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Unread 06-24-2013, 12:15 AM   #1
Charley3
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1999 XJ Cherokee 
 
Join Date: Mar 2013
Posts: 3,618
My 99 XJ is getting terrible gas mileage

Low rpm power is fine.

Mid rpm power is slightly weak.

High rpm power is weak.

Gas mileage 13.8 mpg highway driving 65 mph. Haven't tested city mpg.

This sucks. I have 30 x 9.5 R15 tires. Gas mileage and power were poor with stock 225/75R15 tires, and are azlittle worse with the 30s.

My prior two Jeeps (96 XJ and 06 LJ) hax 4L engines and got better gas mileage and power.

I have already had sparkplugs changed, which helped a little; and had plug wires, cap and rotor changed, which didn't make any noticeable difference. I also changed air filter, which made no difference.

What else is left that might be causing lousy gas mileage and weak power at mid and high rpm?

Fuel injectors need cleaning? Oxygen sensor? Fuel map sensor? Something else? I'm stumped.

This 99 4.0L has 35K miles on it. It doesn't burn a drop of oil.

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Unread 06-24-2013, 05:40 AM   #2
nickcospo
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I have the same problem and would like to know to
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Unread 06-24-2013, 08:57 AM   #3
Evildriver-3
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You haven't made any changes and it suddenly just went to crap mpg ?

OR are you saying this happened after you removed the 225/75R15 and went to a 30"?

If this happened after the tire switch you're problem is the increase of diameter by almost 2" but more likely the weight of that tire is quite a bit different.

Tire weights need to be looked into, changing a tire and going even a few lbs heavier per wheel is Killer on perf and mpg.

.
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Unread 06-24-2013, 12:59 PM   #4
Charley3
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The gas mileage has been crap since I bought it a few months ago. Gas mileage was approx as bad with stock tires.
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Unread 06-24-2013, 01:04 PM   #5
Stegs
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Quote:
Originally Posted by Charley3 View Post
The gas mileage has been crap since I bought it a few months ago. Gas mileage was approx as bad with stock tires.
replace both 02 sensors

i had similiar issues, and did upper and lower sensors for a big increase

run half can of seafoam thru vacuum line
other half in throttle body

run for a few days then replace the 02 sensors....

i went from 14ish to 16ish now....also 89 octane fuel seems to be helping me a little bit
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Unread 06-24-2013, 01:09 PM   #6
nickcospo
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Whrre could I get an engine diagram of whrre the sensors are
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Unread 06-24-2013, 02:01 PM   #7
Charley3
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Quote:
Originally Posted by Stegs View Post
89 octane fuel seems to be helping me a little bit
I noticed that too.
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Unread 06-25-2013, 08:27 AM   #8
Stegs
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Quote:
Originally Posted by nickcospo View Post
Whrre could I get an engine diagram of whrre the sensors are
02 sensors is your exhaust

go underneath your jeep....there will be a sensor sticking out in front of your cat and then one after....

just unplug them, screw them out, screw the new ones in and plug them in

if you can turn a bolt, you can change them...

tip: find a time when you know your going to change them. The night before soak them in pb blaster or wd-40....they will come out easier

make sure you get the right ones too....there is a upper and lower

rock auto has the cheapest price on oem

have fun and report back
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Unread 06-25-2013, 08:57 AM   #9
LtCdrLange
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the 02 before the cat is the upstream and after the cat is the downstream.. important terms to know if you order parts.
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Unread 06-25-2013, 09:20 AM   #10
nickcospo
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Are there performance o2's
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Unread 06-25-2013, 09:40 AM   #11
LtCdrLange
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not really.. 02 sensors just perform a specific job that's to measure the amount of fuel in the exhaust so the vehicle can burn more efficiently. if there were performance ones it'd be a waste of money
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Unread 06-27-2013, 10:59 AM   #12
nickcospo
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Ok I installed my throttle body spacer and now it idles at 1500 rpms and I think carb needs to be adjusted. How do Ido that
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Unread 06-27-2013, 11:13 AM   #13
rjbruzan
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If the idle went up after the spacer and was correct before the spacer that means you have a vacuum leak or the throttle is not closing completely. The is no idle adjustment.

Ron
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Unread 06-27-2013, 11:39 AM   #14
nickcospo
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Well it says it adds horsepower that I can feel
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Unread 06-30-2013, 07:32 AM   #15
Evildriver-3
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The spacer should/will increase signal to the carb and possibly make it work/perform better if the carb is further tuned for the particular spacer, not just putting the spacer beneath the carb and going, and depending what spacer you got, tapered, straight, open all play a different role on the signal and where/how it helps since it effects the plenum area.

.
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