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Old 11-14-2012, 04:12 PM   #1
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Gas mileage all time low

I have a 1999 2.5L 5 speed, my gas mileage has been about 55-60 miles for one quarter of the fuel meter. I have gotten a signifact drop I only recorded 40 miles per every quarter of the fuel meter. My Jeep is mainly stock except BB lift and 31x10.5 tires. My speedometer is not adjusted to the tires. My gears are stock. I shift at about 2500 RPM. I also have a small leak from tranny and transfer case. My tire pressure is 29-30. The Jeep does make funky idle sounds it drops from 1000 RPM when standing in place and gets as low as about 800. Any ideas why this may be happening? Anything I should check?

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Old 11-14-2012, 04:24 PM   #2
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Fuel meters on the Wranglers are so unreliable to be able to figure anything out with. Go by miles/gallons used but add back in for the % increase of your 31" tires over oem stock tire size...probably about 10-12%.

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Old 11-14-2012, 04:27 PM   #3
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Quote:
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fuel meters on the wranglers are so unreliable to be able to figure anything out with. Go by miles/gallons used but add back in for the % increase of your 31" tires over oem stock tire size...probably about 10-12%.
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Old 11-14-2012, 04:46 PM   #4
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I heard of cats getting clogged and that also puts the gas mileage down. I'll check out whats going on in there after work.
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Old 11-14-2012, 05:00 PM   #5
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I'm not sure if this is the best way to check mileage but this is what I do.

When the light and chirp sound letting you know you need gas I believe that sound goes off when there is a certain amount of gas in the tank....

So I will fill up to a certain amount of gallons from there and reset the milage and when the light and chirp sound comes on. I'll take the milage and divide by the gallons used the fill up on the last chirp...

Seem right for my set I get about 13ish for stop and go driving in traffic. Hard to get highway due to all the damn traffic here lol
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Old 11-14-2012, 05:14 PM   #6
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Easiest/most accurate way to calculate mileage..
1.) Fuel up to a full tank
2.) Reset trip odometer and/or record the mileage at the pump
3.) Drive untill you are below 1/4 tank
4.) Fuel up to a full tank again
5.) Divide the number of miles you went since previous fill up(e.g. 185 miles) by number of gallons used to fill up THIS TIME(e.g. 13 gallons)
185 miles divided by 13 gallons is 14.23 miles per gallon!

For more accurate numbers, do it for a longer period.
Fill up full and record mileage(e.g. 121,121) Drive around untill you need to fill up; write down how many gallons you needed to be full again. Then drive around until you need to fill up and again write down the gallons needed. Do this a third time and the divide the miles by total number of gallons, that way you can get a more average number..
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Old 11-14-2012, 05:26 PM   #7
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A bad 02 sensor is a common cause of a sudden loss of mpg. When an 02 sensor goes bad, it can erroneously report that the air/fuel mixture is too lean which causes the computer to keep making the mixture richer until it finally reaches full rich. That causes a loss of both mpg and performance. And like spark plugs, 02 sensors wear out and need to be replaced.
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Old 11-14-2012, 05:42 PM   #8
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A bad 02 sensor is a common cause of a sudden loss of mpg. When an 02 sensor goes bad, it can erroneously report that the air/fuel mixture is too lean which causes the computer to keep making the mixture richer until it finally reaches full rich. That causes a loss of both mpg and performance. And like spark plugs, 02 sensors wear out and need to be replaced.
question

