IF you have an analog high impedance meter, it should narrow down to a tight range in closed loop at 2500 rpm, like .35 to .55 V with a very tight rapid swing. Any codes? Define poor mpg and any mods....
On HO rigs, I have never heard anything good about any O2 sensors except NTK. Is the exhaust manifold cracked????
13.266MPG overall average (with a 75/25 mix of Highway/City driving) with one outlier of 11.2mpg (on 5th fill up) and one of 16.6mpg (straight highway for a whole tank)
Mods
235 tires
"Upcountry" suspension
2 inch reciever hitches front and rear
No armor or aftermarket bumpers or anything like that
Parts replaced so far,
Full tune up with air filter, plugs, cap, rotor, wires ect
O2 sensor with the aforementioned Bosch
Both CCV Elbows
Sections of nylon vacuum line that had cracks.
Waiting to be replaced,
New STANT t-stat with OEM Mopar TSTAT because I'm getting huge temp swings currently so thinking that STANT may have been bad out of the box
Parts tested and checked good-
Coolant temp (in the thermostat housing)
Intake air temp
Vacuum (pulls 20-21 inches at idle with no fluctuation of the gauge)
Tests waiting to be done due to time
MAP sensor testing
Smoke testing of Vacuum, brake booster and EVAP systems. Doing the propane torch method showed nothing but I still want to do the smoke test.
No codes using the key on-off method of pulling OBD1 codes
I've attached a chart below from the app I use to keep track of mileage, service, ect. This is my 4th XJ and the only one so far that hasn't gotten high teens or even 20mpg on the highway.