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Average Gas Mileage for 99 XJ?

A

AG74683

NAXJA Forum User
Location
Hickory/Boone, NC
4.0l engine. Its got a 3 inch lift on 31's.

Im getting around 10 mpg city (very rarely do high way driving). I think this is around 4 mpg low.

Ive looked around at how to calculate the difference for my tire size, but I cant seem to find out how exactly to find the accurate gas mileage without using the trip meter or MPG read out on the overhead counsel.

Any ideas as to the cause of the low MPG's? Gonna begin replacing plugs/wires here soon.
 
remove your mechanical fan and run 2 stock electric fans, k&n intake plugs all that stuff, maybe a better flowing exhaust. i have 33s and get 22 mpg highway and 18 city. the best way to figure mpg is fill up and reset your trip then drive then go fill up and figure how far you went by how many gallons used
 
Using the mileage from your odometer will make it seem lower than it is. What ever the number is for how far you've gone between fill ups, multiply it by 1.11. 31's are about 11% off from stock, this will be how far you have actually gone. then divide by gallons.
10 is really low, do a search on oxygen sensors.
 
Just checked both O2 sensors. Both seem to be hooked up fine, wires look okay.

Ive read about how to test them, but Im not sure I feel comfortable with it, plus I dont think I have the equipment needed to do that here.

Does one generally fail over the other? They both are clearly the stock sensors, and from the way the wires run to the before cat sensor, I can definitely see where the wires could get torn up.

The spark plug wires are clearly stock as well, and I would hazard a guess that the plugs might be as well, although it looks like replacing the plugs on this is a several hour long process, the number 1, 4 and 6 cylinders look like a PITA to remove.

My tires are also on the way out, but I wouldn't think they would have as huge of an impact on losing close to 7-8 miles per gallon like I am losing...
 
The plugs and wires are really easy to change. Takes about an hour for me and im really slow. You just need the right tools. IF your O2 sensors were bad i would expect a check engine light.

Doing basic maintenance will ensure that you are getting the proper gas mileage for your vehicle unless there are bigger problems.

If everything looks original its time for a tune up. These parts are meant to be changed at regular intervals. I would start with Fuel filter, air filter, plugs, wires, cap and rotor. Get the OEM style parts at any local parts store. Then check tire pressure and get an alignment. If your check engine light is on, pull the codes using a scanner which can be used at most auto parts stores and get the problem fixed.

Doing the basic maintenance will give you a good platform to measure your actual fuel economy. Remember you on are bigger tires so follow lightfoot's advice
 
1 basic maintenance listed above
2 remove lead-filled shoe soles :)
3 k&n, larger exhaust etc.

you should be able to get a lot better gas mileage out of it, but your getting a little better than you think too.
do you pack a lot of extra gear around in it in the city. extra weight can do a lot to kill your mileage.
 
Either you drive like a dick or there is something wrong with your xj.

I get 13 city and 17 highway with an auto, 39.5 rockers, unimog/9" with 5.32 gears, K&N air filter and clock in at 4500lbs driving around town. If i go above 60mph my milage drops off quick.
 
I dont stomp on it or drive like a maniac so I know thats not the issue.

The plugs/wires definitely need replacement, but I cant imagine that those 2 items could possibly cause a drop that much could it?

Most of the time Im not hauling anything around that would add any weight that would mess with gas mileage.
 
checking the O2 sensor is easy. you just need a multimeter. If you want to get fancy you can do like I did and use some scotchlok inline splice connectors to add pigtails to the o2 harness and run the wires into the cab for easy testing and monitoring while driving.

http://naxja.org/forum/showthread.php?t=1012701&highlight=test+renix+oxygen+sensor

Bad plugs, wires, cap, and rotor can definitely affect mileage.
 
AG74683 said:
Just checked both O2 sensors. Both seem to be hooked up fine, wires look okay.

Ive read about how to test them, but Im not sure I feel comfortable with it, plus I dont think I have the equipment needed to do that here.

Does one generally fail over the other? They both are clearly the stock sensors, and from the way the wires run to the before cat sensor, I can definitely see where the wires could get torn up.

The spark plug wires are clearly stock as well, and I would hazard a guess that the plugs might be as well, although it looks like replacing the plugs on this is a several hour long process, the number 1, 4 and 6 cylinders look like a PITA to remove.

My tires are also on the way out, but I wouldn't think they would have as huge of an impact on losing close to 7-8 miles per gallon like I am losing...
Click to expand...
upstream is the only one that will affect your mileage afaik, downstream will just make the jeep complain if the catalytic converter isn't doing its job properly because it needs to be replaced. Replace the upper one first, sometimes they do go bad or start getting lazy without throwing a code. FYI, the overhead console is going to be using the same miscalibrated distance counting as the speedo/odo so if you go off that it'll still be wrong. Plugs and wires are easy, as you noted the #5/6 can be a little troublesome but it only took me about 45 minutes to do mine the first time and that was even though I had to replace the heater control valve after I broke it in half pushing it out of the way.

Other things - are you reaching operating temp? You should hover around 200-210 or so when driving normally, if you don't get above 170 or so, the engine will never go into closed loop mode and you will be blowing gas out the tailpipe.

I get around 13 w/uncorrected speedo gear (around 15 actual) on my XJ with stock gearing and 33s, and I'm a big fan of the skinny pedal, so something definitely needs work on yours.
 
Bought a filter recharge kit for my K&N filter. Didnt realize these kits are good for more than one recharge, so every time Ive bought a vehicle with a K&N I never bothered with buying the kit :laugh2:

Picked up new plug cables (Bosch. These any good?). New plugs, and picked up a bottle of Lucas Injector/Carbon cleaner and dumped it in.

Oil change is up next, probably will do the A/T fluid and diff. fluid eventually. I need to put my new Diff cover on anyways.
 
I have a 2000 XJ 6cyl auto. I'm not surprised, you could be losing 7-8 mpg. It sounds to me like all of the regular maint. items would easily add up to that loss. Oil helps. I Hope the plugs and wires helped.

I have used a scangauge 2 for about 2 years now and I think I can notice when my tire pressure is 5lbs low. I'm also not surprised to hear the folks running oversize tires are getting decent ~20mpg on a 60% hwy 40% city trek.

I get between 18-20 regularly and have seen as high as 22. I'm using Rusty's airtube, a K&N filter and replaced the catback with a Flowmaster 50 which has a 3" exhaust diameter. I think gas quality has a bit to do with it as well. I'm partial to Marathon and run a tank of premium through every once in a while. The scangauge TPS readout (Throttle Position) is by far the most controllable variable in your control.
 
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