Another gas mileage thread... 12mpg.. what should I do?

I have a '96 XJ, stock with 65k miles and 235 tires (although 2" lift going on this weekend).

Over the last 429.1 miles, I've averaged 12.027 miles per gallon. I do not drive it hard. In fact my '90 w/ 171k gets 15 - 16mpg in town.

What are the first things I should replace to get my mileage up to a more acceptable level? I have a new fuel filter and air filter in already. I just bought this rig, and I don't know if the plugs/wires/cap has ever been changed. I also don't know if the O2 sensors have been changed or not.

What's the first thing I should do?


Also, when I go to reset the computer, after the battery cable is disconnected, do I turn the ignition to the "run" position, or all the way over as if I were going to crank the motor?

Thanks

Start with the basics, new plugs, wires, cap and rotor, used dealer OEM parts for this.
You do have a problem, my 98 gets 22 solid mpg now that I aired my LT235's up to 40llbs, it was 19 when they were around 30....
Clean the throttle body out with a good cleaner, dump a large can of Chevron Techron in a 3/4 full tank of gas. Do the cleaning and additive before you change the plugs.
The next big culprit is the O2 sensor, a bad one will kill milage by reporting back a lean conditon and the engine will run richer/more fuel.

I only get 13mpg according to the onboard computer (are they accurate??) I have new plugs and wires and I think the O2 sensor is good because it just passed smog really, really easy. How do you do the computer reset thing, just disconnect the battery for a few minutes or do you have to do anything special?

Run a can of BG44K though the gas. It will clean the injectors and remove carbon. The stuff works miracles.

What condition is your exhaust manifold? Cracks can screw up the O2 sensor readings. Of course, if the O2 sensor itself is ancient and not functioning it can cause the engine to run rich and suck fuel. Have you done the on-off-on-off-on trick to pull codes yet? I'd definitely give it a thorough tune-up before getting too concerned about mileage.

I didn't think of trying the on off on off on trick.

I just went out and did it, and the check engine light blinked 3 times, paused, blinked 5 times, paused again, and blinked 5 more times.

What does this mean?

Thanks

Think you better have another look, most of the codes I´ve seen are always double digits (they start at 11). Sounds like you got a 12, battery disconnected in the last fifty cycles and a 55 test over.
www.jedi.com/obiwan/jeep/dtc.html has a pretty good list, I printed out and keep in the glove box.

8Mud said:

Think you better have another look, most of the codes I´ve seen are aöways double digits (they start at 11). Sounds like you got a 12, battery disconnected in the last fift<y cycles and a 55 test over.
www.jedi.com/obiwan/jeep/dtc.html has a pretty good list, I printed out and keep in the glove box.

Click to expand...

You sir are correct. Thanks for that list.. I'm printing it now.

I use my XJ for work some, they pay mileage when I use my own. They offer a sub compact. Figured I had to do something about the mileage. After the tune up, I went a couple of tire sizes bigger (30X9.50) and installed a vacuum gauge. Upper scale actual vacuum, lower scale yellow/green/red, to let you know when your using the most fuel. Between the two, it saves about 3-4 MPG. Gas here is more than $5 a gallon.
Think I´m gonna install and air/fuel gauge also. Most of what the vacuum gauge did, was force me to change my driving habits.