really bad mpg

i have a 97 4.0 168,000 4" lift on 31's i used to get around 15 mpg in the last month i have been getting around 10-12 mpg any ideas

o2 sensor

i just replaced them 2 weeks ago

same gas station?

yeah every time

checked plugs, wires, filter, etc?

does it run worse? or just bad mpg?

yeah i just the plugs made sure the gap was right checked the wires it runs just the same as it did

Are all the tune-up parts fresh ? Tested the coil ? Cleaned the Throttle body ? Used some fuel injection cleaner ? Check for a cracked exhaust manifold ? Re-booted the ECU ?

What brand of O2 sensors were installed ? Inspect the sensor wires ? Tested the sensor operation ? Viewed any O2 sensor live data ?

yeah i checked all of that except the ecu i'm trying that tonight

Tim, what is the proper way to test the coil?

Cracked exhaust header/manifold? Mine had a hairline crack at the collector and my fuel mileage went down from 17 to 14 mpg...

I'm not big on JP magazine anymore, but they ran an article on Cherokee gas mileage a few years back, and it's on line. Some pretty good info(and some usual B.C. from JP). Here's the parts:
http://www.jpmagazine.com/projectbuild/154_0808_building_a_fuel_efficient_jeep/
http://www.jpmagazine.com/projectbuild/154_0810_building_a_fuel_efficient_jeep_part_2/
http://www.jpmagazine.com/projectbuild/154_0906_1998_jeep_cherokee_xj_project_mileage_master/

merdock69 said:

Cracked exhaust header/manifold? Mine had a hairline crack at the collector and my fuel mileage went down from 17 to 14 mpg...

Click to expand...

Wow, seriously? I thought it would take a pretty big leak.
Maybe it's time for me to look at getting a header...

A leaky/cracked manifold or faulty O2 sensors will likely cause a 30-40% drop in gas mpg's.

souske said:

Tim, what is the proper way to test the coil?

Click to expand...

The quickest and simplest test is easy, and very definitive.

Remove a plug wire, hold it about 1/4" from ground and have someone crank the engine or even start it. You'll need insulted pliers to hold it.

Check the COLOR of the spark it's a great indication of the secondary voltage. If the insulation inside the coil is breaking down, it limits the voltage the coil can provide.

It should be a SNAPPING BRIGHT BLUE SPARK - any other color indicates a lower voltage. White, yellow, or red is not near enough voltage.

If it's not the desired blue, check the color right at the coil tower. If it's still not blue, replace coil.
If it's blue at the coil tower but not at a plug - replace coil wire!

Secondary insulation breakdown is very common on the Jeep coils.

Aftermarket coils or crappy “Lifetime Warranty” coils from China don't last long.


Test the coil with your volts/ohms meter. You should have about 1.0 OHMS here
.


and about 10,000-12,000 OHMS here
.