Sudden Drop in gas Milage

[vajeeper]

I have a stock 97 xj country. Within the last month my gas milage has dropped from 320 miles per tank to about 240 per tank. I havn't done anymods to my jeep at all. please help me out if you know anything.

 

[vajeeper]

for a little over a year when i start my jeep and first start going the engine will bog down and i can fix it by leting off the gas or by reving in neutral. Im not sure if it is the cause of the new problem or this problem has gotten worse.

 

[joe_peters]

Loss of mpg:

1. O2 sensor causing run rich.
2. Cracked exhaust header upstream of O2 sensor, O2 reads lean, runs rich.
3. Coolant Temp Sensor bad, reads engine cold, computer makes it run rich.
4. Missing thermostat, stuck open thermostat, wrong temperature thermostat, engine is not warming up and runs rich.
5. Belt driven accessory failing, too much drag on engine, poor mileage.

Check these articles in JP Magazine:

http://www.jpmagazine.com/projectbuild/154_0808_building_a_fuel_efficient_jeep/index.html

http://www.jpmagazine.com/projectbuild/154_0810_building_a_fuel_efficient_jeep_part_2/index.html

http://www.jpmagazine.com/projectbu...cherokee_xj_project_mileage_master/index.html

 

[vajeeper]

I think i will start with the O2 sensor and see what happens after that...is it better to replace the front one or the rear to try and troubleshoot the problem...can an O2 sensor be tested?

 

[joe_peters]

I think i will start with the O2 sensor and see what happens after that...is it better to replace the front one or the rear to try and troubleshoot the problem...can an O2 sensor be tested?

Testing the O2 sensor is kind of a PITA--it has different voltage outputs at different temperatures and different o2 levels.

I would replace the the upstream (front) O2 sensor first.

 

[jujuscisyn]

sudden drop in mileage I immediately look for the o2 sensor or a plugged cat