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Old 09-20-2013, 10:53 AM   #1
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Absolutely Horrible Gas Mileage

So I have a 1998 4.0 Auto TJ. I was getting 17 highway last year, 15 average. This year I'm driving back and forth from Maryland to Pittsburgh for school in my Jeep and I'm averaging 11 miles per gallon, all highway miles.


...this is ridiculously low.

What could be the cause of my problem if you were to guess?


(I had a gas leak, but that has been fixed. Need new tires but that wouldn't kill me MPG by that much).

Thank you for your help.

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Old 09-20-2013, 10:58 AM   #2
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just a shot in the dark I would say O2 sensors. Replace with NTK brand ONLY!!!!

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Old 09-20-2013, 11:01 AM   #3
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I would do a tune up. See if that helps. Then go to o2 the maybe TPS.
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Old 09-20-2013, 11:09 AM   #4
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Check your tire pressure too, won't be the main cause but as it's getting colder it could be a MPG or two.
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Old 09-20-2013, 01:47 PM   #5
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Your '97 has two 02 sensors and only the upstream 02 sensor (located just below the header) could cause that issue. In other words the downstream 02 sensor on the catalytic converter doesn't need to be changed. The odds the upstream 02 sensor being the cause of your suddenly poor mpg are very good.
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Old 09-20-2013, 04:35 PM   #6
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TUNE UP TIME!
those o2 sensors cost an arm and a leg.... mine has 4 do the cheap stuff first.
were you driving with the top off?
that will significantly drop your mpg too.
when I got my Jeep I got somewhere around 12 mpg and after all the tune ups i started getting 16 combined steady
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Old 09-20-2013, 04:50 PM   #7
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Quote:
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those o2 sensors cost an arm and a leg.
$33 for the upstream sensor Jerry recommended changing (or $66 for both) isn't what I'd consider costing an arm and a leg.
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Old 09-20-2013, 05:02 PM   #8
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Quote:
Originally Posted by Jerry Bransford View Post
Your '97 has two 02 sensors and only the upstream 02 sensor (located just below the header) could cause that issue. In other words the downstream 02 sensor on the catalytic converter doesn't need to be changed. The odds the upstream 02 sensor being the cause of your suddenly poor mpg are very good.
Just wondering, would the o2 sensor cause the CEL to come on/throw a code?
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Old 09-20-2013, 05:06 PM   #9
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wow, mine cost 250-300 for all 4...
here is the thread I had on mpg.
http://www.wranglerforum.com/f5/20-m...tj-239322.html
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Old 09-20-2013, 05:17 PM   #10
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wow, mine cost 250-300 for all 4...
here is the thread I had on mpg.
http://www.wranglerforum.com/f5/20-m...tj-239322.html
Did you purchase from the dealer? All 4 NTKs from rockauto for my '06 are less than $100.
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Old 09-20-2013, 05:45 PM   #11
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Old 09-20-2013, 05:56 PM   #12
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I'm seeing $70.99 at Advance Auto for the NTK upstream sensor. They run some coupons to bring the price down a bit though, maybe a $20 off $50 or something like that.

What kind of MPGs can we expect on a good day? I'm seeing about 15 mpg around town driving with some highway.
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Old 09-20-2013, 06:08 PM   #13
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Rockauto.com has the best prices on NTK o2 sensors...
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Old 09-20-2013, 06:51 PM   #14
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I get an average of 12 with 90% of that being hwy. I have noticed a slight rattle when I first start her up. Only had her a few months, purchased from dealer. I'm thinking I'm going to start with the plug change and judging from the rattle may be time for a cat replacement sooner than later. Ugh. I sure wish I could see 15 MPG Avg. I'd be happy with that I think.
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Old 09-20-2013, 07:17 PM   #15
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No use to you but $125 for your four NTKs from rockauto.

Quote:
Originally Posted by BTroll
They run some coupons to bring the price down a bit though, maybe a $20 off $50 or something like that. I'm seeing $70.99 at Advance Auto for the NTK upstream sensor.
Quote:
Originally Posted by wwch99tj View Post
Rockauto.com has the best prices on NTK o2 sensors...
^ this

Quote:
Originally Posted by BTroll
What kind of MPGs can we expect on a good day? I'm seeing about 15 mpg around town driving with some highway.
17ish
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Old 09-20-2013, 07:29 PM   #16
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I think these guys really hit the nail on the head with their diagnosis of the o2 sensor. Happen to me on a blazer one time. Buying fuel became an ever 2 day thing until I changed the sensor.
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Old 09-20-2013, 07:37 PM   #17
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My gas mileage is also poor and the exhaust pipe tip is sut black. Would that be a sympton of an O2 sensor problem?
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Old 09-20-2013, 08:04 PM   #18
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I have a crazy idea. Diagnose the problem and fix it. Throwing parts at it and crossing your fingers works sometimes but it sure sucks when it doesn't work.

Check for pending and hard codes to help direct your repair. An o2 sensor response test takes 5 minutes with a scanner. Fuel trim readings will tell you how much (if any) extra fuel is being added (or subtracted).

If you need help processing the data let me know. Ot just post it up and I'm sure people will help
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Old 09-20-2013, 08:40 PM   #19
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Just wondering, would the o2 sensor cause the CEL to come on/throw a code?
Not usually since the PCM has no way to verify what the 02 sensor is telling it on early two 02 sensor systems.

