antibiotics

Sorry for another gas mileage thread? But I am really confused now. My rig (99 XJ, I6, automatic, 4wd) is well maintained, engine pulls strong and transmission shift smoothly. The only defects I am aware of are cracks on exhaust manifold, and some thumping noise when press accelerator.
The highway mileage seems normal as I can get 20 mpg at 65-70 MPH. But the city gas mileage really sucks, only 12 mpg! I have replaced spark plugs, wires, distributor cap/rotor, oxygen sensor, upgrade fuel injector. The fuel pump assembly was also replaced due to the failing pump, so fuel filter should not be a factor here.
Any suggestions?

Thanks.

89Laredo

sounds about right... EPA estimate is around 14 city IIRC.
Add in ethanol and it goes down a little.

xjmuddigger

x2,a new exhaust manifold might help as well

ProudXjOwner

Some people get more and some less. I personally get about 8-10mpg city

antibiotics

That is one reason why I like old-school XJ even it returned terrible gas mileage.

Should I be concerned about the slight thumping noise when press accelerator hard? I checked drive shaft U-joint and it seems fine, no play when turning. Could a worn engine mount cause this?

xjmuddigger

i get about 18ish combined

tjwalker

My 99 gets from 16-21, depending on how I drive it and the outside temperature.

If you can pull 21, I would suspect that there isn't much really wrong. The two sensors that you could test that contribute to fuel/air mixture would be the coolant temp sensor and the map sensor. Both can be easily tested with a meter and a manual.

As for that noise you hear upon acceleration, have you ever heard of the "slip yoke" issue with all XJs 97 and newer? The slip yoke on older ones was lubricated by a bath of automatic transmission fluid, but newer ones don't have this. The factory lube goes away over time, leaving you with a "clunk" noise upon initial acceleration. Very common. I have lubed my slip joint a couple of times already; seems to last about 30k or so.

Here is a link to a thread that gets into the procedure if you are so inclined...

http://www.jeepforum.com/forum/f11/g...ite-up-357569/