rollincoal Posted August 23, 2012 Share Posted August 23, 2012 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! Link to comment Share on other sites More sharing options...
mvusse Posted August 23, 2012 Share Posted August 23, 2012 Don't bother testing it, replace it. Link to comment Share on other sites More sharing options...
cruiser54 Posted August 23, 2012 Share Posted August 23, 2012 Don't bother testing it, replace it. Good idea. They were meant to last for 67,000 miles. Link to comment Share on other sites More sharing options...
mvusse Posted August 24, 2012 Share Posted August 24, 2012 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. Link to comment Share on other sites More sharing options...
rollincoal Posted August 24, 2012 Author Share Posted August 24, 2012 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? Link to comment Share on other sites More sharing options...
MJCARENA Posted August 24, 2012 Share Posted August 24, 2012 3:55 gears and 33" tires could be the reason for lack of power and poor mileage. Had the same combo then went to 4:56 gears and got my power back. Link to comment Share on other sites More sharing options...
rollincoal Posted August 24, 2012 Author Share Posted August 24, 2012 ive been reading about mpg with stock gears and 33's, and most peopple are averaging between 12-15 over on cherokeeforum. Plus they are small 33's, my buddies 31's are almost as big as my 33's lol. Link to comment Share on other sites More sharing options...
HOrnbrod Posted August 24, 2012 Share Posted August 24, 2012 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. Link to comment Share on other sites More sharing options...
rollincoal Posted August 24, 2012 Author Share Posted August 24, 2012 i was using my gps to get my mileage. And i feel like the hesitation when cold has something to do with it? Link to comment Share on other sites More sharing options...
HOrnbrod Posted August 24, 2012 Share Posted August 24, 2012 i was using my gps to get my mileage. And i feel like the hesitation when cold has something to do with it? Then I agree with the others. Replace the O2. Link to comment Share on other sites More sharing options...
skidoo_j Posted August 24, 2012 Share Posted August 24, 2012 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. Link to comment Share on other sites More sharing options...
mvusse Posted August 24, 2012 Share Posted August 24, 2012 IIRC it's 18x1.50, same as the spark plugs. Link to comment Share on other sites More sharing options...
carnuck Posted September 10, 2012 Share Posted September 10, 2012 Vacuum leak? Especially the hose to the MAP sensor leaking or clogged causes trouble. Link to comment Share on other sites More sharing options...
purplexj Posted September 12, 2012 Share Posted September 12, 2012 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. Link to comment Share on other sites More sharing options...
cruiser54 Posted September 12, 2012 Share Posted September 12, 2012 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. Link to comment Share on other sites More sharing options...
Keyav8r Posted September 13, 2012 Share Posted September 13, 2012 He said he has a 4.0 HO in his 87. Does the HO have two O2 sensors? My 88 is Renix and has one. Do HOs generally get better or worse mileage than Renix engines, engine condition, miles and state of tune being equal? Link to comment Share on other sites More sharing options...
Keyav8r Posted September 13, 2012 Share Posted September 13, 2012 Dang! I just noticed I have been elevated to Advanced Member. They don't know me very well, do they? Link to comment Share on other sites More sharing options...
mvusse Posted September 13, 2012 Share Posted September 13, 2012 I have gotten 24mpg out of a Renix cruising 60 mph. Above that speed it drops off in a hurry. Best I have gotten out of an HO was 22, but that was going 70. At 60 I was only getting 21. Link to comment Share on other sites More sharing options...
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