Something i was looking forward to in this jeep is the critically acclaimed "decent/good" gas mileage. My cousin has a bone stock 99 on 245-75-15 mud tires and he gets 20-21 in the city.
I have a 1999 classic 4.0
130k miles, clean af
I am all stock height and wheel/tires.
I have the accel ignition kit. Cap,rotor,plugs,wires. It needed it. The PO hadn't done anything with any of that stuff.
I have a spectre cowl intake.
I have a modded S pipe, no more crush after the header.
I don't spend my entire life on the pedal by any means.
I know the bank 2 down stream o2 sensor has a short. The one by the cat. Could this be an issue/the reason?
It is reluctant to start some times I believe because the fuel filter needs some attention or maybe even the pump. Otherwise I'm kinda disappointed. 15-16 mpgs typically and it sucks with my school commute.
I have the 784 injectors ready to put in just not sure how to clean them properly.
What can I do to keep the gas in the tank at this point?

 

20/21 City seems too good.
Maybe your cousin doesn't know how to calculate mpg.

 

15mpg avg is about normal for a xj,If your cousin is getting 20 hes got a stick and drives like a old lady maybe.

 

Yup. They're fairly average. I'd certainly change out the O2 sensor though. O2 sensors can greatly effect mileage.

 

X2 on the 02 sensor, I had a TJ with a bad 02, replaced it & doubled the gas mileage.
YouTube how to clean the 784's.
If you want to save gas, sell the XJ & buy a Prius. LOL. 16-17 MPG sounds about right for me with 265's, 3.55 gearing & 703 injectors.

 

15-16 sounds prefect for stock shouldn't worry. I get 15.x city and up to 21 hwy depending on miles traveled and how long I can keep it under 2k rpm. Get those 02 sensors, install injectors and find a tire size gear ratio chart on Google. Idk about that 20 mpg with mud tires, doesn't sound right

 

This.
20 MPG on MT's in the city? Probably not, 15-16 is average, as others have stated. I am on 35" Cooper STT's, 4.88 gears, and just did my MPG calculations this week. Getting 14-16 city, and around 15-17 highway. Before I did my gears, cut those numbers in half. You're doing just fine for what you have. If you want better MPG's for your commute, get a small import beater. Definitely replace the O2 sensor.

 

One other thing: If you think about it, 15 MPG is pretty good for a brick on wheels, LOL

 

When my jeep was stock, I purposely tried to get as good as possible from a tank, hipermileling and driving 55 everywhere. I got 20mpg mix of hwy/city. I seriously doubt your cousin is getting anywhere near 20/21. I'm content with my 16/17 average since I've had my XJ.

 

If I get told to get a Honda civic, prius, etc. One more time I'm gonna freak out.
No I did not buy the jeep for sippin gas. Yes I did think they got better milage than this. No my cousin is not an unreliable source, he's easily one of the best gm/jeep mechanics in the state. I might not know the whole story with his xj, that's just what I thought I remember him saying.


Now...that wayyy downstream o2 sensor? The one obviously measuring emission gases or whatever it's giving that po138 code I think where it's being shorted electricity...if it's in the wiring I'm gonna cry but if the sensor itself is bad I'll replace. Just wondering if thats makin me run lean

 

From my understanding the downstream monitors cat performance and doesnt affect trim iirc.

 

That's what I figured it'd be nice if it were the easy culprit but deep down I knew it has nothing to do with it...
My future plans right now are to get my tb bored to 60-62mm get the 784 injectors in there figure out this adjustable map sensor trick and whether it's worth it...and a cat back.

 

after cat 02 sensors have absolutely no effect on driveability or your mpg's it also doesnt help that the xj has the aerodynamics of a brick...if you want mpg's get rid of it now and concerning yourself with that is no way to live if you want a certain vehicle deal with the sacrifices/ consequences lol i sold my 50mpg vw tdi to my fiancee for my 10mpg lifted xj...just bought a stock xj for a dd though...also the accel ignition will not help your mpg's and the cai and injectors won't have much effect either

 

there was a point where I was getting 19 on the highway with 3" lift and 32s. There is no magical trick to getting better mpgs. Everything can affect it. Every xj will be different. Just keep it in tip top shape, and lay off the skinny pedal.

 

Uh-oh....get ready for the freak-out.... LMAO