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Old 03-19-2012, 10:10 AM   #1
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97 stock TJ 4CYL getting 8-10 MPG! Please help

I have a 97 stock 4 cylinder that only getting 8-10 MPG. I changed all the plugs, wires, cap and rotor with no change. there is no obvious fuel leaks. I know my gas gauge is messed up so i reset the miles every fillup to make sure i don't run out of gas. I have read other threads with people having the same issue but didn't see what they did to fix it. Can someone please help me. Also, I am not getting the check engine light. I am thinking O2 sensors or possibly Cat Conv but then I"m starting to get into expensive repairs.

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Old 03-19-2012, 10:20 AM   #2
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Upstream o2 sensor will go bad and not throw a code and its possible your cat could be clogging up www.Rockauto.com has the best prices on NTK brand.
You have stock size tires?

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Old 03-19-2012, 12:49 PM   #3
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When this happened to me on my 94' yj (same drive train) its was a clogged cat. Try replacing that first.
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Old 03-19-2012, 01:00 PM   #4
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Got a '97 here myself - however mine is an I-6, 5 speed. I'm getting between 19.5 and 20 MPG - -

Saying that, mine is totally stock - no lift, no big ole' tires and the original 3.07 gears - I do like the skinny peddle - however gas is expensive - so we all have choices and have to live with them.

I would look into your gearing first and adjust accordingly (if possible)
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Old 03-19-2012, 01:09 PM   #5
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I have a 97 with 33" tires, 4.56 gears and running I6 with the 5 speed. Around town still getting 15 mpg or so. For you I would start with either the Oxygen sensor(s) and the Cat. Problem with either or both can change you mileage that drastically especially on a stock. Now if it was mine, I would just blame all the heavy Body armor I have been installing.
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Old 03-19-2012, 02:40 PM   #6
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Thanks everyone for your responses. Looks like I"ll start with my o2 sensor, easiest to replace. is the upstream sensor the only one or should I look at or the front sensor?
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Old 03-19-2012, 02:47 PM   #7
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Originally Posted by reelchief
Thanks everyone for your responses. Looks like I"ll start with my o2 sensor, easiest to replace. is the upstream sensor the only one or should I look at or the front sensor?
Or you can take it to the auto parts store and hook it up to a scanner. Oreilly should have something you can use.
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Old 03-19-2012, 03:07 PM   #8
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Would removing the cats change mpg?
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Old 03-19-2012, 09:13 PM   #9
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Hey reelchief,my 97, 2.5 not doing reel good either,would appreciate it if u would let me know if u get yours doing a lot better, and what the problem was, Thanks!
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Old 03-19-2012, 09:45 PM   #10
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So i hooked it up to a diagnostic machine and i am getting a P0171, System too lean bank 1. I also notices ALOT of black carbon on the end of my exhaust. and puff of black smoke out the pipe when you hit the gas(telling me its running rich). So would a bad upstream O2 sensor cause all this?
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Old 03-19-2012, 10:26 PM   #11
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Originally Posted by reelchief
So i hooked it up to a diagnostic machine and i am getting a P0171, System too lean bank 1. I also notices ALOT of black carbon on the end of my exhaust. and puff of black smoke out the pipe when you hit the gas(telling me its running rich). So would a bad upstream O2 sensor cause all this?
It definitely can.... Since the sensor is reading lean the computer is going to richen the a/f mixture to try and compensate for it
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Old 03-20-2012, 08:38 PM   #12
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so i replaced the upstream O2 sensor and it ran great didnt get the P0171 code and it wasnt blowing smoke when i accelerated. then after about 20 miles of driving i noticed it started idleing rough again and blowing smoke when i with the gas. One thing i did when i replaced the sensor was unplug the negative from my battery so i am thinking I reset the computer and thats why it was running good at first then went to shit. so what other sensors control air fuel mixture and would cause the engine to run rich?
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Old 03-26-2012, 01:04 PM   #13
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After doing some reading and some suggestions i replaced the Bosch O2 Sensor i put in with a NTK one and I"m still getting 9 MPG and running rich. I checked the exhaust manifold for visible cracks or leaks but i couldnt see anything. does anyone have a way to check for an exhaust leak between the Cat and exhaust manifold?
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Old 03-26-2012, 01:36 PM   #14
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Originally Posted by reelchief View Post
After doing some reading and some suggestions i replaced the Bosch O2 Sensor i put in with a NTK one and I"m still getting 9 MPG and running rich. I checked the exhaust manifold for visible cracks or leaks but i couldnt see anything. does anyone have a way to check for an exhaust leak between the Cat and exhaust manifold?
A quick way is to have someone fold up a towel and use it block off the tailpipe for a second or two while you listen for leaks. You may have to lay underneath slightly, so make sure you have your e-brake on and scotch your wheels.
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Old 04-12-2012, 03:25 PM   #15
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I just wanted to follow up incase some searches the forum with the same problem I had. It turns out that after I replaced the Bosch Sensor with an NTK one I still needed to reset the computer. Once I unplugged the battery and reconnected it I started gettign 14-16 MPG thru 2 full tanks, Not great but ALOT better then the 8-9 I was previously getting. I plan on dropping the fuel tank this weekend and checking the condition of the fuel filter. The jeep sat in a garage for 2 years so I am hopeing a little TLC and maintenance will help get it running more efficently. I would like to get up to the 20-22 MPG range that other 4cyl stock jeeps are getting. Thanks for all the help and someone should really tell Bosch to fix their O2 sensors
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Old 04-12-2012, 05:42 PM   #16
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alot of it will depend on 2 things-your driving habits and the terrain. lead foot and/or alot of hills won't do your mileage an good. i would also pop a plug and see what it looks like(maybe clean/regap em) also check your tire pressure. a lot of little things can, and do add up. i get any where between 18-21 mpg-usually depending on my foot.

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