Why Am I Getting Such Bad Mileage

Hello,

I have noticed within the last couple of months, my fuel mileage has gotten significantly worse. I used to be able to get around 14-15 MPG in city driving and around 22-24 MPG on the highway. Recently it has gone down to 10-12 MPG in the city and 18-20 on the highway. Nothing has changed mechanically on the Jeep in the past few months, so I am not sure what to check next.

The Jeep is a 2000 4-Door Classic. 4.0L, automatic with 1-year old tires on it. I have always mathematically calculated the mileage, not relying on the computer readout. I have my tires at 36 psi which is 3 psi higher than recommended. What things should I be checking? Is it possible that worn wheel bearings would cause more rolling resistance? Or is it something like a bad O2 sensor that is causing the Jeep to run rich?

Thanks for your help guys.
Ryan

My bets -
air filter. Have you replaced it recently?
O2 sensors. These have been known to reduce gas mileage before they throw a check engine light, due to fouling and wear, check them.
Temp sensor and/or thermostat - is your engine reaching proper operating temp? It should sit at about 210degF in normal operation, if it doesn't reach 170 degrees you are wasting gas and also washing your cylinder walls with it. Not good.
If your wheel bearings were bad enough to increase rolling resistance noticeably, you wouldn't be able to hear yourself think, and they'd probably be quite hot!

Also check to make sure your e-brake adjuster is set properly.

JP Magazine series on XJ mileage:

http://www.jpmagazine.com/projectbuild/154_0808_building_a_fuel_efficient_jeep/index.html


http://www.jpmagazine.com/projectbuild/154_0810_building_a_fuel_efficient_jeep_part_2/index.html


http://www.jpmagazine.com/projectbu...cherokee_xj_project_mileage_master/index.html


Good read.

As the others have posted--general maintenance/tune up? Air filter, plugs, plug wires, cap, rotor, o2 sensors, any exhaust leaks upstream of the first o2 sensor? Is the engine reaching normal operating temperatures? Longer warm-up times? Does your state mandate E10?

In those JP Mag articles I was amazed at how much of an impact the failing PS pump had on the XJ's mileage, but any belt driven accessory can be failing and causing a lot of drag before you know its going out. Speaking of, does the fan clutch funtion correctly? They can fail and lock-up, the only indication would be extra noise from the, and you might not notice that with the windows up and the heater blower running. Multi-grade oils will give you better mileage, using synthetic multi-grade will give you more. When you buy a cap and rotor, pay extra for brass, don't waste your time on the aluminum junk.

Forget the good information on those links. The girls in the bikini's distract me too much to read the whole article! LOL

winterbeater said:

Forget the good information on those links. The girls in the bikini's distract me too much to read the whole article! LOL

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I would call you a "dirty old man", but we are too close together in age! LOL!

Has your station changed to a "winter blend" of gas in the last few months (use higher ethanol percentage)? When they do this in CA I notice a nice 2MPG or so decrease. Not sure what other states do.

the_weirdo said:

Has your station changed to a "winter blend" of gas in the last few months (use higher ethanol percentage)? When they do this in CA I notice a nice 2MPG or so decrease. Not sure what other states do.

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i notice this problem every winter. you would think the colder temperatures would relate into better mileage. i get about 300 miles a tank, and after a few years i see this number going down. i just run it really hard, REALLY hard for a couple of tanks. sometimes as little as 100 miles a tank... usually does the trick. got to blow all that carbon out. its a good excuse to get new tires too.

E10 is problematic as an automotive fuel, and lowers gas mileage, can cost about 10 percent MPG.

Add in the reformulation for winter, with E10 blended in, and you can expect to lose another 10 percent MPG.

kastein said:

My bets -
air filter. Have you replaced it recently?
O2 sensors. These have been known to reduce gas mileage before they throw a check engine light, due to fouling and wear, check them.
Temp sensor and/or thermostat - is your engine reaching proper operating temp? It should sit at about 210degF in normal operation, if it doesn't reach 170 degrees you are wasting gas and also washing your cylinder walls with it. Not good.
If your wheel bearings were bad enough to increase rolling resistance noticeably, you wouldn't be able to hear yourself think, and they'd probably be quite hot!

Also check to make sure your e-brake adjuster is set properly.

Click to expand...

I just replaced the air filter. My engine temp is right on 210. I will have to check the O2 sensors and the e-brake. It may just be the combination of winter fuel and 10% ethanol gas, plus the fact that I drive up hill a lot in 4WD because of the snow and warming up the jeep a lot. But thanks for the suggestions.

joe_peters said:

JP Magazine series on XJ mileage:

http://www.jpmagazine.com/projectbuild/154_0808_building_a_fuel_efficient_jeep/index.html


http://www.jpmagazine.com/projectbuild/154_0810_building_a_fuel_efficient_jeep_part_2/index.html


http://www.jpmagazine.com/projectbu...cherokee_xj_project_mileage_master/index.html

Good read.

Click to expand...

thanks!

kndrewa said:

thanks!

Click to expand...

Hope it helps.

RSchurig356 said:

It may just be the combination of winter fuel and 10% ethanol gas, plus the fact that I drive up hill a lot in 4WD because of the snow and warming up the jeep a lot.

Click to expand...

Very possibly.

Be sure to get "multiple samples" (at least 3) of your gas mileage numbers before formulating an opinion.

No mention of when spark plugs were last changed. If more than 30k, get a fresh set in there. There are no wires, cap, or rotor on your year XJ but those plugs still need regular changes.

As mentioned by others, the oxygen sensors can get a little "lazy" over time and can tweek the mileage down a bit before it gets slow enough to set a code. The "upstream" sensor before the catalytic converter is the one that generates an input signal that helps determine fuel/air mixture. I have found the lifespan of the oxygen sensors to be between 100k-150k, varies quite a bit. They can be tested if you know what you're doing but most guys just swap in new ones. If you do replace them, always buy high quality OEM style. Stay away from universal sensors and many ebay style el-cheapo sensors. You do get what you pay for here....

I will try buying gas at different places, but this whole winter I have been buying gas all over the place. My spark plugs were changed about 20K miles ago, so I don't think that those have gone bad yet, but I will check.

The Jeep only has 61K miles on it, so hopefully my O2 sensors are still good, but I am going to check those next.