Make sure to pump in a full tank of gas. Drive it for at least one full tank.. running it down close to the empty mark before filling it back up a second time.

When the PCM is in learning mode the monitors do things at specific points. The majority of what they need to do is all done at the level of an 85% full gas tank. The rest of what needs to be done happens somewhere around when the gas tank level is at 15% - 33% full.

Normal everyday driving technique works. It needs a little of each: Some steady cruise RPMs at varied speeds, some idling time, and some stop & go driving to fully accomplish what needs to be done. If the monitors do not get to fully accomplish something they need to do? They will just come back to it at a later time when the gas tank is at the correct level for them where it needs to be... They can skip around to accommodate the driver in order to fully complete the process.

These are the reasons why I mentioned saying it as needing, "one full gas tank" worth of driving time in order to be able to achieve a full recalibration of the systems.

The biggest factor in MPG is the component behind the steering wheel. If you're driving aggressively you're going to take a hit on MPG. You mentioned doing 70-75 on the highway, this is not "lead foot" level, but it's certainly more aggressive than the "grandma" level I'm referring to.

thats the speed limit here

X2 on the NTK sensors.

This same thread comes up about once a week or two . There’s always someone somehow that says normal is something asinine like 18-22mpg or 25 even . I have owned about half a dozen 4.0 jeeps ranging from 93-05 and they ALL did terrible on gas mileage and all ran as intended it’s common knowledge with a 4.0 . They seemed to all range around 15 mpg often times less hardly ever more . If you want mpg you bought the wrong vehicle , period.

Agreed.
Like my buddy says. "You are basically driving a billboard down the road".
I figure in my '96, with Federal Emissions, I used to own I got 15 MPG around town.
And my around town is very short trips.
She seemed to do much better on longer highway trips.
But I never drove it far enough to bother to calculate the MPG
My '00, with CA Emissions, that I own now I am thinking about 13 MPG around town.
Again short trips with frequent starts and stops.
Now on the highway I have done almost 22 MPG on longer trips.
Calculated when I did a 360 mile round trip to Jersey and 300 mile round trip to the Cape.
Of course to buy Jeep parts. LOL.
And this is doing everything I could think of to improve gas mileage.
The Jeep was a kind of running not really driveable impulse purchase.
Once I straightened everything out and performed all the maintenance I set my sets on improving MPG.
I did not really care but I figured I would do what I could and it is what it is.
So she needed a new exhaust and got new O2 sensors.
Cleaned the throttle body and IAC.
New plugs, synthetic in my diffs, and brand new tires. Cooper Discovery HT's.
And of course just making sure that everything is operating as it should.

I found this below.. It is recommended to do for XJs after any major change has been done to the Jeep. I do not believe you are required to do the headlamps thing on a WJ, but it is necessary to do it like this way for on an XJ.. from some of what I read.

It applies for several differing aspects of the PCM memory functions. Anytime a performance part, or some of the sensors get changed out, etc... This procedure applies. In my mind, the oxygen sensors finally coming back alive on this Jeep makes this apply. What do you guys think? Do you think this could further help his situation more by doing this next?


The PCM (computer) memory is stored in different capacitors for various things such as the PCM having learned the operators driving technique, fuel mapping, etc...

(copied and pasted):

The Electronic Control Module, or ECU as its commonly called, stores the information for your vehicle and your driving habits in its memory. The memory is maintaned by a series of capacitors that hold a charge and maintain power for the ECU when the vehicle is turned off. The following procedure will allow you to reset the memory and allow the computer to 're-learn' how to drive by adjusting to the drivers habits and the vehicles modifications.


1. Disconnect the battery by removing the positive battery cable. Remember, the positive battery cable.

2. Ground the positive cable to the negative cable for 30 seconds to erase the electric charge in the capacitors.

3. Reconnect the positive cable to the battery.

4. Turn the key to on but do not start.

5. Turn the headlights on.

6. Turn the headlights off.

7. Turn the key off.


The ECU has now been reset to its factory settings. For the next 50 warm-up periods the ECU will re-learn the engine and driving habits for your driving conditions and the newly installed parts.

(end copy and paste)

When I was driving my Jeeps with the 4.0, I was averaging a little more than 5,000 miles per month every month this past year.. The least miles I ran one month on the WJ from between January through Sep/Oct timeframe this year was when one month I only put on around 3,000 milea during that one month. The WJ saw 5,000 (+/-) miles every other month though...

