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Old 10-21-2012, 01:42 AM   #1
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91 injected 2.5 only 12 mpg

I have a 91 yj 2.5 with 31" bfg all terrains and 4" lift. I only get about 12 mpg. And to boot my speedo reads 55 at 50. Soo it's prolly more like 11 mpg. I'm not sure why It could be so thirsty. That's around town and highway miles. What can I do to get better milage.? And what can I do to fix my speedo/odometer?

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Old 10-21-2012, 03:47 AM   #2
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To fix the spedo you need new spedo gear setup for your tire size and gear ratio. As for your fuel issue.
Have you done full tune up?
New O2 sensor?
What temp is your engine running?
Have you replaced your ECU temp sensor?

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Old 10-21-2012, 12:00 PM   #3
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I haven't done a o2 sensor. Just an oil change and an air filter. It doesn't seem to miss though, it has miss matched plug wires. Maybe I should try a cap rotor plugs and wires. How does the 02 sensor effect milage?
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Old 10-21-2012, 03:11 PM   #4
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After your engine warms up to temp 210 degree the ECU goes in to a close loop mode. If it doent reach 200-210 it will not go in to close loop mode and will burn extra fuel in attpempt to get to operating temp. When its in the close loop mode it relies on signal from the O2 sensor and adjust fuel/air mixture. When an O2 sensor fails its default is to run the engine rich to avoid damage to the engine. Its better to run your engine rich then lean. I had my O2 sensor go out and i was getting 8mpg after I replaced it I was back up to 14mpg.
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Old 10-22-2012, 11:55 AM   #5
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The engine runs at 210. Could I still need a o2 sensor?
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Old 10-22-2012, 12:34 PM   #6
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If you have a yj with an original O2 sensor than yes you do need one. I have a 95 with a 2.5 and I only get 16mpg on average which is pretty lame for a little 4 cyl! You can always pull of a vacuum line and stick the hose in a can of seafoam, it will suck it up like a straw, and if there is any room for improvement in the induction, you will likely see it in your MPG. I got +2 mpg after I did that trick 3 times and it also woke up the engine some.
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Old 10-22-2012, 05:37 PM   #7
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Here's a list. Go through and replace all of the parts:

-Vacuum lines
-Crank Case breather (dealer part only)
-Fuel filter
-Air filter
-Injector cleaner
-Remove your throttle body and scrub it clean
-Top-end cleaner
-O2 sensor

And because it's a good idea to do this every few years:

-Rear diff fluid
-Front diff fluid
-Transmission oil
-T-case fluid
-Flush/replace coolant
-New 195* t-stat (test it with a boiling pot of water)
-New radiator cap

You should be good to go from there.
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Old 10-22-2012, 11:38 PM   #8
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Thanks guys I will try these things. I have a feeling I'm gunna end up with gears or a different jeep 4.0...
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Old 10-23-2012, 12:25 AM   #9
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Quote:
Originally Posted by Yellowdude View Post
I have a 91 yj 2.5 with 31" bfg all terrains and 4" lift. I only get about 12 mpg. And to boot my speedo reads 55 at 50. Soo it's prolly more like 11 mpg. I'm not sure why It could be so thirsty. That's around town and highway miles. What can I do to get better milage.? And what can I do to fix my speedo/odometer?
the speedometer is reading 55 and your only going 50?...a speedometer will only read higher than the speed your going if you downsized your tires from what the speedometer was calibrated for.....either the speedo gear is really messed up, the linkage is bad, or whatever you used to determin what speed you were going was wayy off...i averaged 16-18 mpg with 31" super swampers under my 2.5L 5speed....and my speedometer was off just the opposite of yours, the speedo said 50, i was going 55-56.(used both the gps on my phone, and my brothers new dodge journey to confirm)
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Old 10-25-2012, 09:20 PM   #10
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I don't know why it reads high. Person before me must of had 35s with the stock gearing lol. It's a dog with 31s. Everything is cool except for any slight incline.

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