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1999 Sport Fuel Mileage... Lost!

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Old 02-12-2013, 07:11 PM
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would it be possible that there's a vacuum leak somewhere? not sure if that would throw a code or not.
Old 02-12-2013, 08:24 PM
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Originally Posted by Mendax13
would it be possible that there's a vacuum leak somewhere? not sure if that would throw a code or not.
You would hear it... I believe...
Old 02-12-2013, 08:57 PM
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A buddy of mine was stuck in the mid 11's mpg-wise. We looked over/cleaned and replaced all the parts you listed and nothing helped. Until he noticed his motor mount on the drivers side was pretty worn. Not broken completely, but well on its way. We replaced them with brown dog mounts and oem replacement for the trans mount. He jumped up about 1.5 mpg immediately. It didn't magically give him 20 mpg, but with our rigs every bit helps right !
Old 02-12-2013, 09:13 PM
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Hmm... Is your jeep coming up to temperature ? Running in the 195 210 area... If its running to cold that will also have bad gas mileage ...
Old 02-12-2013, 10:44 PM
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Originally Posted by daves01XJ
A buddy of mine was stuck in the mid 11's mpg-wise. We looked over/cleaned and replaced all the parts you listed and nothing helped. Until he noticed his motor mount on the drivers side was pretty worn. Not broken completely, but well on its way. We replaced them with brown dog mounts and oem replacement for the trans mount. He jumped up about 1.5 mpg immediately. It didn't magically give him 20 mpg, but with our rigs every bit helps right !
Interesting! I don't think mine are in great shape, but this could explain why i have such poor economy in the city (say 16 in the summer) but can pull 28 on the highway, stocker 5-speed.
Old 02-13-2013, 06:42 AM
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A lot of car have "adaptive driving" meaning that a car will learn its driving conditions and adapt to either fuel milage or performance. you can reset the ecu(by doing this, you also reset the fuel trim). give it a try and see what happens
Old 02-13-2013, 07:04 AM
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trans mount was replaced and took care of the banging from the exhaust but no mileage improvement. Motor mounts are for sure on the list of one of the next things. the ECU was reset when the 02sensors and injectors went in. and have run two tanks since then so definately enough time to get the computer to relearn the trim

Temp is good, even when running the computer to take a peek at live data its running steady about 200-205, so i know the computer is seeing the correct temp.

this one has got me puzzled since all the normal culprits of poor mileage have been taken care of,
Old 02-13-2013, 08:38 AM
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Originally Posted by Guntar13
I would love to be able to change that but unfortunately on the 97+ it's in the tank as part of the entire assembly. Not serviceable
It is certainly serviceable. You just have to drop the tank. I have plans to replace the whole thing due to my gauge jumping around and not really being accurate.
Old 02-13-2013, 08:47 AM
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you are right is is serviceable but does anyone really want to drop the tank for a filter? I understand your position being how the gauge is off or not working correctly. What i was saying is that in a normal maintenance schedule of things changing the fuel filter isn't really a feasible idea.
Old 02-13-2013, 12:42 PM
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Originally Posted by Guntar13
you are right is is serviceable but does anyone really want to drop the tank for a filter? I understand your position being how the gauge is off or not working correctly. What i was saying is that in a normal maintenance schedule of things changing the fuel filter isn't really a feasible idea.
True. There's a youtube video somewhere of a guy that takes the whole pump assembly apart, but aint nobody got time for that!
Old 02-13-2013, 12:53 PM
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Originally Posted by PocketsEmpty
True. There's a youtube video somewhere of a guy that takes the whole pump assembly apart, but aint nobody got time for that!
Heard that! But I'm subscribing as well. My 96 is riding on 2" of lift and on the same set of tires. I'm around 14 mpg. My 88 I was kicking it with 18 mpg's.
Old 02-13-2013, 05:06 PM
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How was your mpgs before those 784 injectors? I heard their flow rate is to high and even after the ecu it'll spray too much fuel
Old 02-13-2013, 05:49 PM
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Originally Posted by xj guy
My 96 is riding on 2" of lift and on the same set of tires. I'm around 14 mpg.
In winter months? Be happy with that, that's not bad at all
Old 02-13-2013, 06:06 PM
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How can a bad motor mount reduce MPG?

O2 sensor is often blamed for bad MPG.
Old 02-14-2013, 07:10 AM
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Originally Posted by iron horse
How was your mpgs before those 784 injectors? I heard their flow rate is to high and even after the ecu it'll spray too much fuel

honestly it hasn't changed at all, still crappy!


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