XJmanXJ

i have 1999 Xj with 95k with a 3in lift and 31's and have been getting 10mpg... even before i lifted it it got 10mpg i have replaced plugs, wires, cap and rotor and still no help any ideas would be greatly appreciated!!!

Roblaza

check all your u-joints. if they're bad and dried out, or if some of the needle bearings are jammed up it'll create extra friction meaning your engine will have to work harder to turn the wheels. my 99 ram experienced that, 12-13 MPG on the highway before i did the u-joints and 19-21 MPG highway after i did them.

EvstaG

Check the vacuum line going from the throttle body to the MAP sensor on the firewall. If there is any sort of a crack or puncture in this tube, the vehicle will run rich and use fuel unnecessarily. Have your oxygen sensors been replaced recently?

XJmanXJ

as for u-joints there is no play in them but for the small price i might as well replace them and cross that off the list and i have checked all vacuum hoses

EvstaG

That figure seems very unlikely, especially because most driving is done with the wheels straight, when (theoretically) no needles should be moving. I'm not saying a difference won't be noticed, but A difference of nearly double your fuel economy...

XJwonders

My 99 has a 3 inch lift and running 235/75, and I get about 11-12 mpg. I know for a fact that my tranny is slipping a bit and that contribute to loss of mpg. U can do the 784/789 upgrades. It should help add 2-3 mpg, u also can do cold air intake, sourcing the sir outside the engine bay, it help put in 1 mpg. Some people said putting a throttle body spacer and it ll help add 1-2 mpg but I don't believe in that but it doesn't hurt to try though. Also check your tranny oil for signs of wear and do a tranny flush. U can change the mechanical fan over to dual electrical fans, as it put less load on the engine. And remove unnecessary weight from jeep. For 31's. it would be great idea to regear the axle to 4.10.

EvstaG

I think the OP should be focusing on returning things to their "stock" condition, rather than trying to add aftermarket parts. An injector swap is always a good idea, but unless you have a very frequent lead foot, 10mpg's seems very low. Are any brakes dragging, do you have a clogged cat? (I know there is a good way to test this that is outlined somewhere on this forum.)

XJwonders

I'm merely suggesting any available ways to do it, stock or aftermarket. I'm interested into how to test the cat converter. How do u do it?

XJwonders

Oh yeah. Avoid wearing steel toed boots, wear sneakers instead or go barefoot

EvstaG

Oh sorry, I certainly agree that what you said is an acceptable way to increase fuel economy, (except for the TB spacer), but 10mpg seems excessively low. Also, to the OP, did you use a GPS to calculate mileage, or just the speedo? Because if you have bigger than stock tires, your mileage will be calculating lower than it is actually if you have larger tires.

XJmanXJ

i used the odometer but even when i was sitting all all stock suspension and tires i was getting 10

XJwonders

I lost 4 mpg when I ran my 31's on my stock 3.55 gears, so I went back to 235/75s and plan to get 4.10 gears installed then I will put my 31s back on. So if u are running 31s all the time, it probably be a good idea to regear also. With that and all other fixes on engine's tune up, the mpg will be even more, but not the throttle body spacer, it really does nothing.

alpine.adrenaline

Seafoam everything, regear axles to 4.56, and get an alignment.

Roblaza

I ***** you not. Mind you, those numbers were going by the overhead readout so accuracy is going to vary obviously, but it was a big change nonetheless. I could feel the difference in the truck not working half as hard as it previously had to. When I got around to changing them, my driveshaft was about to fall out, and about 75% of the needle bearings were gone. The other 25% of them were flattened right out or bent on top of themselves.

Dan91

My '99 gets 10/15 city/highway but it's at 270,000 miles, and I've got 33" tires on stock gears. I've replaced plugs/wires/dist cap/rotor/air filter and a load of mechanical stuff. I'm thinking O2 sensors next...