Why the bad fuel economy? - JeepForum.com

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post #1 of 17 Old 01-14-2013, 11:53 PM Thread Starter
Jeep318
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Why the bad fuel economy?

I have new plugs, new plug wires, new fuel filter, new map sensor, new CAS and new CPS, new O2 sensors, new exhaust, new air intake, cleaned IAC and throttle body, entirely new cooling system and full fluid flushed, heavier steel wheels and 30" tires. I'm averaging 10-12mpg and I drive very conservatively and 60% freeway, I get maybe* 250 on a full tank but more recently 230ish. Any idea on why its so bad? I use 89 octane. I haven't changed my cap or rotar or my coil yet.

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post #2 of 17 Old 01-15-2013, 12:28 AM
AVR2
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Quote:
Originally Posted by Jeep318 View Post
Any idea on why its so bad?
It's a 17-year-old Jeep with a 5.2L engine.
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post #3 of 17 Old 01-15-2013, 01:15 AM
Josef86
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No idea. Do the injector swap in FAQ? That's pretty bad. With a 3.5" lift and 31s I still manage 18 hwy and 13city with the 5.2L. But I've no front driveshaft currently.
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post #4 of 17 Old 01-15-2013, 04:48 AM
PolkaPower
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Having the front drive shaft out doesn't help anything for mpg. The axles, drive shaft, and gears still turn.


Check for a plenum pan gasket leak. That will cause poor MPG. If it's never been repaired before then you can be 99% sure it's bad.
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post #5 of 17 Old 01-15-2013, 05:36 AM
ratmonkey
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Plenum pan possibly.
Did you use Copper core plugs like you should? Platinum plugs don't work as well in these.
What brand o2 sensor? Anything other than ntk or mother mopar and the computer will hate you.
89 octane won't help you with anything other than a lighter wallet.



And accelerate quicker, these tend to get whose mileage when you take it easy.

'97 zj 5.2, some stuff, some other suff, and some things that even work sometimes.

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post #6 of 17 Old 01-15-2013, 07:22 AM
dougmorgan99
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Quote:
Originally Posted by Jeep318 View Post
I have new plugs, new plug wires, new map sensor, new O2 sensors, new exhaust, new air intake, cleaned IAC and throttle body, entirely new cooling system and full fluid flushed, heavier steel wheels and 30" tires. I'm averaging 10-12mpg and I drive very conservatively and 60% freeway, I get maybe* 250 on a full tank but more recently 230ish. Any idea on why its so bad? I use 89 octane
Your Jeep specs are very close to mine. My o2 sensors I changed were Mopar a couple of months back with about .75 mpg change. I say look at the plenum like PolkaPower suggest. I plan on changing mine this summer anyways for don't know if it ever was. But again... its a Jeep.

Other threads and sites cover it, see:
http://www.jeepforum.com/forum/f13/a...-5-2l-1157923/
http://www.fueleconomy.gov/feg/bymod...Cherokee.shtml

So as you see from the fueleconomy.gov site it is within spec for my year. Plus with 3.5 lift, heavy front/rear bumpers and roof rack... I am running at 11.5mpg with 80% in town driving.
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post #7 of 17 Old 02-04-2013, 03:34 PM
davidmatlack
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Plenum?

What is it? Where is it? Easy to replace?

Thanks
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post #8 of 17 Old 02-04-2013, 05:21 PM
Snowfan2
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Quote:
Originally Posted by davidmatlack View Post
What is it? Where is it? Easy to replace?

Thanks
Plenum=Intake Manifold

I have the same year and engine as you and average 18, mostly rural highway. I hope you find the problem.
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post #9 of 17 Old 02-04-2013, 05:28 PM
um7267
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clogged cat? wheels and tires that weigh more could cause some mpg loss but now that much. did you reset you battery after the new o2 sensors where installed?

