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Old 02-03-2011, 06:39 PM   #1
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Gas Milage Problem

Ok, so i know the common answer to the gas milage question(dont drive a jeep) but I'm getting really bad gas milage and would like to fix the problen soon. it has only been going on this week. I seem to be getting about 150 miles to the tank. my normal miles per tank is 220 to 240. keep in mind this is a 99 tj with lift and 33x12" tires.

I had an oil change about 3 weeks ago( and oil was still clean before it was changed), the air filter is clean, I pulled and checked all of the spark plugs today, they look fine and the gap is correct.


any advice on what to look at ???? how many miles per tank do you get ?? just seems like a big enough mpg change to throw the question out there...

Thanx, Prettyboytoys

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Old 02-03-2011, 06:52 PM   #2
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That is a big drop. Any codes? Any other symptoms? Stalling etc.? Tire pressure correct?

Could be a bad O2 sensor telling the PCM you're running too lean. That should eventually throw off a code though.

I get about 265 - 285 between fill-ups (usually takes about 16-17 gallons).

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Old 02-03-2011, 07:01 PM   #3
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yes Tire press is good at 35psi, yes stalling has been a little bit of an issue when I got to accelerate.....kinda strange it seems to stall as I'm taking off in first gear.( i have had this jeep for 4 months and havnt stalled it once untill yesterday....stalled twice on me

I havnt checked for a code yet,



to check for a code you hold it the milage reset button and rock the key 3 times right ?
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Old 02-03-2011, 07:02 PM   #4
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rtiburon, what size tires do you have ?? that seems like fantastic gas milage wow
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Old 02-03-2011, 07:18 PM   #5
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The tires are 30x9.5 w/no lift, it's a 97 Sahara with about 145k. It's my daily driver.

I'm not sure if the changed the OBDII system in later years but mine will display any codes in the odometer if I just do the key on/off thing. Here's a helpful link FAQ - TJ Error Codes - JeepForum.com
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Old 02-03-2011, 07:23 PM   #6
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Also, if the key trick doesn't work on yours, a lot of auto parts stores (I think Auto Zone) will let you check them right there in their parking lot with a reader.
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Old 02-03-2011, 08:09 PM   #7
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ya, i checked for codes but nothing, the key thing worked but no codes
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Old 02-03-2011, 08:50 PM   #8
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Shoot. Well, you could also have an exhaust leak which might fool a good o2 sensor into reading a lean condition. You mentioned you checked the plugs, how are the cables good still?

Another possibility is that your thermostat valve is stuck in the open position, which will make your engine run cool way longer than usual. If this is it, it's great news because they're cheap and easy to swap out.

Hope this helps some.
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Old 02-03-2011, 09:11 PM   #9
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Funny that you mention this because I noticed the other day that I was getting really bad mileage with this last tank of gas. At first I thought that I may have reset the tripometer by accident, but I hadn't. Once I started thinking about what had changed, I realized that for the first time since I got my Jeep, I had to drive in 4wd (snow and ice) all week and I was letting it warm up longer than usual in the mornings. Is there anything similar going on with yours? Just a thought.
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Old 02-03-2011, 09:55 PM   #10
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it is cold here in traverse city and yes I do run in 4wd once in a while but this week has been the same as normal.
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Old 02-03-2011, 10:20 PM   #11
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if its auto, it could be a bad torque converter.

also, any vacuum leaks? all your vents working properly?

