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-   -   Why the bad fuel economy? (http://www.jeepforum.com/forum/f13/why-bad-fuel-economy-1463918/)

Jeep318 01-14-2013 11:53 PM

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.

AVR2 01-15-2013 12:28 AM

Quote:

Originally Posted by Jeep318 (Post 14791921)
Any idea on why its so bad?

It's a 17-year-old Jeep with a 5.2L engine.

Josef86 01-15-2013 01:15 AM

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.

PolkaPower 01-15-2013 04:48 AM

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.

ratmonkey 01-15-2013 05:36 AM

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.

dougmorgan99 01-15-2013 07:22 AM

Quote:

Originally Posted by Jeep318 (Post 14791921)
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.

davidmatlack 02-04-2013 03:34 PM

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

Thanks

Snowfan2 02-04-2013 05:21 PM

Quote:

Originally Posted by davidmatlack (Post 14906921)
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.

um7267 02-04-2013 05:28 PM

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?

newfieZJ 02-04-2013 05:54 PM

Didn't see mention of a new cap and rotor :dunno:

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

torchd 02-04-2013 09:21 PM

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

Dyn0mitemat 02-04-2013 09:33 PM

Quote:

Originally Posted by ratmonkey (Post 14792258)
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)

speedbucket 02-05-2013 10:00 AM

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

Jeep318 02-05-2013 02:20 PM

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.

davidmatlack 02-05-2013 02:48 PM

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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