12 mpg?

I have an EZScan 6000 which reads enhanced chrysler data and have come up with these figures:
Readiness Test Results
MIL Status Off Evaporative System Not Supported
Misfire Monitor Not Supported Secondary Air System Not Supported
Fuel System Monitor Not Supported A/C Refrigerant Not Supported
Comprehensive Component Completed O2 Sensor Not Supported
Catalyst Not Supported O2 Sensor Heater Not Supported
Heated Catalyst Not Supported EGR System Not Supported
Operational Data
## Parameter Name Value Units (if applicable)
1 Readiness Monitors Complete Yes
2 Throttle Position 0.00 %
3 Coolant Sensor 0.90 Volts
4 Throttle Pos 0.80 Volts
5 Min Throttle Pos 0.80 Volts
6 Battery Voltage 13.70 Volts
7 AIS Position 29.00 Steps
8 Oil Press Sensor 1.88 Volts
9 Oil Pressure 36.00 PSI
10 MAP Sensor 1.70 Volts
11 Total Advance 11.00 °BTC
12 Fuel Sensor 2.76 Volts
13 Engine Vacuum 426.00 Torr
14 Desired Idle Speed 664.00 RPM
15 Desired AIS Position 30.00 Steps
16 Actual AIS Position 30.00 Steps
17 Purge Duty-Cycle 8.00 %
18 Injector Pulse 1 4.00

Anyone know if these readings are up to snuff? Jeep runs really well but not as good as I think it could and I'm only getting 12 mpg and i'm not beating on it.
2000 xj with 108k on the clock running 30x9.5 15's and 3" of lift
New plugs and oil changed consistently every 5k

12mpg in the city? Auto or manual? If you take it easy you shouldnt have any problems but my city with a little spirited driving gets me around that with 235's.

Automatic in the city. Don't do too much highway probably around 25%, so mostly short trips. I'm thinking the 02 sensor might be bad. Haven't replaced it since I got the jeep with 40k on the engine. Need to check it though. There are a couple posts about the test but mostly revolve around the renix. I need to do some more research about the test and how the 02 sensor works.

No matter if I'm on ther highway , city or gravel raods/wheeling , I get a steady 16 . Not 15 . Not 17 . 16 .

Weird ....

wow...you've got something wrong....i'm running 31x10.50's and get about 16 or so straight city, and 17 almost 18 on the highway...all with A/C ON. Stock I could get 17-18 city, and 19-20 on the highway..again A/C ON.

Running good quality gas? I've found that the lesser stations tend send my readings down the crapper (the ole "you get what you pay for" type thing).

I know you say new plugs, but did you do a full tune up, or just throw some plugs in? Other things to check are air filter, tire pressure...etc etc.

edit: When you moved to the 30's, did you recalibrate your speedometer? If not, then you're not getting a true reading of mileage....

that sounds around right for an automatic in the city. Mine is stock with 235 AT's and if I drive it around town, with only a bit of highway I'll be quite lucky to avg 12MPG. Highway driving I pull down good numbers, probably around 20-21, but in the city, with an automatic slushbox you can count on the thing just sucking gas. Ive given up on mine, the cost of gas is just getting too high. I live at the top of a long hill (12 blocks or so long), and to drive from town at the bottom of the hill to my place costs me around $3 each and every time. It adds up, fast.
I drive a fullsize ford with a V8 for work, and on average that fullsize pickup would cost me only $25 more per month in gas if I did the same driving. Seems a little ridiculous, but the inline 6 is just a pig on gas no matter how you look at it. I'm selling mine for something with a more efficient 6 cylinder, maybe even go down to a 4 banger.

tsill said:

Automatic in the city. Don't do too much highway probably around 25%, so mostly short trips. I'm thinking the 02 sensor might be bad. Haven't replaced it since I got the jeep with 40k on the engine. Need to check it though. There are a couple posts about the test but mostly revolve around the renix. I need to do some more research about the test and how the 02 sensor works.

Click to expand...

There is a post I made near the end of a thread I started, called the "ReniX Files" where I detailed the difference and similarities in exactly how to test the Renix, and the newer 1991-2002 XJ O2 sensors. You need an analog (old style, not digital, it has a needle like the fuel gauge) volt meter. Then back brobe the two smaller wires (not the fatter wire that carries 14 volts to the O2 heater) and look for it run back and forth across the set point of 0.45 volts. If it stays on either side of 0.45 volts with out crossing 0.45 volts about 1 or 2 times a second, or if it is stuck closer to 0 or 1 volt then it may be bad. Before you replace it make sure it has 12 to 14 volts at the third wire. If not, then look for a wiring or O2 heater relay problem, fix it and retest the O2 sensor.

