XJ Fuel Mileage Help

[ozzy702]

97' XJ, ~ 145k on it, 3" lift, 31's, stock gears and just getting crap mileage, around 10mpg mixed driving. If 100% highway I may be pulling 13mpg. I'll be doing the following but am looking for thoughts and ideas from you guys on anything else to look at that could be causing the poor mileage and anything I can do to improve.

- Fuel Filter

- Air Filter

- Spark Plugs (which ones?)

- Air Pressure in tires

 

[N-Smooth]

That is lower than I saw when I had 31's for sure. I'd suggest copper plugs because the 4.0 typically doesn't respond any differently to fancy plugs. I use champions. If your alignment is jacked that will cause poor mileage as well but you should see it in tire wear.

 

[skippy]

The best Modification i ever did to a Jeep to get better mileage was buy a civic

 

[ozzy702]

The best Modification i ever did to a Jeep to get better mileage was buy a civic

Ha! I don't commute in it, but would like to get better mileage on long trips out to the desert. Civic doesn't cut it for getting into the back country, although a Subaru Outback would be decent for quite a bit and get much better mileage, financially it just doesn't make sense for me to buy one though since I work a mile from home.

 

[N-Smooth]

Skippy is right. Buying a Subaru is definitely not the solution either...

Do the normal tune-up stuff and see if it helps. Chances are it needs it anyways.

 

[ozzy702]

Skippy is right. Buying a Subaru is definitely not the solution either...

Do the normal tune-up stuff and see if it helps. Chances are it needs it anyways.

As I've been looking at the options keep this turd of an XJ just makes sense, even at 10mpg it's cheaper than going the alternative route and it's a hell of a lot more capable than an outback. Past normal tune up stuff any ideas? Fuel injectors? Throttle body? IAC?

 

[DaveB]

Our 4.0s do much better than that. Do you have any error codes showing up ? If the heat sensor, oxygen sensor are messed up or if you have a cracked exhaust manifold that is leaking badly it may be running rich.

 

[ozzy702]

Our 4.0s do much better than that. Do you have any error codes showing up ? If the heat sensor, oxygen sensor are messed up or if you have a cracked exhaust manifold that is leaking badly it may be running rich.

That's the thing, it isn't throwing any codes. It does have a decent amount of miles on it and I don't know what it's maintenance history is like.

Ok, so here's what I have so far.


Oil Change - going to try some 5W-30 Moblie one

Champion plugs (gap?) , I believe RC12LYC

Air Filter

Air Pressure in tires

Possibly heat sensor and oxygen sensor.

 

[Bobzilla]

could also check and replace, cap rotor & plug wires, check catalytic converter plugged?

 

[idahoyj]

Maybe check the fuel pressure? Mine got terrible mileage when the pump was going out. The fuel wasn't atomizing properly since the pressure wasn't spraying out of the injectors.

 

[SAVAGE]

install a 4bt. Just kidding. You could also do a quick check on all the connections on your mass air flow sensor, throttle position sensor, ect. Also as mentioned above heat sensors are famous for destroying gas mileage.

 

[bryson]

What gears do you have? Are you correcting mileage for the tire size? Maybe you are not getting as bad of mileage as you think...

 

[jeeper]

I would get about 10 out of the xj's I had. Only solution I found was putting a block of wood under the gas petal to limit throttle.

 

[RogueJeepr]

I put 4:10's in w/31's and gas milage went up. Maybe I'll post the official results when I get my new speedo gear here soon.

 

[ozzy702]

What gears do you have? Are you correcting mileage for the tire size? Maybe you are not getting as bad of mileage as you think...

3.55's. Yes, I'm actually using GPS to track mileage. If it wasn't getting such bad mileage I'd consider 4.56's and 33 10.50's but at this point unless I can figure out why it's getting such bad mileage I may just pop some 235's on the thing (it needs new tires).

 

[Jay5.9L]

Does it smell or sound funny like your running rich or have a cracked exhaust manafold? How's your alignment?

 

[ozzy702]

If the temperature sensor was bad would it show on the temperature gauge? Mine temp gauge seems to be working fine. What degree thermostat do you guys run/recommend?

 

[SAVAGE]

I'm thinking you might have 2 sensors. One is just for your gauge. And the other goes to the computer to tell when the engine is warm/hot/cold ect. If the temperature sensor on the motor that is connected to the computer is bad, then your computer will always see that your jeep is cold. That would then run the motor in open loop running the engine way rich to help it warm up. That certainly would destroy your gas mileage. So I guess whip out a chilton manual and see if you do actually have two tempurature sensors, or crawl all over your motor and look in all the freeze plugs and the outlet/inlet necks. Hope that helps ya. Oh and we run a 180 degree in our 304 cj5.

 

[DaveB]

OBD1 4.0s had two temp sensors, one in the front of the engine and one at the back of the head. The one in the back runs the temp gauge and the one up front is for the computer. On the OBD2 motors there is only one sensor which is the one up front. 96 was a change over year so my guess is that you only have one in your 97.
Most of these motors use a 180-190 degree thermostat.

 

[SpeedyVic]

Just for reference, I just towed my 6x10 enclosed trailer to the desert race with my XJ. From Logan, through Saratoga Springs, over Eureka, and back the same way. Fighting a little head wind on the way down and I got 13.7 mpg when I filled up at Smiths in Saratoga Springs. I stopped there again on the way back and filled up. Got 17 mpg then. Tail wind helped a bunch but it is a slight uphill most of the way back. I have not filled up since I got back to Logan to get the complete trip average. I am also fighting a slight overheating problem so was pretty conservative on the gas pedal but my results are way better than I thought they would be.

My XJ is a 96 auto with 4.56 gears and 32" tires. The speedo gear has been changed to the correct tooth count.