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Old 12-03-2013, 11:07 PM   #1
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Fuel efficiency issues

Hey guys,

To start, I have a 1997 2.5L 4cyl wrangler. About 162k miles. As with a lot of tj's, my gas gauge isn't always reliable, so I judge my fuel based on the odometer. I can go about 200-215 on a tank.

For the last 2 or 3 weeks, the jeep can barely roll over 150 miles on a tank. I haven't done anything differently, driving wise. Any ideas as to what the issue may be? I did notice a little...thing...under the hood that looks like it's spews a little bit of fuel every couple of days, but it doesn't seem to be enough to warrant such a change! I'll get out and snap a few pictures tomorrow.

Just wondering if this is a common problem orrrrr.....??
Thanks!

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Old 12-03-2013, 11:14 PM   #2
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Kevin063:

Winter formulation of fuel.

--Kevin

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Old 12-04-2013, 12:47 AM   #3
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Possibly a bad upstream 02 sensor. That sensor checks the air-fuel mixture & it can go bad and starts erroneously indicating too lean of an air/fuel mixture. That will cause the computer to start adding more fuel trying to correct the ratio which was really ok to begin with. Like spark plugs, 02 sensors need replacing at regular intervals.
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Old 12-04-2013, 11:43 AM   #4
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change your air filter to a paper one maybe clean your throttle body before anything these are fairly easy to do before you get into more diffcult things.
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Old 12-04-2013, 02:56 PM   #5
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I would be more worried about something under the hood that seems to be spewing fuel than a drop in mileage. That's not normal.
Wouldn't a bad O2 signal the check engine light?
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Old 12-15-2013, 06:06 PM   #6
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My check engine light is literally ALWAYS on...I thought that just might just be a thing jeeps do (-__-) I'm kinda stupid....
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Old 12-15-2013, 09:42 PM   #7
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Quote:
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Possibly a bad upstream 02 sensor. That sensor checks the air-fuel mixture & it can go bad and starts erroneously indicating too lean of an air/fuel mixture. That will cause the computer to start adding more fuel trying to correct the ratio which was really ok to begin with. Like spark plugs, 02 sensors need replacing at regular intervals.
Will a bad upstream sensor trigger the CEL? Also, any idea on how "regular" that interval is (in miles)?
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Old 12-15-2013, 09:53 PM   #8
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Fsm says to do o2 sensors at 85k and a upstream wont always trigger the cel.
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Old 12-15-2013, 10:12 PM   #9
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Fsm says to do o2 sensors at 85k and a upstream wont always trigger the cel.
Hmm... I wonder if that's why I'm getting about 10.5 mpg on 3.73s and (roughly) 32's.

They're cheap enough to swap out. I'm gonna see if I can run some diagnostics on them first with my scan tool.
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Old 12-16-2013, 09:26 AM   #10
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I have the same era of Tj as the OP with the 2.5l and I get around 13mpg. I replaced both o2 sensors, replaced spark plugs in a quest to improve mpgs and didn't see an improvement on my gas mileage. Perhaps the air filter swap might help, doubtful you'll see much improvement from that though.
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Old 12-16-2013, 11:25 AM   #11
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That fuel economy doesn't sound good at all. My 2.5 with 31s and 4.10 (I'm pretty sure) is right around 18.5mpg. I'd think something else is either drinking up the gas or you're losing some along the way.
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Old 12-16-2013, 12:57 PM   #12
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Should both bank 1 sensors be showing the same voltage spikes and valleys? I read that the sensor will constantly bounce back and forth between .1 and .9 volts as it does it's sensing. I assume both top sensors would do this. If so, I think I found a bad sensor.
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Old 12-16-2013, 01:34 PM   #13
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Both upstream sensors should be going back and forth. The downstream sensors should be staying relatively constant (this tells you the CAT is working)

From what I understand you should have a bank 1 and a bank 2 sensor that bounce back and forth, as well as one bank 1 and one bank 2 sensor that stays relatively constant.
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Old 12-16-2013, 01:36 PM   #14
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Bank is relative to the cylinder numbers. On a v8 bank one might be left side of the motor, bank two the right side. So:

