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Old 09-24-2007, 02:24 AM   #1
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lower fuel milage after tune up?

Changed my o2 sensors, plugs, wires and air filter and now im gettin only 6 mpg while before I was getting 10.....did I do something wrong? lol

this really sucks

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Old 09-24-2007, 05:08 AM   #2
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Just a guess, but perhaps the computer is learning the new air flow with the new o2 sensors? FWIW I get @13-14mpg with 35's and 4.56's

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Old 09-24-2007, 05:31 PM   #3
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thats horrible something isnt right. what gears do you have??

what plugs did you install??
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Old 09-25-2007, 04:31 AM   #4
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Crap man, I get 12 mpg pulling a boat up a steep grade hill with 33" tires at 8000 feet in my 4cyl( at 50 mph, pedal to the floor) 6mpg= big problem
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Old 09-25-2007, 06:18 AM   #5
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Disconnect the battery and ground out both leads to the frame WITH THE BATTERY DISOCNNECTED, IN FACT MOVE THE BATTERY ABOUT 10FT AWAY.
You want to drain any residual voltage from the system so you will loose your radio presets, etc. That way the ECU will go back to defaults and relearn.
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Old 09-25-2007, 07:43 AM   #6
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Or someone is doing a lot of in-town driving or 80-90 on the freeway.


If you did wires, why didnt you do a distributor Cap and Rotor.?
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Old 09-25-2007, 07:25 PM   #7
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did you gap your plugs correctly?
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Old 09-25-2007, 10:02 PM   #8
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thats horrible something isnt right. what gears do you have??

what plugs did you install??


stock gearing...not sure what it is exactly but i have the d35

i put bosch plugs in
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Old 09-25-2007, 10:03 PM   #9
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Disconnect the battery and ground out both leads to the frame WITH THE BATTERY DISOCNNECTED, IN FACT MOVE THE BATTERY ABOUT 10FT AWAY.
You want to drain any residual voltage from the system so you will loose your radio presets, etc. That way the ECU will go back to defaults and relearn.

did that....didnt help
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Old 09-25-2007, 10:04 PM   #10
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did you gap your plugs correctly?



plugs were suposto come pre gaped
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Old 09-25-2007, 10:05 PM   #11
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Or someone is doing a lot of in-town driving or 80-90 on the freeway.


If you did wires, why didnt you do a distributor Cap and Rotor.?



ran out of money thats next week
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Old 09-25-2007, 10:05 PM   #12
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Old 09-25-2007, 11:10 PM   #13
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6 mpg seems to be a bit low even for a jeep. if you don't have a cel, and the engine's running smoothly, maybe there's something wrong with the odometer or the speedometer gear.

You're not running around in 4Hi right
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Old 09-25-2007, 11:30 PM   #14
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FWIW. A while back I did a similar tune up my 98 TJ (less the O2 sensor), new wires and put some of the Bosch platinum 4 plugs in. Created all sorts of problems, including awful mpg (but not as bad as you're getting). Took to a buddy who is a ASE Master Tech at DC and he laughed at how much I just spent on the Bosch's and said the 4.0 didn't like them and I should have got the $1.50 Champions. He proceeded to tell me he's solved lots of unknown minor engine issues in the 4.0 by replacing the high-dollar plugs with some good ole Champions. FWIW.
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Old 09-26-2007, 06:13 AM   #15
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FWIW. A while back I did a similar tune up my 98 TJ (less the O2 sensor), new wires and put some of the Bosch platinum 4 plugs in. Created all sorts of problems, including awful mpg (but not as bad as you're getting). Took to a buddy who is a ASE Master Tech at DC and he laughed at how much I just spent on the Bosch's and said the 4.0 didn't like them and I should have got the $1.50 Champions. He proceeded to tell me he's solved lots of unknown minor engine issues in the 4.0 by replacing the high-dollar plugs with some good ole Champions. FWIW.
The 4.0 and 2.5 both prefer plain old copper core champions, people tend to get defensive and then offensive when you tell them they wasted $30 or so on one of those exotic plugs. I ran NGK plats in my trooper and ended up with burned valves in less than 120miles.
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Old 09-26-2007, 08:25 AM   #16
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I had a similar problem, but it was caused by champions. Threw in a/c delco and problem fixed
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Old 09-26-2007, 09:42 AM   #17
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I had a similar problem, but it was caused by champions. Threw in a/c delco and problem fixed
I have not worked on tuning up anything newer than a 99, did jeep stick with champions when they went to the coil on plug ? I've also used autolite and ac/delco but the champion copper core truck plugs are more common around here. Plus the dealers sell them for maybe $.25 cents more than the parts stores so it's just as easy to get the plugs, cap, rotor, wires at one stop as I change them all when I do a tune up on either the XJ or the TJ's.
One thing I do intend on doing more often is replacing my injectors, last year I had the #3 stick open and it took out #3 cylinder, washed it right down at 80mph on the turnpike, had 330,000 miles on it but ran perfect till that happened. I put a jasper in and got new injectors from four oh motor sports, going to do them every 100K from now on. If I had not been in a hurry I would have gone with a stroker but I needed it faster then 3 weeks.
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Old 09-26-2007, 01:21 PM   #18
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must have bein the computer learning the new air flow with the new o2 sensors cause I just filled up today and im gettin 13 mpg and thats driving really hard on the skinny pedal trying to keep up to my friends on a road trip who drive little sports cars....hopefully a week of driving normal again it will be at least 15 mpg
thanks for all the responses
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Old 09-26-2007, 02:46 PM   #19
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Good deal.

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