) caused mine to get worse gas mileage by being peppier in the higher rpms, I had to flog it harder to get normal acceleration. It's a low rpm OHV engine. Vacuum leak too, I smoke checked mine with a cigarette and found a leak immediately. How's the alignment, toe in toe out wise? Just writing some things I could think of, not trying to run over anyone else's recommendations.
Best improvement I got was when I found that the vacuum reservoir had a crack in it and I plugged the line to it. Check for leaks.
I have a 95, with 32" tires, 3.55 gears, 4.5" lift. I get between 12-13 in town.It is stock for the rest.
I got it fixed it was a stuck valve and a broken fule injector
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