Bank 1, Sensor 1 should be upstream bank one
Bank 1, Sensor 2 should be downstream bank one
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Old 12-16-2013, 01:52 PM   #15
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1x1 is bank 1, sensor 1 = upstream.
1x2 is bank 1, sensor 2 = downstream.

if you don't have the 00-06 4-sensor & 3-cat system, you've only got one bank, two O2 sensors & one cat.

the upstream should bounce between .1 and .9V. The downstream should remain relatively level.
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Old 12-16-2013, 02:24 PM   #16
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Whoops. Thought bank 1 was both upstream sensors and bank 2 was both downstream.
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Old 12-16-2013, 09:56 PM   #17
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B1s1 looks wrong. The cross count should have no flat spots. Not saying thats the sole issue but that looks like a good sensor to replace. B2s2 is showing that the mixture is on the rich side, should be in the middle of the graph relatively flat. But a bad upstream will cause the down to read off. Id replace the upstream with a ngk o2 sensor.
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Old 12-17-2013, 12:21 AM   #18
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B1s1 looks wrong. The cross count should have no flat spots. Not saying thats the sole issue but that looks like a good sensor to replace. B2s2 is showing that the mixture is on the rich side, should be in the middle of the graph relatively flat. But a bad upstream will cause the down to read off. Id replace the upstream with a ngk o2 sensor.
Thanks for that info. I'm not too sure on how to read the data I'm looking at. From what I've read, I thought the 1x1 sensor looked ok. I'll post a pic of both upstream and both downstream, side-by-side tomorrow.
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Old 12-17-2013, 06:27 PM   #19
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A good o2 should look close to this http://images.picoauto.com/titania.png
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Old 12-17-2013, 06:47 PM   #20
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I only had 1 upstream o2 sensor(federal emissions). I have the split manifold as well. I actually pm'd Jerry to see what he thought about my crappy idle and shitty mpg's. Replaced the upstream o2, reset the pcm per fsm, and problem solved. I had no cel. Just a random misfire at idle, and steadily decreasing mpg's. Btw thanks Jerry, she's running great
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Old 12-17-2013, 06:50 PM   #21
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That sensor doesn't look bad to me. The flat spots are just because of the poor resolution of his monitoring device (looks like bluetooth OBDii to a phone). The computer takes a split second to adjust to the sensor value and add/remove fuel...it's not going to instantly go back and forth between hi/lo
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Old 12-17-2013, 07:09 PM   #22
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Quote:
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Should both bank 1 sensors be showing the same voltage spikes and valleys? I read that the sensor will constantly bounce back and forth between .1 and .9 volts as it does it's sensing. I assume both top sensors would do this. If so, I think I found a bad sensor.

What app is that you're using?
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Old 12-17-2013, 08:48 PM   #23
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What app is that you're using?
It's Torque, only available for android. And I'm using a cheapie Bluetooth obd2 reader off of amazon.
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Old 12-17-2013, 09:44 PM   #24
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Took this sitting idle
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Old 12-17-2013, 09:45 PM   #25
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Thanks!


They make phones besides Android?
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Old 12-17-2013, 09:58 PM   #26
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Thanks! They make phones besides Android?
Yeah they make this kind of phone that goes the whole day without needing the battery to be pulled. It's called an iphone!
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Old 12-18-2013, 12:45 AM   #27
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Lol!
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Old 12-18-2013, 10:53 AM   #28
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Should the sensors be changed by mileage or time, or just when the fuel mileage changes or engine light comes on?
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Old 12-18-2013, 10:59 AM   #29
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02 sensors are like spark plugs, I'd replace them per the schedule. And a bad 02 sensor rarely turns on the CEL, especially on older TJs where a bad 02 sensor is much harder to detect.
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Old 12-18-2013, 02:14 PM   #30
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I replaced the second upstream one a while back. Maybe I'll change the first just for grins.

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