2004 Lexus Es330, alive; 1999 Cherokee Sport
1998 Grand Cherokee, sold
1997 Grand Cherokee; Black and Gold, Limited 4.0....RIP 2005
to the good times and the bad, the family we have and lost, little brother 1988-2011
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post #10 of 17 Old 02-04-2013, 05:54 PM
newfieZJ
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Didn't see mention of a new cap and rotor

You may have a brake drag from a bad caliper .

Original cat ? how many K ? may have partial restriction .

The plenum plate is under the intake , commonly called a kegger because of it's shape . It's a stock steel plate bolted to an aluminum intake . Due to both metals having different expansion rates , the seal between the 2 is a common issue .

The job entails
draining the coolant
removing the front components out of the way alt and AC
removal of the intake , after you unplug everything (not as complicated as you may first think , the wires all falls into place )
replace plate and gasket (alum. plate , fel-pro gasket)

keep all the bolts in order and it's just take it apart , put it back together . http://dodgeram.info/tsb/2000/09-05-00.htm
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post #11 of 17 Old 02-04-2013, 09:21 PM
torchd
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That's an easy question to answer.


It happens every year to me in the WINTER time... The gas is crappier gas, more water is in the gas. Starting late Nov. to mid March I get around 11 mpg to 13 mpg.


Then when SUMMER hits, they use the better gas and my mileage increased. It goes from 15 mpg to 17 mpg.


Nothing changes between summer and winter besides motor oil weights

1998 Jeep Grand Cherokee Laredo
2" BB Pucks | SkyJacker Hydro Shocks | Rubicon Wheels | Cooper 245/75/16 Tires

2002 Trans Am
Cam, Full Exhaust, Stock Heads, Stock LS6 Intake and Stock 10 bolt with 3.42s
420 RWHP and 382 RWHTQ
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post #12 of 17 Old 02-04-2013, 09:33 PM
Dyn0mitemat
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Quote:
Originally Posted by ratmonkey View Post
Plenum pan possibly.
Did you use Copper core plugs like you should? Platinum plugs don't work as well in these.
What brand o2 sensor? Anything other than ntk or mother mopar and the computer will hate you.
89 octane won't help you with anything other than a lighter wallet.



And accelerate quicker, these tend to get whose mileage when you take it easy.
X2 on all of this, especially the last part. Sitting on a full (proper) tune up, 4" lift and 31's and I had a hard time keeping it in the double digits. Learned to quit driving like a granny (which would help in most vehicles) and get upto speed a little faster, and it made a big difference.

Its like spending 15sec @ 7mpg to get upto speed, or 5sec @ 5mpg, you'r more efficient actually getting into it a bit more.

And heavier wheels wont help anything but to use more gas to get upto speed. 1# of rotating mass is like adding 10# of weight to the car (or something similar to that effect I remember)

09 Rubi 2dr:MetalCloak2.5", 37"MTZ's/Slabs, EVO Stff, ORFab Cage, Chromo's, Driveshafts etc
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post #13 of 17 Old 02-05-2013, 10:00 AM
speedbucket
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Im at 5", 33s, steelies, roof rack, full size spare on roof, 5.2, 266k miles, and weigh in at 5100lbs with added armor, bad plenum.....and i average 12-13 overall........
Do a compression check before you start replacing anything

http://www.jeepforum.com/forum/f22/speedbuckets-zj-build-1361322/
V8 zj at 6" . Double tri rear and true 3 link front. Stretched to 108"
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post #14 of 17 Old 02-05-2013, 02:20 PM Thread Starter
Jeep318
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I used copper plugs, mopar o2 sensors, and the catalytic converter is a brand new magnaflow from kolak. I do have heavier steel wheels but I doubt that would kill mileage all together.
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post #15 of 17 Old 02-05-2013, 02:48 PM
davidmatlack
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EGR Valve and Solenoid

These have never been replaced on mine. Do they wear out? If replaced can they have a positve effect on MPG? Curently getting 18 on HWY?

thx
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