Why am I stuck with 12mpg? Help Appreciated

[srmitchell]

Hey dudes. I won't bore you with my complaints about how my rig only manages 12mpg, but really... wtf. I'm looking for answers, and help.
Don't tell me to buy a honda. I have a Dodge dart which manages 27mpg, all day long. This is about range for expedition travel.

I use my jeep for travelling- I like rock crawling, but I usually only see dirt roads, rocky trails, and long distance backroad travels. I love exploring the redwood coast where I live, but the geographic distance between attractions is huge, so I need to ind out how to make this thing have a longer range.

2001 xj- 4.0, AW4. 190,000 miles.
33inch kevlars, 4 inch long arm... Lots of armor, but no roof racks or anything. Weighs 4200lbs.

4.56 gears... Only engine mods are an AFE intake, Flowmaster 40 dumped at the axle, 4 hole grand caravan injectors, 60mm throttle body.

I hate the aw4, and want to swap it for a NV3550... Im sure having a manual would boost my mpg average...
Any luck with the Banks header? (I have pre-cats).I tried electric fans once, two stock 97 fans. It worked, but I had a chronic issue with the fan controller and I once started overheating because of the bad controller. That spooked me and I went back to a clutch.

I have run BG44k twice in 2,000 miles. I run rotella 5w40, and I have changed the spark plugs in the last 15K miles I think.

I have never pulled the fuel pump to look at the filter... Maybe I should do that? 190K and 16 years of build up? How about the timing chain?

I'm sorta at a loss. I drove 70 miles to a coastal town north of me this weekend. I clocked 85 miles before I topped off my tank... It took 6.8 gallons for a whopping 12.5 MPG.

I consistently, with ZERO variation, get 200 miles to a tank. That is shit! Any and all help is appreciated. I know, I know... XJ's suck fuel... But what if I need to go somewhere long distance, and need the range? 200 isn't enough, especially if you must drive far to, and from a trail, where mileage will be even worse.

The thirsty junk-
Redwoods by Sean Mitchell, on Flickr

 

[PhotoJared]

Hey dudes. I won't bore you with my complaints about how my rig only manages 12mpg, but really... wtf. I'm looking for answers, and help.
Don't tell me to buy a honda. I have a Dodge dart which manages 27mpg, all day long. This is about range for expedition travel.

I use my jeep for travelling- I like rock crawling, but I usually only see dirt roads, rocky trails, and long distance backroad travels. I love exploring the redwood coast where I live, but the geographic distance between attractions is huge, so I need to ind out how to make this thing have a longer range.

2001 xj- 4.0, AW4. 190,000 miles.
33inch kevlars, 4 inch long arm... Lots of armor, but no roof racks or anything. Weighs 4200lbs.
This is probably part of the reason; geared or not you're still pushing around 33s and "lots of armor"

4.56 gears... Only engine mods are an AFE intake, Flowmaster 40 dumped at the axle, 4 hole grand caravan injectors, 60mm throttle body.

I hate the aw4, and want to swap it for a NV3550... Im sure having a manual would boost my mpg average...
Any luck with the Banks header? (I have pre-cats).I tried electric fans once, two stock 97 fans. It worked, but I had a chronic issue with the fan controller and I once started overheating because of the bad controller. That spooked me and I went back to a clutch.

I have run BG44k twice in 2,000 miles. I run rotella 5w40, and I have changed the spark plugs in the last 15K miles I think.

I have never pulled the fuel pump to look at the filter... Maybe I should do that? 190K and 16 years of build up? How about the timing chain?
Do the fuel filter just because. I pulled mine off with 165k and it was all junk filled

I'm sorta at a loss. I drove 70 miles to a coastal town north of me this weekend. I clocked 85 miles before I topped off my tank... It took 6.8 gallons for a whopping 12.5 MPG.
Another large factor is elevation gains. If I go up hill in my XJ I loose a lot of mileage too

I consistently, with ZERO variation, get 200 miles to a tank. That is shit! Any and all help is appreciated. I know, I know... XJ's suck fuel... But what if I need to go somewhere long distance, and need the range? 200 isn't enough, especially if you must drive far to, and from a trail, where mileage will be even worse.
What if? Buy an extended tank and some gas cans. Extended tanks are like 28 gallons and 2 5 gallon gas cans is 38 gallons total with an almost 500 mile range.

The thirsty junk-
Redwoods by Sean Mitchell, on Flickr

 

[blondejoncherokee]

Replace o2
Sensors, tune up,
Exhaust leaks, bad cat,
Intake leaks, vacuum leaks,
All filters, plugs, tps, tire pressure, Calculate mpg as
Miles driven over gas used not by the computer.
I get 17mpg in my mj on 33s and like 14-16 in my
Xj on 40s

 

[XJLI]

Replace o2
Sensors, tune up,
Exhaust leaks, bad cat,
Intake leaks, vacuum leaks,
All filters, plugs, tps, tire pressure, Calculate mpg as
Miles driven over gas used not by the computer.
I get 17mpg in my mj on 33s and like 14-16 in my
Xj on 40s

Yup. Tune up tune up tune up. New coil pack, new plugs. I think you can use a Viper coil? Theres a thread on that somewhere... Switch to synthetic oil in your diffs, but at that point you are really trying to squeeze out everything.

I've found that some XJs will just get better MPG, but you're a little low. I got 12 in my 3/4 ton swapped 5100lb pig.

 

[MoparManiac]

Yeah that definitely seems low.