Is the CEL the best way to determine to change or would you recommend changing after an interval of miles? I had light come on 2 times for 2 sensors I reseted the light and hasn't come back on in the past 2k miles.
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Old 11-14-2012, 05:44 PM   #9
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A bad 02 sensor won't necessarily light up the CEL. I don't recall the recommended 02 change interval but every 80K miles rings a bell. Two of my Toyota's 02 sensors died last month and it had just a hair under 80k miles on it.
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Old 11-14-2012, 06:01 PM   #10
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^ Ok thanks!
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Old 11-15-2012, 01:27 AM   #11
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I actually forgot to mention my check engine light is on...last time it was on it was the 02 sensor, I replaced it but I Heard there are 2 02 sensors and I only checked one of them?? I'll go to autozone tomorow and let you guys know what it was. Thanks for the help guys!
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Old 11-15-2012, 07:08 AM   #12
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Could be the winter blend of crappy fuel we are buying now. My TJ has lost about 2 mpg since the gas stations have switched to the winter blend. I did find some real gas at a Marina, paid out the a$$ for it but the TJ got 5 mpg better fuel economy.
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Old 11-15-2012, 11:54 AM   #13
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Quote:
Originally Posted by ob269 View Post
Easiest/most accurate way to calculate mileage..
1.) Fuel up to a full tank
2.) Reset trip odometer and/or record the mileage at the pump
3.) Drive untill you are below 1/4 tank
4.) Fuel up to a full tank again
5.) Divide the number of miles you went since previous fill up(e.g. 185 miles) by number of gallons used to fill up THIS TIME(e.g. 13 gallons)
185 miles divided by 13 gallons is 14.23 miles per gallon!

For more accurate numbers, do it for a longer period.
Fill up full and record mileage(e.g. 121,121) Drive around untill you need to fill up; write down how many gallons you needed to be full again. Then drive around until you need to fill up and again write down the gallons needed. Do this a third time and the divide the miles by total number of gallons, that way you can get a more average number..
This is only "fairly" accurate if you have the correct speedo gear that is for the tire size. Like I said above....if you run bigger tires and the speedo gear is for stock 205 or 225 tires, then the calc will be off. The 14.23 mpg would be more like 16ish if you had 31" tires.
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Old 11-16-2012, 08:49 AM   #14
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Quote:
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This is only "fairly" accurate if you have the correct speedo gear that is for the tire size. Like I said above....if you run bigger tires and the speedo gear is for stock 205 or 225 tires, then the calc will be off. The 14.23 mpg would be more like 16ish if you had 31" tires.
That is a very good point, I did not account for oversized tires..
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Old 11-16-2012, 01:50 PM   #15
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Hey guys thanks for the awesome help. So here are the codes for the check engine:
PO133
PO135
PO171
Looks like they're related about the 02 sensor, its the same sensor which I changed in mid July, the guy at autozone said I have the 1 year warranty on the product. So for now I will do the exhange of 02 sensors. It is a big chance it isn't the 02 sensor but its a start. Any other things you guys can tell me about these codes? Do I smell a tune up soon?
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Old 11-16-2012, 02:31 PM   #16
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The first two codes are referencing the O2 sensors. The third is throwing because the computer thinks your fuel mixture is too lean. This is measured (I believe) by the O2 sensors, as well. I would start with swapping out the O2 sensors. When I changed out the ones in my 97, they were fairly easy to get to, and only cost about $20. Also, if I remember correctly, Jeep had a catalytic converter recall in 2005, which affected my 97, so it would stand to reason that it may be affecting your 99. A clogged cat could cause you to lose power, mileage, and that flucuation in RPM's.
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Old 11-16-2012, 03:52 PM   #17
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This is how my 4 month old o2 sensor looked like, had a power steering leak for a few days and some how must have oiled up the sensor. Just put in the new o2 sensor. How do I know that its not the other o2 sensor thats in the rear end of the jeep?
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Old 11-17-2012, 11:17 AM   #18
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Sensors before the catalytic convertor are for sensing the fuel mixture and will throw the lean codes when they go out. This causes the poor mileage. The ones after the cats are for checking the cats themselves. These will throw codes for catalytic effeciency, but will not affect mileage.

*edit* - Some vehicles are pickier about O2 sensor brands than others. I don't know if Jeep has a preference or not. Maybe one of the resident experts can chime in on if this is a concern or not.
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Old 11-17-2012, 07:17 PM   #19
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I just recently changed my upstream O2 sensor in my 97 TJ w/ 2.5 L4. Jerry B. recomended the NTK model 23506 from Rock Auto. Shipped, it was less than $34. I didn't see what year and engine you have but RockAuto.com is pretty easy to use. BTW, NTK is the OE maker for these models of Jeeps so this is the best replcements for the job IMO.
Hope this helps.

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