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  #1  
Old April 18th, 2011, 19:43
blistovmhz blistovmhz is offline
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Another boring MPG thread

98 XJ - 5spd. 4.5" lift on 30"/10.5", 4.10 gears.

I've got my OBDi port plugged into a bluetooth adapter which is feeding data to my phone. According to my phone, I am averaging 18MPG but when I fill up it's closer to 12MPG.
The O2 sensors both seem to be working fine. Backported and both bounce between 0.1-0.8V. Air filter is clean, just changed the oil, not throwing any codes.
The oil in my gears/trans/tranny is probably pretty old and I'm sure it could use an oil change, but I doubt that's goona give me 6-8MPG. Now I know most guys say 12-15MPG with my setup, but I've had 6 Jeeps and they all averaged between 18-22MPG on the highway. I am absolutely positive of this as I drive from Edmonton to Vancouver (1100km) on exactly two full tanks of fuel (Pulling into Van on fumes). This puts me at between 8.5 and 9.3kpl (19.99-22MPG). Of the 5 Jeeps I've driven to Van, only one required more than 2 full tanks (150L).
Doesn't seem to matter what grade of fuel I run. Exhaust doesn't smell rich or lean.

Why am I getting 12MPG? What should I check? Assume I've checked nothing as I've gotta be missing something obvious at this point (unless that gear oil will hurt by 6-9mpg).

Last edited by blistovmhz; April 18th, 2011 at 19:54.
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Old April 18th, 2011, 20:15
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Tim_MN Tim_MN is offline
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Re: Another boring MPG thread

So which is wrong, your phone app or your mathematic calculations ?

Did you install a new speedo gear or are you correcting the odometer readings for the larger tires ?

4.10 gears are a bit tall for 30's.
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Blanket caught in the rear liftgate latch. Now I can't open.
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Last edited by Tim_MN; April 18th, 2011 at 20:24.
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Old April 18th, 2011, 20:25
blistovmhz blistovmhz is offline
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Re: Another boring MPG thread

Clearly the phone. 300km to 54L is 5.5kpl no matter what my phone says .
The odd thing though is that the phone is grabbing OBDi data, which would suggest that the data is wrong (or the application is calculating incorrectly).

Either way, exhaust smells good, jeeps got the power I expect (same as all my other jeeps). My 90xj with 500,000km and my 92 xj with 450,000km would do 9kpl easily. Edmonton to Van on under 2 full tanks. Something's horribly wrong here .

I immediately suspect my O2 sensor, but backporting it shows good voltage fluctuation. That said, it could still be throwing bogus data, but again, exhaust smells good.
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Old April 18th, 2011, 21:35
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Tim_MN Tim_MN is offline
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Re: Another boring MPG thread

Suspect a lazy O2 sensor or a failing Coolant Temperature Sensor.

A lazy O2 sensor would be slow to respond or could provide incorrect data. A lazy CTS would provide bad data and the PCM would cycle in and out of the warm up mode, either one being lazy or on the edge of failure would use extra fuel by running rich.

Neither failure will trigger a Check Engine Light until the sensor is faulty enough to stay out of the normal data range for and extended period of time.
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Blanket caught in the rear liftgate latch. Now I can't open.
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Old April 19th, 2011, 00:11
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heyhar heyhar is offline
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Re: Another boring MPG thread

X2 on the CTS. I replaced it on my '96 a few weeks ago, and within a week, MPGs were noticeably better. For a week. Tested it again, new one took a dump. Another new one, this week it's back up. When I tested it, I also tested the voltage coming from the computer. Five point something, so I guess not too bad? Anyway, try the CTS. If it works, it will give you back its cost in a few weeks.
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