• Welcome to the new NAXJA Forum! If your password does not work, please use "Forgot your password?" link on the log-in page. Please feel free to reach out to [email protected] if we can provide any assistance.

damn gas mileage, whats wrong here

J

Jays00XJ

NAXJA Forum User
Location
Harleysville, Pa
So lately ive noticed a decent drop in gas mileage. I havent changed a thing. Its a 2000 XJ sport, 4.0, aw4, 2'' RC BB, and 31x10.50 STT's, soft 8's, flareless and cut fenders. When I first bought the Jeep it was bone stock, with 156,000. When I bought it I changed the plugs, air filter, changed the oil, rear diff fluid, put a homebrew cowl snorkel on it, and a thrush glasspack, bought the lift, wheels and tires a little later and threw them on. When it was stock it got great gas mileage, after the lift and tires it dropped a little but nothing major, it was just the larger tires i figured. So now the lift and tires have been on for about 4 months. All of the sudden like last week my mpg just got really sh*tty and nothing has changed. What could be the cause of this?

Jay
 
your driving a box on wheels. what do you expect?
 
1. If you are buying ethanol mix, expect to lose up to 10% of MPG.

2. During winter the fuel reformation, and ethanol mix, expect to lose up to 20% MPG.

3. Is your o2 sensor functioning correctly?

4. Is the engine reaching normal operating temperature, about 205~210?

5. Did you recalibrate for the larger than stock tires?

6. Did you read what Stewie posted?
 
Did it go from 30 to 22 mpg? Or 7 to 4 mpg? Saying that it just "got real sh*tty" doesn't really help us to help you.
 
o2 sensor is what i would assume is the culprit.
 
The downstream o2 sensor doesn't do anything but rat out the upstream o2 sensor--kind of like a rat-faced little sister running to Mom to tell on her big brother.

Seriously, the upstream o2 sensor is the worker.
 
Seriously, all it does is make sure the upstream O2 sensor and cat are doing their jobs :D

Mine dropped from 18 to 13-14 in the last three weeks in my stock MJ, right back where it was when the tstat was stuck open (and the exhaust smells rich again.) Got a Bosch O2 sensor on order, hopefully it arrives this week. The PPO (previous previous owner) was not big on maintenance so I'm betting it still has the factory O2 sensors in it at 208k.
 
I was figuring it was something like the 02. I never recalibrated my speedo for the tires but that really doesnt matter I can still tell there was a drop from before. I was at around 17mpg now im at 10-11.
 
If you didn't recalibrate the speedo, the odo is off as well. Just sayin'. You're still a bit low, taking 31s into account.
 
Jays00XJ said:
I was figuring it was something like the 02. I never recalibrated my speedo for the tires but that really doesnt matter I can still tell there was a drop from before. I was at around 17mpg now im at 10-11.
Click to expand...

are you determining poor gas mileage from 1 weeks worth of mileage? could be a bad tank of gas. you really need several tanks of gas to determine mpg.

or simply, diagnose/test all the components that control air, spark, fuel. dont just guess.
 
Yea the od will be off, but it will still give me mpg reading and will tell me when there is a drop. That shouldnt matter. So is everybodys vote O2 sensors then?
 
Jays00XJ said:
Yea the od will be off, but it will still give me mpg reading and will tell me when there is a drop. That shouldnt matter. So is everybodys vote O2 sensors then?
Click to expand...

vote on the bad part? it could be, but, with larger tires the odometer is also off by the same percentage. where do you think the computer gets the info for the mileage driven to compute the avg mpg?

before replacing the o2, check the fuse for the o2 heater curcuit. then test or have it scanned for codes.
 
Need at LEAST 3 samples before you can even begin to formulate an opinion. Calculate your gas mileage pump to pump, miles driven divided by gallons consumed. No shortcuts or extrapolation of miles driven to a tank.

As mentioned, cold weather and fuel formulations can have a pretty significant effect.

On a 2000 with no check engine light that is running reasonably well, new plugs and possibly a new UPSTREAM oxygen sensor may help.

Periodically, the use of a quality fuel injector cleaner like BG44K, Lucas, Techron etc can be helpful in optimizing injector spray pattern.
 
Next time I fill up I will use some fuel injector cleaner, and it does run well no problems at all, once in a while it will start up and run on 1 or 2 less cylinders but goes away after about 30 seconds of driving. but that happens like once a month if that. When calculating mpg i do divide my miles driven by gallons used. I replaced the plugs at around 160,000 im at 178,000 now. I replaced them with autolite platinums, i know there is no need for the platinums on a 4.0 but walmart only had enough plugs in the platinum.

Where is the fuse for the 02 heater circut? In my fuse block under the dash or in the fuse block in the engine compartment, never looked to see if there even was a fuse block in the engine compartment. Does anyone know what slot it is?

Thanks for the help guys!!

Jay
 
ha i get 10, and 5-7 wheelin and drivin like its stolen lol but its to my new knowledge that the wiring and the second o2 which is like 6 months old liked to rub on the drive shaft so now all the wiring is bare.
 
Jays00XJ said:
When calculating mpg i do divide my miles driven by gallons used
Click to expand...

Yes.

Try to reduce the numbur of variables as you can.
Try to fill up at the same station/pump/time of day. Try to fill up at about the same tank level every time (I usually fill it at or near 1/4 tank). Fill up, and don't add any past the initial pump click-off. Re-set you trip odo. Next time you get gas, divide the miles on trip odo by amount of gas you just added (the first fill is just a starting point). Repeat, repeat, repeat, etc. (larger sample size yields better/more reliable results).
 
Jays00XJ said:
and it does run well no problems at all, once in a while it will start up and run on 1 or 2 less cylinders but goes away after about 30 seconds of driving. but that happens like once a month if that.
Click to expand...

ummmmm.....this needs to be explored a bit. The engine should run smoothly in all conditions, all engine temperatures.

So it runs rough on startup periodically but ONLY when engine is cold? Never does this when the engine is hot? Do you ever notice extended crank times (having to crank longer than normal) when the engine is cold?

Do you currently have or have you ever seen a check engine light? If so, you have stored codes which should be pulled with a code reader or a scan tool for guidance, then post the exact code numbers here for comment.
 
Last edited:
You must log in or register to reply here.

Similar threads

J
Cranks, Fires, Dies - 1991 Briarwood Restoration
Replies
17
Views
3K
Jam Master Jay
J
G
Chrysler 8.25 wheel studs for aluminum rims
Replies
10
Views
2K
kastein
kastein
L
Gas Leak from Top of tank?
Replies
16
Views
2K
Anak
Anak
T
XJ shackle with custom springs question.
Replies
11
Views
203
XJlimitedx99
X
P
AC Compressor Only Engages When Relay is Jumped
Replies
10
Views
3K
Irene6A
I
Back
Top