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-   -   My 99 XJ is getting terrible gas mileage (

Charley3 06-24-2013 12:15 AM

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.

nickcospo 06-24-2013 05:40 AM

I have the same problem and would like to know to

Evildriver-3 06-24-2013 08:57 AM

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.


Charley3 06-24-2013 12:59 PM

The gas mileage has been crap since I bought it a few months ago. Gas mileage was approx as bad with stock tires.

Stegs 06-24-2013 01:04 PM


Originally Posted by Charley3 (Post 15601519)
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

nickcospo 06-24-2013 01:09 PM

Whrre could I get an engine diagram of whrre the sensors are

Charley3 06-24-2013 02:01 PM


Originally Posted by Stegs (Post 15601549)
89 octane fuel seems to be helping me a little bit

I noticed that too.

Stegs 06-25-2013 08:27 AM


Originally Posted by nickcospo (Post 15601570)
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 :cheers2::highfive:

LtCdrLange 06-25-2013 08:57 AM

the 02 before the cat is the upstream and after the cat is the downstream.. important terms to know if you order parts.

nickcospo 06-25-2013 09:20 AM

Are there performance o2's

LtCdrLange 06-25-2013 09:40 AM

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

nickcospo 06-27-2013 10:59 AM

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

rjbruzan 06-27-2013 11:13 AM

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.


nickcospo 06-27-2013 11:39 AM

Well it says it adds horsepower that I can feel

Evildriver-3 06-30-2013 07:32 AM

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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