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increase fuel mileage

sandsgg

NAXJA Forum User
I have an 88 mj 4x4 5 speed and i am trying to find some tops on increasing my gas mileage since I'm only getting like 10 city and 14-15 highway
 
going assume its a 4.0l? if so, there are lots of threads showing how too, but if the mileage is that low, might want to change the o2 sensor, it might be bad.
could also change it to a HO model, but main things to search and read is cold air intake, bored throttle body, injectors, 2.5 exhaust, highflow cats and mufflers, etc.....
 
Yes it's the 4.0, and i keep hearing 20+ mpg that i just don't see lol, it's new to me and the only thing the po did was put A K&n filter on it
 
The only time I've seen 20+ is if your highway driving. Again if I was you, check your o2 sensor, iirc it the pre cat o2 sensor that will give bad mileage.
 
88 = 1 o2 in the header.
My biggest mileage gains came from a ho header swap with a custom made 2.5" down pipe. Our renix rigs have a log style manifold, the how got a tubular manifold..it's a header basically. The 2.5" down pipe helped sound and power, noticeably.
Relocated your iat sensor..free of charge and few minutes of your time. Intake manifold heat shield for around 12 bucks. I have a 60mm throttle body and it feels great, but saw no power gain at all. If it helped or hurt mileage it wouldn't of been enough to notice on paper..so that be a last resort type of mod for me again..
703s or 700s or 710s will help you out..19-20lbs between the all, helped idle ALOT. also no more stumbling when crawling off idle slowly..I've got a 5speed, so. Slipping clutch with no gas it would almost die before, now it holds strong. These were done after the header/down pipe and on paper I saw no gain, but I also went from falken all terrains to some nasty mud terrains within days of this mod. I lost no mileage with the MUDs, which means I would of probably gain 1mpg or so had I kept the AT's. the mudders took any gain they gave me.

Fwiw, mines a 87 xj, ax15, 231, 3.55s, 31s and about every single bolt on you'd imagine, but also done CHEAP, or when needed..header cracked, I grabbed a APN for a ho and got a custo, down pipe with a flew section added..no more cracked header:)
Injectors were yanked and cleaned/rebuild by myself. Did a lot of stuff to help keep the exhaust hot and intake cool. 2 cans of vht 3000* ceramic paint..about 7 bucks a can. Coated, heated in oven for 1hr at 200*, let cool, recoat & bake at 300* for 1h, let cool, repeat at 400* and repeated at broil or 500* except I went 2hrs at that temp. It's didn't flake, I knocked on it with a ratchet will removing other parts, no chips. Stayed stronger. I later replaced the gasket and wrapped the header, 13 months or so later. Stayed strong!
I could get my hand close to it. Without it being to hot, while the uncoated stock log manifold would bake your hand within a few inches.
The heat shield is actually a foil type stuff from summit. Sticky on one side, shiny on the other. Dropped intake manifold enough to place hand on the manifold. Before you would be burned after a few seconds.
I'm also running a 2.5" flowrasper with a 100 cell magnaflow cat. Dumped after the axle.


I'm getting 17mpg city and 21highway. Pushing the 350k range on the original motor. Doesn't look to have been rebuilt but who knows. It actually just started knocking/rattling. So russ has started on a stroker. Even still she holds 21psi at warm idle & has decent compression, cyl 6 being a wee bit low..mph is with .2mph of my gps which can't get any closer, and I'm hand calculating. I can pull more if I draft on he highway I'm sure. It drops to around 14/18 if I'm driving hard. I run 65-70 normally.
 
