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horrid mpg

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ok i have a 87 mj with a 4.0 HO in it. I'm getting 8.3 mpg...it has 3.55 gears and 33's.

has new plugs rotor cap wires

new map sensor and IAC

new alternator

new fuel pump

new exhaust


It also has a hesitation off of idle when i first start it up, but after driving for 5 minutes, it's good to go. also the power feels like its lacking.

o2 sensor? how could i test? thanks!

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Have you installed the correct speedometer gear that matches your new tire size diameter vs. your existing gear ratio? If not your speedometer / odometer is no where near correct. And how do you deriving your MPG figure? If it's not by using GPS it's AFU.

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i odnt have any lights on the dash, the engine has 100k on it. which one to replace? the upper or lower? and is the o2 even the problem?



a renix system only has one O2 sensor. It's located near the manifold above the connector, its easier accessed if you remove your front drive line if 4x4. It's a pain to get out usually you have to heat the bung up until it's glowing orange and use a end wrench and a bfh to get it out. Some people try to keep the o2 sensor cold with a wet rag so the metal on it doesnt expand. When I tried it it was just too much to control, not burning the crap out of the intake/ egr/ clutch hoses etc while yanking on a wrench that's hot as hell and keep cycling a wet rag to the o2 sensor. I guess more stuff could be removed to improve access even more.


I did chase my threads once I got the o2 sensor out, but I don't recall what tap it was.

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  • 3 weeks later...

If it is a vaccum leak do your brakes feel weak too. Anyway those gears are way off. I recently averaged 16 mpg pulling my 700-900 lb MJ trailer to and from PA from NY. 65-70mph most of the way with some city/rural . 31's and 4.10's with no lack of passing power even with the trailer.

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Cherokees and Comanches (and YJs?) had a 50,000 mile timer to turn the maint light on to let you know it was time to replace the O2 sensor.


67,000 mile timer. I remember it well because we would do the 60,000 mile service, and then about 7,000 miles later the light would turn on and customers would be mad.

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