i'm having the same exact issue. I get 120 miles per tank, so about 8-9mpg.
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Old 02-03-2011, 10:26 PM   #12
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wow i you guys getting 250ish per tank are making me feel dumb. The only time i have ever gotten 250 was driving to NC from NJ all highway miles. Right now im averaging about 150 or so mixed higway and city. WTH? lol
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Old 02-03-2011, 10:27 PM   #13
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I have a 97 Sport, 31" tires, and my best is 180miles to a tank. 220-240 seems like a dream!
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Old 02-03-2011, 11:56 PM   #14
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i have 31" tires everything else stock. 4.0L automatic and I get 200 miles to a tank (took 14.5 gallons last fill up. so Im guessing i could have gone farther but the gas light was on for about 20 miles.
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Old 02-04-2011, 12:02 AM   #15
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I would try filling up with a higher octane fuel, and running a lucas fuel additive through. The winter blends will destroy gas mileage, but it shouldn't be that big of a drop. It's worth a try though.
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Old 02-04-2011, 01:58 AM   #16
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I use mid-grade fuel (otherwise it knocks) and no lift and 30x9.5 helps but the other thing to remember for those that have bigger tires is that your odo will under report miles. If for example you increase the tire circumference by 30% from stock, you need to divide your trip odo miles by .70 before you do your mpg calculation. For Prettyboytoys, he saw a major decrease in MPG from one week to the next without having changed his setup.
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Old 02-04-2011, 06:48 AM   #17
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Cool

Quote:
Originally Posted by ABSOLUTnie View Post
if its auto, it could be a bad torque converter.

also, any vacuum leaks? all your vents working properly?

i'm having the same exact issue. I get 120 miles per tank, so about 8-9mpg.




Its a 5 speed manual. no vacume leaks.
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Old 02-04-2011, 06:52 AM   #18
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I use mid-grade fuel (otherwise it knocks) and no lift and 30x9.5 helps but the other thing to remember for those that have bigger tires is that your odo will under report miles. If for example you increase the tire circumference by 30% from stock, you need to divide your trip odo miles by .70 before you do your mpg calculation. For Prettyboytoys, he saw a major decrease in MPG from one week to the next without having changed his setup.






thanx for the info. I was doing some reading. do you think something like the pcv valve or Iac valve could be doing this ??? I'll throw in some mid grade gas today before I head to boyne tomm for some skiing. I'll also add some injector cleaner but not sure if that will do much for this case
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Old 02-04-2011, 07:10 AM   #19
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I always recommend checking your odometer with a GPS unit. I went from 225 75 to 235 75's. My speedometer didn't change much, it is accurate until about 60 mph and then it is only off by 2mph. However if I drive 50 miles with my GPS my odometer only shows 42 miles. So if I calculate my MPG's by the odometer it is lower then what it actually is.
Mine is a 99 SE 2.5 auto and I get around 200 miles (odometer) per tank so really I am getting about 250 miles, if I gps'd and entire tank.
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Old 02-04-2011, 08:25 AM   #20
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I am with you all having the aweful mileage. I have a 99 Wrangler 5spd with 33 X 12.50's. Last time I checked mine I got 8.8. I got 141 miles out of a tank. I do have a vacume leak that I cant locate. My heater will switch to defrost when I am runnign it hard and cold air pours out the passenger floorboard. I put a new vacume pod in it, took the switches out and checked them, traced every line I could without removing the dash again. I can drive my 6-door f-350 on 40's cheaper.
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Old 02-04-2011, 09:46 AM   #21
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I am with you all having the aweful mileage. I have a 99 Wrangler 5spd with 33 X 12.50's. Last time I checked mine I got 8.8. I got 141 miles out of a tank. I do have a vacume leak that I cant locate. My heater will switch to defrost when I am runnign it hard and cold air pours out the passenger floorboard. I put a new vacume pod in it, took the switches out and checked them, traced every line I could without removing the dash again. I can drive my 6-door f-350 on 40's cheaper.
I have an identical issue. Post up if you figure it out.
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Old 02-04-2011, 12:13 PM   #22
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thanx for the info. I was doing some reading. do you think something like the pcv valve or Iac valve could be doing this ??? I'll throw in some mid grade gas today before I head to boyne tomm for some skiing. I'll also add some injector cleaner but not sure if that will do much for this case
I don't think the idle air control valve could be the culprit. It regulates how much air is allowed to bipass the throttle plate when it's closed and you haven't mentioned having rough idle/hard-start/surging issues.

