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Old 11-30-2013, 09:49 PM   #1
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Angry 10.5 mpg?!!!

So first off I just look at my VIN and found out I have a 97 not a 98 wrangler (my bad) Talk about feeling DUMB!!!
Now my 97 Wrangler has been getting 10.5-11.8 mpg. I like others did not buy a Jeep for fuel mileage but come on!!! I have been checking this through 6 tanks of fuel. I do not consider myself as a lead foot by any means and %90 of my driving is on the highway with a 24mile round trip each day at 60mph. some things I have noticed that may or may not be related are longer then normal crank time to start the jeep and what I would call a fuel knock under acceleration. the previous owner of this Jeep did install a new cap plugs and wires.
Air filter is new and I guess I do not know where to take my diag next. I have searched on here and read people getting anywhere from 13-20mpg
4.0 engine
auto trans
stock axles and 3.55 gears according to tab on rear axle
31X10.5XR16LT tires

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Old 11-30-2013, 10:00 PM   #2
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Hate to say it, but I don't know anyone with a 4.0 getting anywhere close to 20 mpg. My normal is 8-11 mpg's with my 05 LJ. You also have larger than stock tires, with gearing that is pretty low. That definitely isn't going to help, especially on the highway.

Like most others say, if you bought a Jeep for the gas mileage, take it back and get something else.

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Old 11-30-2013, 10:04 PM   #3
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In my 2001 4.0 auto with 33's and 3.55's I'm averaging about 10 mpg 😐
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Old 11-30-2013, 10:06 PM   #4
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Have you ever changed the upstream 02 sensor? That is what the computer relies on for accurately setting the fuel mixture and a bad 02 sensor can erroneously continuously report an excessively lean mixture. That would cause the computer to continully dump excessive fuel into the engine to try to get the mixture back to where it wants it. That of course would wreck the mpg.

No guarantees but if your power is down too, the upstream 02 sensor (located immediately below the exhaust header) would be high on my probability list.
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Old 11-30-2013, 10:10 PM   #5
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I am going to look at the speedo gear to see if I have the right on. Found a very helpful right up with a chart for selecting the right gear. I was hoping for about 15mpg. how about the fuel knock? I don't know how else to explain it but I can really hear it under acceleration...
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Old 11-30-2013, 10:12 PM   #6
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Have you ever changed the upstream 02 sensor? That is what the computer relies on for accurately setting the fuel mixture and if it's bad, it can erroneously continuously report an excessively lean mixture. That would cause the computer to continully dump excessive fuel into the engine to try to get the mixture back to where it wants it. That of course would wreck the mpg.

No guarantees but if your power is down too, the upstream 02 sensor (located immediately below the exhaust header) would be high on my probability list.
I installed that just a few months ago last owner cracked it when installing the cat back Banks exhaust.
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Old 11-30-2013, 10:19 PM   #7
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When is the last time you changed you spark plugs?

How about giving your throttle body a good cleaning?
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Old 11-30-2013, 10:23 PM   #8
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Are you sure the 98 you have wasnt built in 97?

Sometimes the mfg will put the month and year on the door panel. I had a windstar that was a 2001, but the dom was 11-00.
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Old 11-30-2013, 10:29 PM   #9
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When is the last time you changed you spark plugs?

How about giving your throttle body a good cleaning?
Spark plugs were changed by the last owner over the summer along with the cap
Throttle body appears to be clean
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Old 11-30-2013, 10:58 PM   #10
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Quote:
Originally Posted by 98fargojeep View Post

Spark plugs were changed by the last owner over the summer along with the cap
Throttle body appears to be clean
I'd pull a plug and check...the Wranglers can be picky on plugs
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Old 11-30-2013, 11:19 PM   #11
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Something definitely isn't right that's way too low with those gears and tires.I still average 13-14mpg with 3.73 and 33s even with a large evap leak.
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Old 11-30-2013, 11:58 PM   #12
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Quote:
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Something definitely isn't right that's way too low with those gears and tires.I still average 13-14mpg with 3.73 and 33s even with a large evap leak.
You're lucky. I have the same gears and 32's (they've worn down) and I barely squeak out about 10.8. Mine is an auto and I don't drive with a heavy foot.
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Old 12-01-2013, 12:28 AM   #13
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Check your speedometer against a GPS and see if it is reading right. You may be getting better gas mileage than you think, and the math is just off because of faulty readings on your speedometer.

Also, the date on the door tag is the manufacture date and not the year model. Remember, most new model year cars came out about September of the previous year, so they were in production before that.
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Old 12-01-2013, 08:12 AM   #14
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I would use your odometer against the amount of fuel you put in to figure MPG.
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Old 12-01-2013, 08:15 AM   #15
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I have a 2001 4.0 TJ and was getting close to 10 mpg. I changed the plugs, all 4 O2 sensors, cleaned the throttle body, changed the TPS, and now I get 16 mpg. YMMV!
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Old 12-01-2013, 08:45 AM   #16
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Don't fall for any CAI, Chips, etc upgrades. They are all snake oil products on TJ's. Do a search on this forum and you will understand.
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Old 12-01-2013, 08:55 AM   #17
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I changed my plugs and O2 sensors since I was installing new cats, headers, and exhaust and what a difference that made in how the engine runs. The engine is burning cleaners and the whole power band is smoother. I am averaging 13ish mpg which is good in my book.
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Old 12-01-2013, 09:13 AM   #18
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i get 15mpg most weeks, got 19 on the way to tuttle creek (4 hour drive very few stops). 05 tj, 6 speed 32" tires

maybe i should quit whining about 15mpg during the week? haha
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Old 12-01-2013, 09:22 AM   #19
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When I was commuting 100 miles a day RT I was able to get 19 MPG on a regular basis driving like the old man I am.

