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horrible mileage

C

Cavalry

NAXJA Forum User
Location
ny
I have a 2000 cherokee 4.0 auto 4WD... its getting arounf 12 MPG. Tires are inflated properly, no brakes are hanging up, air intake is clean and unbstucted, exhaust is not plugged. Truck seems to have the proper amount of power, runs fine except it gets lousy mileage.

I scanned the computer, no codes, nothing really odd in the data stream numbers.

Any ideas or things I have not checked
 
Oxygen sensors. It's not unknown for them to fail without throwing a code.

Tried resetting the computer yet? Disconnect the battery overnight (preferably after filling up with a fresh tank of gas), reconnect it in the morning, and let it re-learn. I've had similar symptoms after bad fuel before.
 
casm said:
Oxygen sensors. It's not unknown for them to fail without throwing a code.

Tried resetting the computer yet? Disconnect the battery overnight (preferably after filling up with a fresh tank of gas), reconnect it in the morning, and let it re-learn. I've had similar symptoms after bad fuel before.
Click to expand...


which one of the pile o' o2 sensors? lol. I will compare the voltages tomorrow to see if one is more dominant.

This has been a trend for the last 3 tanks of fuel. I have only had the the jeep that long
 
Cavalry said:
which one of the pile o' o2 sensors? lol. I will compare the voltages tomorrow to see if one is more dominant.

This has been a trend for the last 3 tanks of fuel. I have only had the the jeep that long
Click to expand...
The pre-cat O2 sensors are what the PCM uses to adjust fuel.
You say you've only had the jeep for a short period and the mileage has been bad from the start? What size tires and gear ratio in the diffs? I've got 31s and 3:55 with the AW4 and I get about 13mpg (I am a bit heavy on the pedal though). If I drive like a grandma then I'll be lucky to average 14.5 or so mpg.

Kyung
 
I just picked up a 2000 XJ 4.0, Auto, 4x4 with 88,000 miles and have run two tanks through. First one netted 14.5 most all city driving and the second tank was 18.4 with mixed city/highway. When I got smogged, it was running REALLY clean with a couple of the numbers showing up as 0.0 in the test.

My guess is bad O2 or even a cracked exhaust manifold, which would make the computer think it was running lean and would add fuel to compensate. That's my .02 worth. YMMV
 
Check your spark plugs, they may be eroded. My wife's 00 Cherokee Classic has the same coil on plug DIS ignition, I recently did a tune up and found the center electrodes on every plug were eroded and round. I think its high spark voltage burns through plugs pretty quickly, 30k is a full life for a set on this ignition system.

I put on a set of bosch platinum +2's
and got 335 miles to a 16 gallon fuel up, averaging 21 mpg.
(Champion plugs will do the job as well, and fit the coil pack standoff height better, the bosch's were slightly taller.)
 
Most of the Jeep Cherokees already had the exhaust manifold replaced due to a factory recall of the manifold cracking, so chances are it is already fixed. I bet the O2 sensor is going bad if you have close to 80K miles. Last time I checked it should be replaced around that time. I'de definately replace the plugs 1st, then the O2 sensor. My 2001 on 32's and 6.5 lift gets 17 hwy and 14 city with the auto and 3.55 gears.
 
How is the operating temperature? If the thermostat is sticky and stays open or opens too early, the ECU will keep the fuel mixture rich to get the car warm, and if it never warms up, it keeps these rich fuel maps and sucks down the gas.
 
I'd check the cat for clogging and DON'T run the cheapest fuel around! The later models have fuel sensors that note when alcohol is present and dump extra fuel in to make up for it!
A bottle of Lucas injector cleaner may help too.
 
Cavalry said:
I have a 2000 cherokee 4.0 auto 4WD... its getting arounf 12 MPG. Tires are inflated properly, no brakes are hanging up, air intake is clean and unbstucted, exhaust is not plugged. Truck seems to have the proper amount of power, runs fine except it gets lousy mileage.
Click to expand...

If you just got it used, do the "normal routine"... plugs, filters, etc.

I was amazed. In the fall, I replaced my air cleaner (it didn't look terribly dirty) with a K&N flat-panel, and put in a new set of plugs (Champions cheapies... replacing Bosch Plats with 30k). My mileage jumped considerably... though I haven't put a calculator to it yet. I had been averaging 230-240 mi/tank back when gas priced jumped (causing the Heep to get parked for the summer while I drove my high-mileage car)!. Now I'm getting around 300. That's close to 25%!

Also '00 4.0 AW4 4wd
Den
 
xjaholic said:
Most of the Jeep Cherokees already had the exhaust manifold replaced due to a factory recall of the manifold cracking, so chances are it is already fixed.
Click to expand...
What? I've never heard of any factory recall on the manifold. You mean, replaced under warranty? If in fact you had a newer XJ, that was still within the mileage and months, then the warranty should take care of it.
 
Check the Sparks. I was having VERY similar issues with my '01. MPG started going down for 18 to 15 to 13 to 12 with in the spane of about 6 weeks. Plugs were replaced prior to 50k checkup and it had 75k on when this started happening in November. Took them out and the head were rounded and thin and the gap was almost .05. Put in new Champions Now getting close to 20mpg and that was the ONLY thing I DID. ITs the Plugs.
 
DenLip said:
If you just got it used, do the "normal routine"... plugs, filters, etc.

I was amazed. In the fall, I replaced my air cleaner (it didn't look terribly dirty) with a K&N flat-panel, and put in a new set of plugs (Champions cheapies... replacing Bosch Plats with 30k). My mileage jumped considerably... though I haven't put a calculator to it yet. I had been averaging 230-240 mi/tank back when gas priced jumped (causing the Heep to get parked for the summer while I drove my high-mileage car)!. Now I'm getting around 300. That's close to 25%!

Also '00 4.0 AW4 4wd
Den
Click to expand...


routine maintance was all done less than 1 yr ago (500 miles) new air filter, new plugs
 
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