I have a '91 XJ with 235/75r15, Auto, 4.0 HO. And every fill up I'm getting around 13.5 to about 15 (If I'm lucky) I changed plugs, wires and cap. The plugs aren't fouling and its not burning any oil, so i'm confused as to what i need to fix or tune. I had a '90 before this with the renix/renault system and it was getting close to 20 on average, with the same size tire.
On another note I do feel vibration past 45 miles an hour so my balance or alignment could be out.
Any help or advice would be greatly appreciated!

SCOUTS OUT!!!!

 

Jeepers...was your 90 an auto as well?

 

Yeah it was.

 

how many miles on your jeep?

 

Clean the throttle body and make sure your tires are properly and evenly inflated. That should help some

 

It isn't the cause of your problems, but new differential, transfer case, and possibly transmission fluid will help your MPG.

 

the jeep has 300,000, but the motor and transmission were replacedabout 20k ago. It does pull to the left a little, if i were to get an alingment done would that help?

 

As mentioned, proper tire pressures, alignment will help. Vibration could also be front propeller shaft 'u'-joints going. Also, consider changing out your air filter. Also, consider going to a higher grade gasoline for a tank, or so. Bad fuel, stiffled air, weak spark, all could contribute to poor mpg. How is your compression? Have you cleaned all your grounds, at fuel pump, engine compartment? Good luck.

 

new O2 sensor will help.

 

Oh.., yeah I forgot, I add 1.5--2 ounces of ACETONE, (from a metal gallon container), with a turkey baster at fill-up, and get an extra mile, or two on average, per gallon.

 

what does acetone do ? and why from a metal container?

 

I dont know if that really works or not, but I heard the acetone does something with the ethanol and makes it run better. All the gas we get now has about 10% ethanol in it and it kills the MPG we should be getting.

 

To get your mpg back
Start with fluid changes front to back, engine, trans,diff, front and back, I prefer synthetic,
New air filter (I not a 5 knock off)and plugs if you can swing it I have great luck with e3 diamond fire ones
Clean your throttle body, air intake temp sensor (I can get cruddy and cause a mis reading on intake temp) and new 02 sensor (a major factor)
If you want to go farther,
Sea foam your engine, using the stall out method, also in your crank case before your oil change
Run fuel system cleaner, I use techrons concentrate, then gumout With regan , you should notice a huge difference by half way thru the regan tank
Also if its high mileage use a good hm synthetic,
If you decide to seafoam I can explain further, I just got my jeep about a week ago, great shape 200k with slight tick when I got it, and about 16mpg it has 31's after a front to back tune up like described I now get 21 highway (I may be off due to tire size but still an improvement by a good bit) after the idle plunger got cleaned it idles smooth as silk and runs amazing,
the key to good mpg is efficiency, I could go on forever haha

 

My renix gets 15.5 pretty much every tank right now, with winter gas. When I bought it it was getting 18. Shows how much of a difference that makes...

 

I hear in the winter you actually get worse mpg, cold dense air is awesome for efficiency, but cold dense air also makes it that much harder to power a brick through it haha