Now that I was downsized and semi retired and only drive around town I stopped looking and when the low fuel light comes on I stop for gas. My guess is I am getting around 12 around town.
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Old 12-01-2013, 09:37 AM   #20
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05 TJ, 4.0, 6spd, 3.73, 31x10.5x15's, getting 16 - 17 without fail in mixed driving.
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Old 12-01-2013, 10:01 AM   #21
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First as others posted model year is a vin diget not the date of manufacture a specific model year starts production several months early so the new models can hit the showrooms about Oct of the previous year

Not absolute but 97 has 3 groups of cowl vent openings behind hood and in front of windshield and 98 up has only one group of cowl openings
They fixed a leak prone system that way ( or at least improved it)

Your mileage is poor but believe able and the tune up and checks suggested by Jerry are a good idea. However you have a 3 SPd non Od auto and with those gears you will run higher then ideal mpg rpm at highway speed
Lock up torque converters help mpg but they are not an Od and all the manuals and all the newer autos have an Od

The good news is that 3 SPd you have is the most reliable auto put in a TJ.

My younger daughter has a yj 4.0 HO with the same 3 SPd auto and damn near 200k trouble free miles

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Old 12-01-2013, 10:17 AM   #22
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The eighth digit from the right of the VIN is the official year.
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Old 06-11-2014, 03:26 PM   #23
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If I turn OD off every time for city driving would that help my gas mileage? 04 TJ 4.0 Automatic. 33x12.50 tires, 3" suspension lifet, don't know gears. I think I am getting 10 mpg.
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Old 06-11-2014, 03:30 PM   #24
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What are all 4 O2 sensors?
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Old 06-11-2014, 03:31 PM   #25
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What are all 4 O2 sensors?

Quote:
Originally Posted by WranglerPete View Post
I have a 2001 4.0 TJ and was getting close to 10 mpg. I changed the plugs, all 4 O2 sensors, cleaned the throttle body, changed the TPS, and now I get 16 mpg. YMMV!
What are all 4 O2 sensors?
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Old 06-11-2014, 03:38 PM   #26
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I get about 16-18 mpg.

Pretty sure I can squeeze another mpg if I get my rough idle figured out
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Old 06-11-2014, 04:01 PM   #27
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What are all 4 O2 sensors?
2003 and later have a "50 State" compliant emissions system, which uses two pre-catylitic converters and one large, main one. Also, instead of two oxygen sensors it uses four, two before the pre-cats and two after.

This is how the pre-2003 TJs that had the "California Emissions" compliant system were set up after a certain year, but I do not know which. Eventually this became the norm, so all of them got it and the name changed to "50 State" compliant.

The older version that was the norm had one cat and two O2 sensors. It was easier to work on - fewer parts to crap out on you...
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Old 06-11-2014, 04:10 PM   #28
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Fargo, I have a 2003 Sport 4.0L and get about 17 mph on the US highway (at a steady 60 mph) and less on the Interstate (at a steady 75 mph).

I have the P0171/P0174 codes that say I have a lean condition in both banks, and cannot find the problem. It seems to be an intermittent thing, though. I get that very decent (for a TJ) mileage about half the time.

The rest of the time I average about 11 or 12 mph on the Highway or Interstate. So maybe that is your problem. You could have an intermittent vacuum leak, perhaps a small manifold crack or a gasket leak. This could trigger a lean condition response (having the computer feed in wasteful amounts of gas to compensate for a lean condition).

I have replaced all four of my O2 sensors TWICE this past year and the condition is only worsening. So I am guessing that my exhaust manifold or its seal to the engine is compromised.

Hope not. But there it is.

Have you had a smoke check done to your vacuum system?

And for anyone who is reading, can a smoke check be done to test the manifolds for leaks? Anyone???

Good luck, Fargo!
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Old 06-11-2014, 04:21 PM   #29
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Check your coolant sensor...that will cause lean/rich conditions too and not always throw a CEL Light......The O2's base themselves on that sensor....
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Old 06-11-2014, 10:13 PM   #30
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i get 15-19 on average with an 03 x 4.0.
130k
30x9.5 tires

when i purchased was getting less than 10mpg

between a combination of the following: (PO did no maint.)
Bosch 4 hole fuel injectors
Autolite XP iridium plugs
Throttle body spacer - yes, yes, i know its supposedly does nothing.
massive throttle body cleaning
new IAC (Idle Air Controller)
changed fluids in both diffs
changed fluid in transfer case

each one may not make a noticeable difference but together it was very very noticeable.

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