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Old 12-29-2013, 11:57 PM
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10 mpg on highway stock yj

OK everybody this is my dilemma. i have a 91 wrangler 4.0 5 speed that is stock. when i got it the whole exhaust was in need of replacement so i went to the local muffler shop (where i take all my vehicles and have never had a problem) to have it replaced and since i live in Arkansas we don't have smog testing i had him cut my rusted out cat out and just put new pipe and a new muffler in. i really think the jeep should at least get 13 or 14 on the highway.i only get 10 mpg. i don't drive it hard and keep it under 2500 rpm all the time. I'm thinking we must have done something with the o2 sensors and the jeep is running rich but i don't recall him unplugging any o2 sensors. any help would be very appreciated!

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Old 12-30-2013, 12:17 AM   #2
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If you keep your top speed at or under 55 mph, this might help with mpg (depending on what your problem happens to be). That and slow starts w/30 psi in the tires. Hope this helps. Not sure about the O2 sensors...

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Old 12-30-2013, 12:34 AM   #3
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I'm not sure if you are wondering where the o2 sensor is located.......On my '91 4.0L, the "upstream" o2 sensor is just below the exhaust collector near the crossover section. IIRC its a 4 wire bosch component. There is only 1 o2 sensor.
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Old 12-30-2013, 12:55 AM   #4
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What condition are your plugs and wires in?
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Old 12-30-2013, 02:10 AM
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Thanks for the location. And my plugs and wires were replaced in April and I haven't put 1500 miles on it since then
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Old 12-30-2013, 08:54 AM   #6
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My 94 4.0 stock height on 30s will do 14 to 17.5. I would check that O2 sensor and maybe the fuel filter and air filters as well.
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Old 12-30-2013, 11:28 AM   #7
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my 91,4.0,auto gets 17
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Old 12-30-2013, 12:22 PM   #8
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Mine got 19mpg on 31s running 70mph or so. Cold air and magnaflow exhaust... Not so good with the 35s...

You definitely got something weird going on...
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Old 12-30-2013, 02:15 PM   #9
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92 stock yj 5 speed 2.5 gets 21 all day long average driving
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Old 12-30-2013, 07:11 PM   #10
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92 2.5 lo with 5 speed running 33s gets 10 city and thats with me taking it easy our area has alot of steeper hills here in eastern ky,,
i know it guzzles it down while im wheeling,,lol
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Old 12-30-2013, 07:22 PM   #11
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I have found as soon as you lift it mpg goes out the door and and a biger tire than an 31
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Old 12-30-2013, 07:30 PM   #12
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You said thay you took off the cat. If you did that you may not have enoff or any back pressure at all you need to check that first go to local parts store tell them you want the cheapest cat they got I got one for 40 bucks half the size of the old one have them install that that is the only thing I can think of because that happed to me one time
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Old 12-31-2013, 12:07 AM
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i can probably just go back the the guy that did my muffler and have him throw one on. he really is the cheapest in town and uses name brands stuff, i dont know how he makes a living
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Old 12-31-2013, 07:34 AM   #14
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Not having a cat IMPROVES mileage not decreases it. That's why they disappear
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Old 12-31-2013, 07:38 AM   #15
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Don't put the cat back on the 4.0.
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Old 12-31-2013, 09:34 AM   #16
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Little help here:

What sized tires?
What axle ratio?
is speedo / odo accurate?

Driving my 4.0 / 6spd / 3.73 / 31x10.5x15 TJ got 16-17 over the Christmas drive to my folks and back.
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Old 12-31-2013, 02:00 PM
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30 inch tires
i think 3.55 gear ratio. the jeep feels under powered but this is the only manual jeep ive driven so maybe its how they all are
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Old 12-31-2013, 02:21 PM   #18
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I have 3.07 with 30s and the 4.0 seems fine. It's not a sports car by any means but it should be more than enough. Have you checked the speed with gps if everything seems to be running right .
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Old 12-31-2013, 02:32 PM
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Ill check the speed and I know it's no race car by i guess I just got used to driving my truck (it gets better gas mileage, weighs more, and will do a burn out go figure) while I was replacing the ball joints on my jeep. It just seems that the way I hear everyone talk about the 4.0 I should be getting better fuel economy and more power. Who knows it may blow up next time I drive it. I don't drive it hard it usually sits at about 1500-1600 rmp when I'm cruising and never gets above 2300 to 2500 when I'm having to pull out onto the highway
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Old 12-31-2013, 02:49 PM   #20
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That sounds exactly how I drive mine and it will run 15 average. It's for sure far under powered compared to new trucks. My 5.7 tundra will run circles around it. They get about identical MPG.
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Old 12-31-2013, 02:59 PM
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Ya my 351 Windsor in my 97 f250 and 5 speed absolutely destroys my jeep in ride comfort,mpg,power and it weighs a lot more
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Old 12-31-2013, 03:06 PM
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Does anybody have any suggestion for what I should try to get my gas mileage up
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Old 12-31-2013, 03:20 PM   #23
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Only drive downhill with a strong tail wind.
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Driving a jeep with a top is like screwing wearing a prophylactic. Maybe a little safer but doesn't feel nearly as good.
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Old 12-31-2013, 05:24 PM   #24
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Russ. i get 13-15.5 on the hwy
93, @ 67K. 4.0 auto 3.07 lifted 2.5 31x12.50
and in the area.. they usually don't get very good mileage
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Old 01-01-2014, 12:30 AM   #25
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Tune up and air up

89 on 33s 4" lift 4:88s running 65+ for 150 miles 18 mpg

It's a four banger with heavy bumpers, winch, trail box with parts and recovery gear and a 70 lb tool box. No wind the other day.

Usually when I travel with camping gear it's 15 mpg
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Old 01-01-2014, 06:46 AM   #26
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check codes on the computer, theres a thread on the forum to tell you how. and put a cat on it. check your brakes to be sure one isn't dragging, after driving feel the rotors and drums, they should be warm not hot.

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