transman

1999 XJ cherokee 4x4 . 100,000 miles .Just bought . Full tune up . Runs excellant . Starts fast . Changed all fluids . Checked replaced all brakes . I have owned 2 Wranglers, and two cherokees .This one is the worse on gas .16-17 mpg combined driving . No lift kit and all air dams in place . Old plugs were clean . Can it be the alcohol in the winter gas . Live in Michigan ?

ZINGGG

I'd replace the upstream o2 sensor, run some injector cleaner and give the throttle body a good cleaning

DFlintstone

Assuming it's a 4 door with auto you aren't doing too bad. And yea, I think they did away with the ethanol subsidy. Maybe our mileage will come back up...

You know the 02 sensor does final tuning of the mixture. I guess as they age/fail they go rich. 60,000 miles is a number I saw.....No Idea if you might have an issue there.

Just on GP everyone want's to be sure that little MAP line from the rubber plug on the throttle body to the MAP back up on the firewall is A1. (mine disintegrated). The MAP will tell it to run rich and cause all sorts of fuss if that like leaks at all, or get's plugged.

The Rooster

I would change both o2 sensors that should increase it a little bit

tjwalker

Oxygen sensors can get "lazy" and inefficient long before they trip a code. In my experience, the average lifespan of oxygen sensors is approximately 100k - 150k and a lot depends on the efficiency of the engine.

If you're going to keep the vehicle, consider installing at least a new upstream sensor. I usually replace both while you're doing it. Try to find NTK sensors; they are the OEM supplier of oxygen sensors to Jeep and are widely available at parts stores.

Ethanol contains less energy than gasoline, so ethanol blends get fewer miles to the gallon. Ethanol advocates contend the drop is 2 percent or less for E10 (which is common through the country for oxygenated fuel), but large scale studies don't agree. An Iowa State University Professor, after reviewing multiple independent studies using fleet vehicles, estimates that E10 delivers 96.4 percent of the miles per gallon of gasoline, a drop of 3.6 percent.

An approximate 4% reduction in gas mileage in the 4.0 seems to be pretty accurate; this from what I have studied and my own personal tests.

My 99 gets between 16-22 mpg, depending on outside temperature, % of highway vs city miles, and how it is driven.

Bottom line is that 16-17 mpg in a Michigan winter is not at all unusual. Don't get too obsessed with trying to find another 1-2 mpg.

transman

Thanks . I will check it out with scanner . Think i will replace . This thing runs great ! Has to be something simple !

541_halseth

16-17mpg thats something i dream about. Then i take the wifes Civic for a drive and remember why i love my XJ

DirtRoadXJ

id love to get 17 mpg. im in the 12-13 mpg range and ive done all the tune up, o2, clean throttle body stuff. im only runnin 31s and ive just acepted the fact that thats just how its gonna be.

The Rooster

it won't throw a code until it stops working completely... they do get lazy and do not throw a code or anything

transman

Update on my posting . Used scanner and watched front and rear 02s .They are well with in specs . Had the factory muffler and tail pipe replaced . BOTTOM FELL OFF . Original . Cat looked good . No blocking . My last fill up showed 15 mpg . 90 % free way . Checked out map sensor . 90 degree rubber line clear , cleaned throttle body . Now solid map code . Voltage hi . Cleared . Solid code . Bought a new one . $90. bucks . Installed and cleared lite . No more lite . Hoping this thing just went bad causing . I will know next fill up . I drive 65 miles per hour . Light accells . My other 3 jeeps I had did much better . 99 xj , 94 yj , 2003 tj .