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See that i would be happy with lol, i actually have an edelbrock header that i had originally on my 95, i just haven't swapped it over cuz i can't figure out how to adapt it to my remix engine
 
I know some headers will hit the 6th runner on the intake I guess..so it's hit or miss. But my. APN work out just fine, you will need either a factory ho down pipe or get a nice custom made 2.5" one with a piece off flex pipe added..hat way if the motor rocks the flex pipe moves vs the header cracking:)
 
That pipe comes off the header around runner #2 and has a peice of flex pipe that connects to the intake. It "helps" heat the intake in cooler weather..that's its intention. Less emissions by not running cool/rich I guess. Ditch it man.
I'm in Cali and I passed without it..hell they arent really looking for "performance" oriented mods on our jeeps. Just pass the obd2 scan if applicable and pass the visual and sniffer. I only have the sniffer and visual because my year. My header is very clearly wrapped, throttle body spacer, and when I slogged last I actually only had a elcheapo autozone filter off the stock intake pipe because in my 87 had he shoty style radiator setup, I swapped to the newer style, and it would rub the intake box and squeaky something nasty. So I pulled the stock box and still haven't thrown it back..I'm now running a Eric'sxj version intake:D
Still passed with that and that runs over the engine with 2.5" exhaust pipe into a flex pipe & filter located in the cowl lol. Smog tech just ask wtf I did it for, told him its LOUD and its a simple way to gain space & raise he induction point so I'm not sucking in water. My jeeps passed 6 years I've had it..well 3 times, every 2 years. Only retested 1 time when I very first got it, cracked header issue made me fail.
 
I used to get 22 pretty regularly, better if it was all highway.

That was with 31" street tires and 3.55's with an AX-15.

What tires and gears do you have?
 
Read my old thread the RenX Files for lots of good ideas!!

http://www.naxja.org/forum/showthread.php?t=905849&highlight=renx+files

My largest MPG issues on the first 87 I bought were the O2 sensor, TPS and IAT that were killing the mileage. Next issue was no, or poor TC lock up. My last MPG issue is age, low compression compared to new OEM IMHO.

Worst tank of gas was 6 mpgs. I can get about 18-20 on a straight highway run now. Averaging 15.5 in Houston, mixed DD now. The 89 has always had better MPGs, it has a lot less miles too.
 
I was reading around and I'm thinking the thermostat is either stuck open or nonexistent as well i think that is also causing some of it
 
mud terrains are going to nuke your mileage. Seriously.
14-15 with mud terrains larger than factory with factory gears?

sounds about right.
Find you some 3.55 geared XJ axles and you'll do wonders for your cruising power and mileage. The 3.07's were too tall for the factory tire size anyway, they get really sucky with anything larger than a 235.
 
You think it would be chapter to regear or just get new axles? I was thinking of deleting the cad anyway, so a new front was a possibility but it actually works still lol.
 
If u swap out the rear axle I would highly recommend a Ford 8.8 from a 98' or newer Ford Explorer. you get 1.5" diameter axle shafts and rear disk brakes. You will need to purchase a bracket kit for spring pads and shocks oh and also make some new axle brake lines but it's worth it. That Axle is stronger than a D44 and if your patient you can pick one up with 3.73 and a limited slip from the salvage yard for about $100!!
 
95 and later is the year split for the 8.8 you want, not 98, but otherwise good advise.

They come with 3.55s, 3.73s, or 4.10s from the yard, 3.73s and 4.10s being the most common at least around here.

I went with an 8.25 because a friend was selling a 3.55 XJ 8.25 that already had MJ perches welded onto it, then threw discs on it from a ZJ, but it's essentially the same plan either way. 8.8 is probably a bit stronger, is a bit narrower, and requires a bit more fab work but either way you're welding to put one in an MJ.

3.55s makes 33s tolerable with an AX15 but 4.10s would be way, way better. 3.55s an AX15 and a 4.0 is an awesome combo though, makes it a dream to drive. And 3.55 XJ axles are everywhere in the yards in every 4.0/auto XJ ever made basically, so they're cheap.

Get a front axle from a 95 or later XJ for the upgraded 5-297/760 size ujoint axleshafts, grab an 8.25 from a 97 or later XJ or an 8.8 from a 95 or later Explorer, weld the right perches on, and never look back.
 
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