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TUNE UP TIME!
those o2 sensors cost an arm and a leg.... mine has 4 do the cheap stuff first.
were you driving with the top off?
that will significantly drop your mpg too.
when I got my Jeep I got somewhere around 12 mpg and after all the tune ups i started getting 16 combined steady
Fine, your '01 requires four 02 sensors which is not even close to the two 02 sensor system the OP's '98 uses. Apples & oranges. His '98 has only two 02 sensors and only ONE of them needs replacing, the upstream 02 sensor, because that is the only 02 sensor on an early TJ that has anything to do with the air-fuel mixture that is causing his problem.

So you're comparing apples to oranges and confusing the OP by trying to convince him to not replace the upstream 02 sensor as was recommended (which was already carefully explained above) based on your newer model TJ's completely different four 02 sensor system where all four 02 sensors can affect the air-fuel mixture.

Bottom line? His '98 system only needs one upstream 02 sensor... not four as your '01 sometimes requires.
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Old 09-20-2013, 09:04 PM   #20
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ok, apples to oranges... fine.

go through your tune up stuff and make sure its working properly.
If you don't have to spend any money to do the job, that would be best right?
-check the gap on your spark plugs.
-check your air filter, maybe it just needs to be blown out.
-make sure your brakes are opening properly.
-make sure your tires are inflated properly.
all this costs no money.
<do the cheap stuff first> before going out and busting your back on replacing frozen sensors.
<<next comment>> yes, maybe they aren't frozen, and yes, maybe it would be easy or cheap for you to do... ok do them...
sooner or later I will learn to word my comments so as not to have any quarrels.
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Old 09-20-2013, 10:01 PM   #21
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Sometimes its better to refrain from commenting!!
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Old 09-20-2013, 10:10 PM   #22
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Chaddg has some valid points.. Checking the basics like tire pressure and brakes dragging only takes a few minutes and rule out some basic issues.
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Old 09-20-2013, 10:53 PM   #23
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A dramatic sudden loss in mpg like the OP described is nearly always caused by a bad 02 sensor.
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Old 09-20-2013, 11:20 PM   #24
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If your o2 sensors are fowling out you'll get a code, don't spend the money until the check engine light comes on and tells you their bad. Not sure if that year model has an in line fuel filter or if its in the tank, change if applicable. Next change your air filter and clean your mass air flow sensor, surprisingly just to MA sensor can give you back 1-2 MPG. Any auto part store should sell a can of Mass Airflow sensor cleaner, it's about 10 bucks and will last you a long time. It doesn't take much. Then pull your plugs and inspect them. Make sure your plug wires aren't grounding out by touching a spot on the block and have melted through. Run a bottle of fuel injector cleaner through your tank with some high octane fuel to really clean off any crud that could built up on your injectors. These are relatively inexpensive things to do and check before spending allot of cash for something that your jeep hasn't even told you is not work yet.
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Old 09-20-2013, 11:48 PM   #25
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Both my 02 are bad, I get 15 in town
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Old 09-20-2013, 11:57 PM   #26
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If your o2 sensors are fowling out you'll get a code, don't spend the money until the check engine light comes on...
As already said several times above, that isn't usually true on older two 02 sensor system TJs like the OP has. An older TJ like his seldom trips a DTC code when an 02 sensor goes bad... the pcm isn't smart enough on older TJs to do that reliably.
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Old 09-21-2013, 12:50 AM   #27
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I agree. The O2 sensor is the most likely suspect in this case based on the info that has been provides by the OP.

however...

A simple review of some readily available scantool data would prove it. Also. Once you have the data you can compare the readings after the repair has been performed to confirm the condition had been corrected.

I'm starting to feel like a broken record on all of these repair threads. I cannot understand why so many "DIY" repair guys refuse to invest a couple hundred bucks in a descent scantool. I bet most of you guys have toolboxes full of thousands of dollars worth of handtools. Spend a few hours online learning the OBD2 basics instead of reading repair threads full of bad info provided by unconfirmed sources. Embrace electronics in your cars or buy something old and carbureted and enjoy the horrible cold starts and constant adjustments/problems.
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Old 09-21-2013, 01:45 PM   #28
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Pssst..... don't forget to read Jerry reminding me of these buggars not having any mass air flow sensors. He is right. thanks! I don't mind being corrected when I'm wrong.
I also do not mind putting my 2 cents in wherever I feel the need. <1st amendment> I love this site! I learn from it, respect it, almost live on it.... check out my cot..... <<<<<HA>>>>
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Old 09-21-2013, 02:45 PM   #29
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What could be the cause of my problem if you were to guess?
Oh, this is an easy one. The answer is: It's a Jeep. If you want great gas mileage, get a Japanese compact. If you want the best off-road vehicle in the world, get a Jeep. NEXT!

"My Jeep won wars, your Honda moves my lawn."

We don't need no friggin' gas mileage, its all about attitude and purpose.
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Old 09-21-2013, 02:55 PM   #30
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Oh, this is an easy one. The answer is: It's a Jeep. If you want great gas mileage, get a Japanese compact. If you want the best off-road vehicle in the world, get a Jeep. NEXT!
To me, that was not a helpful comment at all. When someone's mpg makes a sudden dramatic drop, that's not normal & not just a Jeep thing.

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