4.0L...fuel mileage?!?!

Hey gang,

Haven't had the time or the money for fun stuff lately, but I had a quick question.

My check engine light is on, but the car seems to be running and driving just fine (other than the very, very spongy brake pedal). I've also noticed that my gas mileage is pretty bad considering I drive the Jeep very easy. The family and I drove to San Diego last week, and on a full tank of gas with 90% freeway cruising @ ~70mph, I barely broke 300 miles.

Now, I know this thing has the aerodynamics of Rosie O'Donnell riding on Roseanne Barr's shoulders, but still...why on earth am I netting ~15mpg with almost ALL freeway driving at relatively conservative speeds? I can't afford $200/week on gas.

Do you think the CEL might have something to do with the fuel economy...would it light up for a bad O2 sensor or similar issue, or is it probably unrelated? Also, is there anywhere I can go to scan the CEL for free? Any info is appreciated. Thanks!

yes, CEL would come on with o2 sensor. yes you can go to auto zone and they will scan it for you. is your jeep stock? im on 4" lift, with 31s and i get the same mileage and my junk runs great.

and yes, this should be in the oem tech forum

My bad on the forum choice. But it's 100% stock, no lift. I'll check AutoZone. Thanks!

I get about 12 mpg with mine on 32's with a 4.5" lift...Also, although the CEL would likely light up due to the O2 sensor its not likely it is causing your perceived decrease in fuel economy.

Well, I doubt the O2 sensor alone is enough to cause a stock, low-mileage '01 to get roughly 25% less fuel economy than advertised, but that sure seems like a substantial difference to me!

You say you get about 12mpg...but what kind of driving? How many miles per tank is stop-and-go, and how many highway? At what speed? It makes a big difference!

I was netting 14-15mpg almost all freeway miles, at ~70mph. Advertised MPG for the Jeep is 20 highway. Now, I know it's not realistic to expect 20 all the time (even though that's an average number), but to get 14.5 instead of 20 on a car with only 63k miles?

What do you think could be causing it? I'll probably do all the basics (fuel filter, fuel system flush, O2, air filter, etc)...but the tires aren't under-inflated or anything. Any other ideas? Thanks!

xL8 APEKSx said:

Well, I doubt the O2 sensor alone is enough to cause a stock, low-mileage '01 to get roughly 25% less fuel economy than advertised,

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#1 cause of loss of fuel economy is a bad o2 sensor. the o2 sensor tells the ECU how much gas to put into the engine. if its clogged up it reads less than it should and dumps more gas in to correct what it thinks is a running lean condition. before you replace an o2 sensor pull it out and spray the area past the threads (the part that is inside the exhaust pipe) with eazy-off or some other oven cleaner, it will clean most of the carbon buildup off without harming the sensor. worst case scenario you need a new sensor, which it sounds like you might need anyway


HTH
-mike

GrimmJeeper said:

#1 cause of loss of fuel economy is a bad o2 sensor. the o2 sensor tells the ECU how much gas to put into the engine. if its clogged up it reads less than it should and dumps more gas in to correct what it thinks is a running lean condition. before you replace an o2 sensor pull it out and spray the area past the threads (the part that is inside the exhaust pipe) with eazy-off or some other oven cleaner, it will clean most of the carbon buildup off without harming the sensor. worst case scenario you need a new sensor, which it sounds like you might need anyway


HTH
-mike

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Wow...I never knew that! Thats good to know...

Well, I've had O2's go bad before, but the losses in economy were never this bad. I'll try it and see, it could probably use one anyway.

After I get this sorted out and get some new brakes on, I can finally move forward with the lift. Yay!

CEL = open loop = runs rich. Doesn't really mater why the light is on, it will make economy suffer.

JohnX said:

CEL = open loop = runs rich. Doesn't really mater why the light is on, it will make economy suffer.

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Holy crap, really?!?! I didn't know that! I'll have the code read at the local auto zone tomorrow after work. Damn...thanks!!!

What year?

I wasn't able to read my OBD1 computer with an OBD1/2 scanner even though it was hooked up properly.

Although if you had a 96+ with OBD2 it should be easy as cake to get read!

spongy pedal usually = back brakes.

Id go with an 02 sensor first, but get the scan and find out before you drop the $$ on it, they usually go in 60-80k miles...