I got really good at determining its behavior and learned the average mpg of the WJ very well.. From long-stretches on the highway, around town stop & go, extreme short trip stop & go, and extreme extended idling times. There were days when I would start up the WJ and it would remain turned on and running for over 14 hours straight. Sometimes, it never even moved to be driven more than maybe a couple of hundred yards at best (literally.. having spent 16 hours straight idling on many many days). Other days, it would be on and running for 12 - 16 hours straight.. and it was only driven maybe a mile or two doing strictly short trips of stop and go driving during this time all day long. It would sometimes see 15 - 20 hours straight of driving time all spent on the highway. Etc...

My WJ is well cared for mechanically. It has a good tune on it. Although, the moving parts such as the wheel bearings and such are all still original. The worst I would see for fuel economy during this timeframe was at around 11 mpg. I would say all in all, it averaged right at 15 mpg with mostly idling and stop and go driving throughout. On the highway, I would see a consistent 20 - 25 mpg every time. My WJ is a RWD model with Cooper AT3 30" tires in it.

I realize that now. But I'm stuck with it for a while, unless I find some one wanting to do some swapping

here we go again codes popped up this time po443, po305,po455 pending,

I know it is frustrating to see codes come back again like this happened. In and of itself.. at this point, I do not think any of these particular codes are going to be major issues? These codes may mean nothing at all? The two EVAP codes are also not ussually known to be detrimental, or to cause any type of poor engine running conditions (although.. they can). I will be ignoring the code specific to the engine misfire on the #5 cylinder for the moment too.

The first thing to remember is that the whole system; the PCM memory, air, and fuel are all recalibrating themselves according to your driving it again with oxygen sensors which are just now becoming operational for the first time in probably a long time... With these codes (even the engine cylinder misfire code) it could be directly related to this fact, and this fact alone? The second thing to remember right now is that older vehicles have also been known to erroneously throw codes like these ones for no good reason at all. This is also somewhat more likely to happen this way with your Jeep, since it has recently been running with a lot of abnormal stress being placed on it.

Even if you did not just have the other recent issues like you did. The very first easy thing to do with these specific codes is to check the fuel cap, reseat the fuel cap again, and make certain it goes on securely. Then, see if the EVAP codes go away on their own? Or, reset the codes to see if they come back? Believe it or not.. the most common reason for these EVAP codes is having a faulty gas cap or an improperly secured gas cap.

Here is what I would recommend to do about it right now. Fill up the gas tank again completely topping it off. Check out the gas cap and look to make sure it does not have a crack or anything going on with it, and be extra certain to properly secure the cap after filling the tank. The PCM and monitors will need both, a 1/4 full gas tank and a 3/4 full gas tank, in order to fully resolve any erroneous issues that could possibly be occuring right now after a reset.

Since your Jeep is in sort of an extreme learning condition mode. Use the procedure how I mentioned it above in my last post.. to completely reset the entire system in order to try and help out the PCM. Doing this will initiate the monitor protocols for it. The PCM may simply just be confused?

If the Jeep is not actually displaying symptoms of poor running conditions for you right now? Use the reset procedure from the post above.. and run through two full tanks of gas while continuing to monitor everything.

P0443
Is a generic code that the engine control module (ECM) has detected a malfunction with the purge control valve or its control circuit.

The most common cause is the fuel cap was accidently left off or not tightened securely after getting fuel.

P0440
Is the general code for a problem with the Fuel Evaporative Emission System.

The most common reason for this code is related to the fuel filler cap which may not be sealing properly.

I think mine uses a quart of gas just pulling out of the garage.

P0443 Evap Purge Solenoid Circuit An open or shorted condition detected in the EVAP purge solenoid control circuit.
P0305 CYLINDER #5 MISFIRE Misfire detected in cylinder #5.
P0455 Evap Leak Monitor Large Leak Detected A large leak has been detected in the evaporative system.

I had an EVAP code set once on my Jeep. I looked into it whichever one it was. I think it was P0441 maybe? It only remained active for maybe 100 - 200 miles, before it went away on its own. What I remember reading was the EVAP code having been set on mine was a common code to be set after accidentally overfilling at the filler neck.. how if you pull the nozzle out while there is still some fuel coming out of it, and the fuel overflows going around the filler hole. Sure enough, this was exactly what I had just accidentally gotten done doing too, was spill some fuel out and around the filler hole on the most recent fill-up like that.

Yes that sounds right..
P0441 Evap Purge Flow Monitor Insufficient or excessive vapor flow detected during evaporative emission system operation.

My 89 MJ got 21-22 mpg, and hit 25 on a long highway trip. That was with a manual, 3.07 axles and 225 tires. Lifter 6" with 32s, 4.11 axles, an auto and short 15-minute communtes to work, I get 17-18. So I've had both ends of the range with the same vehicle. :}