SteelBlue99

Ok, guys i have an issue with terrible gas mileage in my XJ.
I know...i know "it's not a Honda Civic, etc" I understand that, but there's now way this mileage is right.
It's a 1999 XJ 4.0L I6. Lifted 4.5" on 32's. Almost all highway with an EXTREMELY light foot and I never leave it running for more then a minute and i'm only getting about 10mpg.
Jeep isn't throwing any codes
In the past 1k miles i've changed the following:
Spark plugs
Rotor
Cap
Plug wires
Neon injectors
Cleaned IAC / throttle body
I rented a diagnostic scanner from VatoZone to see if maybe i had a lazy O2 sensor that just wasn't throwing a code. That wasn't the problem, the O2 sensor came back just fine. However when it tested the catalytic converter, the test came back inc (incomplete ).

Anybody have suggestions? Any and all help is greatly appreciated as always.

HCCAFan

Doesn't sound too far off base for a lift and 32" tires.

SteelBlue99

I'd expect that city, but about 85% highway with a VERY light foot, i would expect more.

The Rooster

I see why you have bad gas mileage.... Lifted 4.5" on 32's light foot or not... sorry


but when was the last time you changed your o2? is your exhaust stock?

I also noticed you have not changed your fuel filter... that could lead to bad mileage..

SteelBlue99

Exhaust is stock, and I haven't changed either O2 sensor.

The Rooster

so your o2's are 12 years old...?
What about your fuel filter?

SteelBlue99

I didn't say they were 11 years old, I said that I haven't changed them. I've only owned the jeep for about 3k miles. Fuel filter, same thing.

The Rooster

Sorry about that i thought you were the org owner...
I would change the fuel filter then.. my gas mileage increased after i changed mine.. and that is way cheaper then o2s and most people forget about it...

jeff2285

i was complaining about the same thing to a co worker and he said he used to get 20-25 mpg on his stock xj ....... told me to change both o2 sensors, the temp sensor and give it a tune up

SteelBlue99

No biggie. Yea, I don't think the O2 sensors are bad, because the diagnostic scanner I rented, tested the voltage and it said they were fine. You have to drop the tank, to change the fuel filter, right?

SteelBlue99

Well, the downstream O2 sensor doesn't effect the mileage at all, it's strictly for emissions. I'll probably end up changing the upstream one though.

The Rooster

im not sure on a 99 but on mine it is on the right side on the fuel line right side before the rear axle...

Irongrave

your running heavy tires on larger then stock wheels added some weight to it and stock gears.

Also with 32s your speedo is off so thats going to throw your MPG way off unless your using a GPS.

to fix it recalibrate your speedo and if you have the money get a set of 4:10s or 4:56s in the axles

widefieldcherokee

i got the same lift and 32in mt/rs and im gettin the same mileage
so sounds normal to me get a better flowin exhaust and maybe even cold air intake to but 10 dont sound to bad to me

tjwalker

Fuel filter on a 99 is part of the "fuel pump assembly" which is located inside of the fuel tank. It is not normally serviced unless symptoms / testing point to it. And your symptom of low gas mileage doesn't point to it. You can verify pressure if you want with a gauge, but if it is running okay, look elsewhere.

Verify your mileage with a GPS, or as mentioned, recalibrate your speedo.

Even if the oxygen sensor checks out, they do have a finite lifespan of between 100k and 150k, depending on engine efficiency.