found something that made a significant difference in my mpg

i have a 99 with 31's aw4 trans ...3.55 gears and 125k miles

after making sure tires are inflated properly, new plugs, wires etc...my mileage went up from 15ish to 16ish

improvement yes...but still not impressive.

cleaned the throttle body...ran smoother, but no mpg increase

well i ordered a new upstream 02 sensor from rockauto...(44 dollars i think)

now my mpg is 17 on the low end...last tank of fuel was just over 19mpg

and thats a mileage app on my phone doing the math...it goes by REAL numbers...not just guessing or rounding off. It tells me even how much it cost me to drive a mile


So, if your mileage is down....try the 02 sensor...my jeep is getting way better mpg and it runs way better too....

It worked for me. Oh and i didnt do the downstream sensor...so replacing that might help (i dont know)

 

When the o2 sensor begins to fail it tells the ecm to rich the fuel mix.

 

The O2 sensor can be "lazy" (switches from lean to rich too slowly) long before it sets a code, and it appears this is the case with yours. I would not change the downstream sensor unless you know it's bad; it will not help your mpg.

 

i didnt think so either...it was the factory 02 sensors...it was lazy for sure

i just posted this to let others know

everyone here knows the one downfall to a xj is the mpg

 

Thanks for the note. I have a 00 with 32s, have not had it long, but I have not gotten more than 14 hwy. and the tail pipe has the tell tail black soot. I am going to replace the up stream O2 and see how much it helps. From everything I have read, my guess is it will aide significantly.
I'll post my results.

 

I replaced my o2 sensor, but i still get 8 mpg

 

Something is seriously wrong with yours. Did you correct the speedometer/odometer for the larger tires? If not, then your MPG is slightly better.

Looks like with a stock tire being a 225/75/15 and your 31x9.5x15s now there is about a 9.5% difference and your odometer will reflect less mileage than actually driven. Your speedo will also show you are going slower than you actually are. When the Speedo reads 55mph you are actually going 60mph.

However, this will only make a small increase and would be probable if you said you were getting 13-15 mpg. You are not highly modified and are not running tires that large. I am running 30" mud tires on my 93 with 4.0, 5 speed, and 3.07 gears, and get close to 19 mpg and my speedometer has not been corrected so my numbers are actually better than that. With an auto I would expect slightly less mpg but not 8 mpg.

Also depends on the way you drive it.

 

big factors in mpg:

how you drive....slow take offs and coast to a stop...braking will hurt your mpg just as much as taking off

weight...get rid of it if you dont need it...

quality fuel...try to stay with a quality fuel...some argue against this...but if i constantly run mobil, bp or shell....my mpg will be better than the local "stop-n-go"

tire pressure.....go down the road...how many cars/trucks do you see driving with a soft tire....make sure you check them atleast 1 time every 2 weeks

good clean plugs too....jeeps love cheap copper champion plugs....12 dollars for plugs....replace them 1 time a year ....12 dollars is cheap (now i know they should last alot longer....but then atleast check the gap

 

I did everything you said, and my jeep still hates me.

 

you got something wrong with the jeep...dirty injector(s)?

fuel filter dirty?

aerodynamics....jeeps are boxes with wheels....lift + rims+tires=mpg loss

 

Mines near stock. Replaced fuel filter, but haven't checked out injectors yet.

 

Check your Manifold Absolute Pressure sensor, dirty MAP and/or bad vacuum can cause this. Coolant Temp Sensor as well I believe. Also check exhaust manifold for cracks.