HORRIBLE MPG...help.

2000 XJ, bone stock for. 150,000 on it.

My MPG went from 16-18 around town to 11-12 overnight. No CEL on, runs fine. Thought maybe O2 sensors were bad, replaced all 4 with NTK's, MPG stayed horrible.

CEL came on the next day with a p0432....Main Catalyst Below Threshold (Bank 2) I cleared it and it hasn't came back in the last few days. maybe I need new front cats which is another issue. I assume it has Cali emissions with 4 O2 sensors. There are precats for Cali and NY Jeeps and others that says for all but Cali and NY(guess they don't have CARB #). Will a non CARB setup work just the same(I am in PA). Big price difference $250 from Rock auto, or $400+ locally.

Can somebody post what good and bad O2 sensors graphs would look like on scanner. I have googled it but haven't had much luck finding out.

Weird thing is, the day my mileage went to hell, I started my Jeep and gas then started POURING out from around the gas cap as though the tank was pressurized. NOBODY can tell me what may have caused this other than" I guess the tank got pressurized .....duh)

ANY idea what I can check/do to hopefully get my MPG back up so I can afford to still drive it. I really don't want to start throwing money into it without a very good idea that it will fix my issue. At this point I think my 401 powered CJ-8 on 35's would be getting better MPG.

Man, and here I thought 10mpg on 30" tires was normal for my 92! I too have no CEL and it runs strong.... Gonna have to look into this... Update of you find anything out

Run a can of BG44K through it. If you can't find that, run Chevron Techron. Dirty/sticking injectors are a big problem that is more common than most think.

I'm really not familiar with the newer type pressurized gas tanks, so I can't add much there.

The last couple of times my mileage fell way off, it was a temperature sensor. Like I said, I'm not real familiar with the newer XJ's, but on most it will only pop a CEL if it is way out of range and not just when it is giving giving the wrong info to the PCM. In other words, a temperature sensor may be telling the PCM it is colder than it actually is and not pop a CEL.

Once was my MAT and the other one was my battery temperature sensor. And don't ask me how my battery temperature sensor caused a significant mileage drop, but it did. It may just be a programming glitch, it may actually have something to do with fuel management, I don't know for sure. I do know after testing it and finding it way out of spec, I replaced it and my mileage improved.

Low tech. but sometimes fruitful when troubleshooting, is to walk around back of your XJ and see how much soot is coming out of the exhaust, after the motor is up to operating temperature at idle. You can do the same thing after a run up the interstate, in a lower gear, keeping the motor above 3000 RPM with a steady pedal. The inside end of your exhaust pipe should be significantly lighter in color.

not familiar with battery temp sensor.
engine temp is in check, not overheating or running cold, heater works great.
I replaced the 0331 head a few years ago.

pajeepman said:

2000 XJ, bone stock for. 150,000 on it.

Weird thing is, the day my mileage went to hell, I started my Jeep and gas then started POURING out from around the gas cap as though the tank was pressurized. NOBODY can tell me what may have caused this other than" I guess the tank got pressurized .....duh)

Click to expand...

Does your rig with California emissions have a evap canister on the firewall? They're supposed to pull fumes from the tank and add it to the intake to be burned. I'm wondering if it's pulling liquid gas and dumping it into the intake. I would think there is some kind of check valve to prevent that.

From Alldata ('97 XJ): DUTY CYCLE EVAP CANISTER PURGE SOLENOID
All models are equipped with a duty cycle EVAP canister purge solenoid. The solenoid regulates the rate of vapor flow from the EVAP canister to the intake manifold. The Powertrain Control Module (PCM) operates the solenoid.

During the cold start warm-up period and the hot start time delay, the PCM does not energize the solenoid. When de-energized, no vapors are purged. The PCM de-energizes the solenoid during open loop operation.

The engine enters closed loop operation after it reaches a specified temperature and the time delay ends. During closed loop operation, the PCM cycles (energizes and de-energizes) the solenoid 5 or 10 times per second , depending upon operating conditions. The PCM varies the vapor flow rate by changing solenoid pulse width. Pulse width is the amount of time that the solenoid is energized. The PCM adjusts solenoid pulse width based on engine operating condition.

The solenoid attaches to a bracket located in the engine compartment near the EVAP canister (Fig. 4) . The top of the solenoid has the word UP or TOP on it. The solenoid will not operate properly unless it is installed correctly.

I'd replace the gas cap to start with.

gas cap was replaced last march. I was getting a code for small evap leak, has not reappearred so I assu,e the new cap is good.
Gf has jeep today so I can't do anything today.

Replace the Coolant Temperature Sensor.

Probe all the O2 sensors wire harnesses for continuity. Test all the O2 sensors with volts/ohms multi-meter.

Reboot the PCM.


This very simple procedure will erase the 1996-2001 “Adaptive Memory” stored inside the PCM and allow a new “Adaptive Memory” to be developed. After performing this procedure the 1996-2001 PCM will re-learn and store into Adaptive Memory your engines performance characteristics. Perform these steps exactly as they are written, in the order they are written. This will cause the adaptive memory in the PCM to be erased and cause the PCM to go into Fast Learn Mode adaptive mode.

Disconnect the POSITIVE battery Terminal and touch it to ground ( not the battery negative terminal ) for 30 seconds. The engine block or the A/C compressor pump will work as a ground. (This is to discharge the PCM capacitors, which maintain the Adaptive Memory).

Reconnect the Battery Cable
Turn Ignition Switch to the “On” position but DO NOT start the engine
Turn Headlight “On”
Turn Headlights “Off”
Turn Ignition Key “Off”


The PCM Adaptive memory has now been flashed, or erased from the PCM. When you start the engine it will be running off a set of pre-programmed tables that come with the PCM from the factory. When you get the engine up to operating temperature the PCM will start to collect data for the “Adaptive Memory”. The PCM will collect data for Adaptive Memory for the first 50 Warm-up Cycles.