My 95 gets around 13 in the city with 30x9.50's.

thanks for the replies. i've got a new air filter, bored my throttle body, run 87 octane mobil1 gas. I have the 231 sye installed and put a new speedo gear on it if i can remember correctly. seemed to be a green colored gear if my memory serves me well. i dont have a analog meter so thats out for now. my tps voltage is .89 at idle and when i floor it, it goes to 4 volts. thought it was supposed to go to 5. need to do some testing.

My 94 gets 13.5 city, with a new o2, cat, muffler, plugs, wires, and air filter.

Wow, thats a lot of lousy mpg. My '87 gets 15+ around town and 19 on the highway with 35's and 6.5" of lift.

How hard are you on the throttle? Do you have a roofrack? How much extra stuff do you carry?

i don't carry anything extra and no drag like roofrack, bumpers, etc...
i used to get 15 mpg about 6 months ago but no more. i'm thinking tps or o2 sensor problems

I doubt the TPS would be a cause for crappy mileage...o2 sensors are definitely a possibility.

how quickly is the o2 sensor supposed to adjust? does it have to be in closed loop?

sorry didnt read above post from ecomike thoroughly. just tested the o2 sensor and it will stay at
.8 volts for 5 seconds
then adjust to .5 for 4 seconds
then .32 for 9 seconds
then .18 for 5 seconds
then back to .8 for 5 and so on.

second o2 sensor, downstream i am assuming is 1 volt or higher and doesn't come below .45 at all.

To determine if it's functioning properly, here is what you should do.

Bring the engine up to operating temperature. Hold it at about 2000-2500rpm's, and watch the readings on your scanner. The first o2 sensor reading should be bouncing all over the place, and he 2nd one should be fairly constant (will still fluctuate a little bit, but not nearly as much as the first one). The 2nd one's job is to monitor the performance of the converter...it plays no role in air/fuel mixture.

But now that I think about it, you have a 2000, and if i'm not mistaken, they have an extra oxygen sensor (possibly two?)...not sure how that changes things, as i'm used to the pre 2000 models where there is only an upstream and a downstream.

If all checks out, it's still possible that it's giving out good readings, but lacking in the performance it once had.

thanks o mighty tim! Just figured out u were the knight who said kni and zi woobie! Makes me think i could lose a couple tires and still fight my way up the hill! Yes I have two o2 sensors but wouldnt the second o2 sensor that monitors the cat tell the computer to set some sort of lean/rich condition? Gonna keep digging and will let you know how things work out after i follow your suggestions.

"NONE SHALL PASS", at least my o2 sensors anyway.

If you have 12 to 14 volts on the third, larger wire at the Upstream O2 sensor then that test data indicates the O2 sensor is shot. Once the engine and sensor are warmed up a bit ( 5 minutes should be plenty of time), it should cross the 0.45 volt mark back and fourth across the .45 volt mark at least once a second at idle, and several times a second above idle, at say 1500 to 2000 rpm. From you data, either the 12-14 volts to the O2 sensor's built in internal heater is missing (thus a wiring or O2 sensor heater relay problem) or the O2 sensor is bad. The second O2 sensor needs to be retested after you replace the first one.

A bad TPS can affect gas mileage at least in an automatic 4.0. Mine would force the AW-4 to shift up at 2000 rpm, and would not downshift at WOT. I could never get the engine over 2200 rpm even at WOT from a dead stop. TPS gives the ECU accelerator (throttle) position data and tells the ECU how much extra or much less fuel to add and how fast to add it during aceleration. In my case I was using way too much fuel at low RPM, not anywhere near the peak power/torque output, peak efficiency of the engine during acceleration, meaning it took forever to get up to speed before it cruised, resulting in very poor city gas mileage.

tsill said:

sorry didnt read above post from ecomike thoroughly. just tested the o2 sensor and it will stay at
.8 volts for 5 seconds
then adjust to .5 for 4 seconds
then .32 for 9 seconds
then .18 for 5 seconds
then back to .8 for 5 and so on.

second o2 sensor, downstream i am assuming is 1 volt or higher and doesn't come below .45 at all.

Click to expand...

the 02 sensor must be bad. heater is functioning properly and i am not getting the switching that is supposed to be happening. far as i can tell i have one o2 sensor on the downtube and one just behind the cat. can i switch those around? just wondering what the second sensor actually does to the fuel mapping if it's not needed, which if it wasn't chrysler would not have spent the money to put it on! Now i know why I became a member of this site! I've gotten tons of great info here now i just need to hook up with you guys to wheel the darn thing!

The one after the cat converter tells the computer if the cat converter is working. Required by EPA since 1996. If the second one stops working it also sets a code. It is also probably on its last legs as well. They are not expensive, best to install a new one, should pay for itself in increased mileage pretty fast. Also both need to be working properly to pass emmission tests.