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Old 06-12-2007, 08:09 AM   #1
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FE not increasing

OK folks....time for a head-scratcher.

I'm having a problem not getting good FE out of Julie (89 Jeep Cherokee, 4.0L w/AW4 auto and 4WD). Now, I understand that she's got all the aero characteristics of a brick wall (or farm tractor, since she moves down the road), but there's got to be something that I'm doing wrong. Since joining this site, I've been keeping fuel logs. (Shortly after purchasing the vehicle, so I don't have much/any back data).

Since joining the site, I've changed my driving habits to include neutral (engine on) coasting. I already drove like a grandma, but have also lowered my speed limits to 55-60 on highways and 60-65 on the freeway. Since this is an older vehicle with auto tranny, I'm not comfortable with the idea of engine-off coasting....plus Indiana is mostly flat...no good places to coast. Unfortunately, this also applies to P&G......no good places to do it.

Is there something that I'm missing here?? Why can't I get my FE over 18.5?? I don't want to start making radical/costly changes to the Jeep, as I'm doing this in an attempt to save money overall. (better FE = less $$ at pump = more $$ in pocket). As of now, I'm looking at a superMID gauge...but don't know if I can raise FE enough to justify the cost. I am also looking at a vacuum gauge, as that may help with FE, but can also tell me several other important things about the motor. (Damn, I hate having a student's budget......makes things pretty tight)


Sorry for the long post, but any help would be great (and much appreciated)
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Old 06-12-2007, 08:20 AM   #2
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Have you tried some of the cheaper starting points:

1) Tune-up the engine (if needed)
2) Check air filter
3) Check (and maybe increase) tire pressure
4) Check front-end alignment
5) Make sure that your brakes are not dragging
6) Ensure you have at least the factory air dam on the front
7) Consider a larger air dam
8) Check power accessories for proper function (not dragging) ie. A/C, Power Steering, Alternator, etc...

Other things that might cost more:

9) If you have AT tires, go with a better "highway" tread, perhaps even a skinnier tire...
10) If you have a mechanical fan, replace it with an electric fan
11) If you have EFI, get a SuperMID (maybe this should be #7)!
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Old 06-12-2007, 10:18 AM   #3
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Looking at your Garage page, it looks like your XJ needs a tune up. Even being boxy, a stock XJ should get around 20-22 MPG
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Old 06-12-2007, 10:26 AM   #4
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yes, I've done #1-3. I've got the factory airdam, (was/am considering a front belly-pan, front bumper -> front axle). I've also been doing some digging for some info/good prices for an e-fan replacement. Will that really give me a significant MPG gain??

*edit*
Also have the new cap/rotor installed...can't quite afford to spring $100 for quality plug wires ATM....shocks and LCA brackets come first, they're in worse shape

<--goes to edit garage entry
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Old 06-12-2007, 10:34 AM   #5
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EOCing is probably your best bet along with inflating the tires to max psi. There's really not a lot you can do... Jeeps are geared so low, you're doomed to bad FE.

Maybe take a look at this thread here.

http://www.gassavers.org/showthread.php?t=3302

Your proposed aero mods are good too. Wheel skirts are also an idea if you don't care to look a little tacky.
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Old 06-12-2007, 10:38 AM   #6
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you will see a diffrence with the super mid. i just dont know how much it will be. when my fe wasent going up i felt the same and the supermid was the first thing i could think of.
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Old 06-12-2007, 10:49 AM   #7
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Quote:
Originally Posted by atomicradish View Post
EOCing is probably your best bet along with inflating the tires to max psi. There's really not a lot you can do... Jeeps are geared so low, you're doomed to bad FE.

Maybe take a look at this thread here.

http://www.gassavers.org/showthread.php?t=3302

Your proposed aero mods are good too. Wheel skirts are also an idea if you don't care to look a little tacky.
EOC (engine OFF coasting) isn't a viable option for me, as this old 4.0 can be notoriously finicky about starting. I'm not taking that risk (or wear/tear on starter) right now....just not cost-effective.

Re-gearing the axles *might* be an option, better than re-gearing the tranny. The only problem is that I've got to re-gear both axles, or kiss the 4WD (one of the reasons for purchasing the Jeep in the 1st place) goodbye.....so, not a good option there. I might have an axle swap opportunity coming up, however...so all hope is not quite lost.

Any other ideas??
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Old 06-12-2007, 10:52 AM   #8
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I think your gaslog says it all. You're getting in the city what you used to get on the highway. Just be patience. Since joining the site there was a slight increase with each fill. The last 3 fills have not been normal driving. What would your mileage been in the past under those conditions? Shift points are very important with an automatic. Try to have the torque converter locked up as much as possible. Just keep pluging along and working on it. Might want to look into sea foaming or using water to clean out the engine with that much mileage on it. The new gas you tried might be E10 also which on some cars hurts mileage. You'll have better numbers before you know it.
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Old 06-12-2007, 10:59 AM   #9
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What gears and axles are you running? I know your front is a Dana 30, but your rear could be a Dana 35, 44 or the Chrysler 8.25. Depending what you have, a junkyard swap + a new speedo gear could be fairly easy. I think XJs had the option of 3.07, 3.55, 3.73 and 4.10 axle ratios
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Old 06-12-2007, 06:49 PM   #10
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If your Jeep has an O2 sensor, you might seriously consider testing and maybe replacing it if you feel the FE is so low.

You might also get some benefit from some grille blocking, and does your Jeep have manual or automatic hubs??
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