I would average about 16-17 in my old XJ. Granted very little armor, but that was on 35s with 4.88s. Stock motor with 230k on it with original injectors and whatnot. I kept up with tune ups and filters, but didn't run any synthetic oil.

 

[ehall]

All of the above plus check compression.

My DD was only getting 11 mpg, found a little drip at the hose clamp on the return line. It still only gets 14 because there's a vacuum leak somewhere that I haven't found (probably in the dash).

My XJ gets 15 MPG in the city

 

[CarbonXJ]

Fighting the same battle, but I'm only 3.5", mildly armored and with 30s on 3.55s and see 13 in the city and 15 on the interstate, MAX! I'm pretty sure I have an unhappy torque converter from the 150k original fluid miles the PO put on it, and I have to do a compression test as well. Engine runs flawlessly except a rear valve cover leak, and I've done every maintenance item you have mentioned and them some.

 

[jk_surgeonfish]

I had a friend that had a 60mm TB and it killed his mileage. If you have the old one put it back on...reset the computer and see if it makes any changes. Won't cost you anything especially if you have the old TB.

 

[old_man]

Old school technique is to hook up a vacuum gauge and see what it reads and if it fluctuates at an idle. Vacuum will tell you tons about your engine.

 

[themauler]

After swapping a ax15 for my aw4 in my 4.0 xj with 33s and 4.56 gears I managed closer to 19-20 mpgs in Virginia mountains with no front drive shaft on since I had to find a shorter one. Put the driveshaft back on the mpgs went down. Sounds shitty and the return on investment is crap but lock outs could make a difference if you don't plan on upgrading to a d44 or a d60 soon.

 

[Tim_MN]

Changed the speedo gear in the transfer case so you have the correct miles traveled ? Average MPG's should be calculated over 3-5 fill-ups to account for weather/temperature/elevation/wind variations.

My 99 got 18.7 hwy when stock on 30x9.5's and 3.55 gears. It gets 17.4 hwy with 3" lift/ 31's/ 3.55 gears.

My 2000 with 5" lift/ 31's/ 3.55 and full armor gets 16.6 hwy.

 

[iluv83vettes]

I get about 15MPG. 99 w/ AW4 and NP242. When I first got it, I would get 19-20 at 70MPH. When I cut the front spoiler thing off under the bumper, it went down to ~18. When I lifted it, it went down to ~17. When I put the ARB on the front, it went down to about ~16. When I put the Rubi take off on, it went down to ~15. I use GPS to correct my speedometer which is about a 15% difference and hand calculate every fill up. I also drive faster at ~75-80MPH.

 

[iluv83vettes]

I tried to edit but it wouldn't let me.

With your nice approach angle, you sacrifice MPG. The more air that moves under a vehicle, the worse your MPG. There was a 2WD XJ on here that the owner added a second air dam and gained ~2MPG IIRC.

 

[srmitchell]

Thanks for the input guys.

Speedo is correct. Either way, if it was a little off, + or - 200 actual miles to 16 gallons that it takes to fill each time? That is right around 13 mpg. The overhead console is dead on at 13.1mpg average.

My O2 sensor were new a few years ago.

I have a spare pre-cat assembly I could throw on. Just did the intake manifold gasket a year ago. Would it be worth throwing a high flow magnaflow cat on there? My cat has been hit quite a few times on the trail. It has scrapes on the casing.

And agin... I know theres a price to capability... I'm just trying to get 300 miles out of the tank again.

 

[ehall]

If your cat is crushed I would definitely replace it. I have had very good luck with the Magnaflow high-flow cats

 

[GordoSmasho]

If your cat is crushed I would definitely replace it. I have had very good luck with the Magnaflow high-flow cats

Do they make a cal smog legal one?

 

[srmitchell]

Yeah, please if someone knows of a 2.5 inch in/out high flow cat that can be sold in CA, I'll buy it.

 

[GordoSmasho]

15mpg should be easily doable. I had a 97 stick with a loft and 35s with 4.10s and i got 17 commuting with alot of heavey tools and a ladder up top. Your issue is deffinately mechanical or sensor related. Wilbur got 15 on my last trip to the rubicon.

 

[Ivan]

Yeah, please if someone knows of a 2.5 inch in/out high flow cat that can be sold in CA, I'll buy it.

Sean,

My Jeep is close to set up, the vintage, and mileage as yours; so the cat being bad may not be out of the question.

My cat would rattle, like it had rocks in it; at start up. Once warmed up, it would quite down. So to get rid of the jalopy sound, I had it replaced.

Since you live in Humboldt like I do....I went to Ray's Old Town Muffler, in Eureka for the best local deal. Way better than Leon's. Guy there said it would be a good idea to do both the cat and the muffler at the same time, since they were both stock. I got a CARB compliant cat, and a magnaflow muffler.

So, for about $550, it was all done; and annoying noise was gone. Not sure if it's the 2.5" size you are looking for, but it is stock and smog Nazi compliant.....

Point of the story was that once the cat was replaced, my mileage jump up about 1 - 2 mpg. Bonus! Now, I'm pulling about 15 - 17 mpg. I do about the same roads and trails as you....

Good luck!

 

[srmitchell]

Hey Ivan!
Awesome. I don't have a tail pipe right now, it's a turn down. Pipe was mangled so I cut it off.
Do you think they could build me a setup that would agree with my gas tank skid/shackle set up also? I'd be willing to just get this done once and for all. I have extra pre-cats from XJensen also. They have 120,000 miles versus my 190. Might be a good idea to do it all.