The I6 uses CCV (crank case ventilation) fittings on both ends of the valve cover instead of a PCV valve. They generally do not need replacing and are pretty much maintenance free. They're 1/4 turn lock fitted into the valve cover with rubber grommets. The little plastic elbow part gets brittle over time and is prone to breaking.

If you want to clean your throttle body and IAC here is a good write up- Cleaning IAC and Throttle body write up (for dummies) - JeepForum.com

How many miles on your rig and have you checked to see how long it takes to get to optimal running temp?
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Old 02-04-2011, 01:00 PM   #23
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I don't think the idle air control valve could be the culprit. It regulates how much air is allowed to bipass the throttle plate when it's closed and you haven't mentioned having rough idle/hard-start/surging issues.

The I6 uses CCV (crank case ventilation) fittings on both ends of the valve cover instead of a PCV valve. They generally do not need replacing and are pretty much maintenance free. They're 1/4 turn lock fitted into the valve cover with rubber grommets. The little plastic elbow part gets brittle over time and is prone to breaking.

If you want to clean your throttle body and IAC here is a good write up- Cleaning IAC and Throttle body write up (for dummies) - JeepForum.com

How many miles on your rig and have you checked to see how long it takes to get to optimal running temp?

I have 147,000 and no, I havnt checked how long but am about to go out on a run so I'll check in a few mins....thanx
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Old 02-04-2011, 01:18 PM   #24
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Ha ha,,,,,,OK so it might not be that funny but, I have found the problem..........



I now feel like a retard and am very sorry for wasting everyones time. I just realized I have no gas cap/////////not sure how this happend but thats the problem......

just got one for 8 bucks,,,,problem solved
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Old 02-04-2011, 01:20 PM   #25
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If you've just run a tank or two with these bad results, I'm gonna guess that you've got bad gas. Add a container of something like STP gas treatment. And buy your next gas at a different station/brand. I recommend hi-test BP. If that's the problem, you'll see a difference really quickly.
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Old 02-04-2011, 01:47 PM   #26
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Ha ha,,,,,,OK so it might not be that funny but, I have found the problem..........



I now feel like a retard and am very sorry for wasting everyones time. I just realized I have no gas cap/////////not sure how this happend but thats the problem......

just got one for 8 bucks,,,,problem solved
Doh! No worries man, that's actually pretty funny.
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Old 02-04-2011, 02:42 PM   #27
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Doh! No worries man, that's actually pretty funny.

thanx, I did learn alot and am trying to figure out milage corrections with my tires.....just drove 10 miles with gps and the odo read 10.2 now for some math to figure out what I really get in terms of mpg
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Old 02-04-2011, 02:44 PM   #28
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I get 14.7mpg on average with mixed miles. 4.0L, 33's, 3.73 gears and the 5speed NV3550. I can't only get 15-16 gallons in mine. I get about 210-220 miles till it beeps at me. Also my speedo is fast. 3 mph off at 70 so my real mpg is lower. I switched speedo gears but as you'll find out. A 33" tire really isn't 33" in diameter.


Edit: No gas cap would show error codes. Do a double check before you think that fixed it. After about 30 miles with my gas cap not fully locked started throwing the leak codes.
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Old 02-04-2011, 03:19 PM   #29
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I also run 89 octane (mid grade). Last i checked with my GPS my destination was 15.7 miles and my trip meter read exactly that.
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Old 02-04-2011, 03:35 PM   #30
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The tires are 30x9.5 w/no lift, it's a 97 Sahara with about 145k. It's my daily driver.

I'm not sure if the changed the OBDII system in later years but mine will display any codes in the odometer if I just do the key on/off thing. Here's a helpful link FAQ - TJ Error Codes - JeepForum.com
If you check for codes and It just says "done" in the Odometer does this simply mean theres no codes?

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