Bad MPG on my 2001 Cherokee

[jfk718]

I have a JCL with 59,500 miles on it. Currently my car seems to be burning gas at a very high rate. The current MPG is 11.1. I recently had the Emmissions computer changed on my car because the old one was not sending the correct signals, and call the car to stall in the middle of me driving.

When i first purchased my car, my MPG was 17 or better, and I would go almost a week before running out of gas. Now I am filling up every 48 hours.

Here is an example of my problem. Earlier my car was on a quarter tank of gas. I drove roughly 12 miles to my home (about 30 minutes in NYC local traffic) and i burned the entire quarter tank. I have "89" gas in my car, and the car just seems to be burning gas at a very high rate. It's to the point where filling up every 48 hours like i said before.

I know there is some stuff I'm leaving out, but if anyone has had a similar problem, please let me know what I can do to get the MPG back up. I had the car checked for leaks, a tune was done recently, and I'm not putting out any black smoke.

[fredjacksonsan]

Wow, that's severe. Check for leaks in the fuel system. After that, I'd check the O2 sensors, as they can fail and cause an increase in fuel usage.

(Of course it could be the computer)

[traceyc]

I am so relieved to hear somebody else has the same problem!! Mine started happening AFTER I had it tuned up, and I've had it in the shop 4-5 times since, two different repair shops. Both say there is absolutely nothing wrong with it, the diagnostics look great. So now what? It was fine before I took it in for the tuneup. If it is the O2 sensor, wouldn't that show up when they run a diagnostics test? Any other ideas on what it could be?

Mine is a 2000 JGC with 59k miles.

[jfk718]

***Sorry for the poor grammar in the original Post***

I was also told that my diagnostics looked great, especially after having the emissions computer changed. I had the car checked for leaks and all.

1. How do I about checking the o2 sensor and pinpointing if that is the problem?
2. Is the o2 sensor an expensive part to replace if it is the problem

Thanks for any suggestions that anyone can come up with at this point. NYC gas prices are "high" to say the least

[traceyc]

I talked to my brother-in-law last night who is a BMW mechanic and he said that a bad O2 sensor doesn't always show up on a diagnostics test. He also suggested that I find out what was done in the tune-up because maybe any chemicals/oils put into the motor during the tune-up contaminated the O2 sensor. So I'm going to do some digging about what was done at my initial tune-up and go from there...

Anybody else any ideas besides the O2 sensor?

[fredjacksonsan]

If larger tires were just put on, that would lower mpg.

jfk718, if you disconnect the O2 sensor and drive a bit, you'll probably see no change in mpg - a clear sign it's not transmitting. The part is usually below $80, and unless it's frozen in the exhaust screws right out. Couple of electrical connections and you are good. Of course traceyc is right, the one that's in there could just be dirty and need a cleaning.

[RCLowIndy]

Originally Posted by fredjacksonsan

If larger tires were just put on, that would lower mpg.

jfk718, if you disconnect the O2 sensor and drive a bit, you'll probably see no change in mpg - a clear sign it's not transmitting. The part is usually below $80, and unless it's frozen in the exhaust screws right out. Couple of electrical connections and you are good. Of course traceyc is right, the one that's in there could just be dirty and need a cleaning.

OK, Off topic, but LARGER tires would decrease gas milage? Go with me here.... if the contact patch, the part of the tire that is actually on the ground, was larger, then yes, due to friction, the gas milage would decrease.

However, if the sidewall height (or over all height of the tire if you will) is larger, then presumably gas milage would improve. Here is my line of thinking.

Let's say that 1 gallon of gas is enough to power a wheel to turn 10000 rotations. The distance traveled would be 10000 x circumfrence of the tire. For the sake of ease, lets assume that a 225 tire has a circumfrence of 3.5 feet, and that a 245 tire has the circumfrence of 4 feet. The 225 tire would allow the car to travel 225,000 feet burning one gallon of gas, while the 245 would allow the car to travel 245,000 feet. Of course the speedometer, odometer and info center would be off, but I could live with that for a little gas savings!

[JDPascal]

Originally Posted by RCLowIndy

OK, Off topic, but LARGER tires would decrease gas milage? Go with me here.... if the contact patch, the part of the tire that is actually on the ground, was larger, then yes, due to friction, the gas milage would decrease.

However, if the sidewall height (or over all height of the tire if you will) is larger, then presumably gas milage would improve. Here is my line of thinking.

Let's say that 1 gallon of gas is enough to power a wheel to turn 10000 rotations. The distance traveled would be 10000 x circumfrence of the tire. For the sake of ease, lets assume that a 225 tire has a circumfrence of 3.5 feet, and that a 245 tire has the circumfrence of 4 feet. The 225 tire would allow the car to travel 225,000 feet burning one gallon of gas, while the 245 would allow the car to travel 245,000 feet. Of course the speedometer, odometer and info center would be off, but I could live with that for a little gas savings!

The contact patch is not the only thing that will cause a decrease in mileage.

Your assumption about the distance that 1 gallon of fuel will will rotate the tires is good as far as it goes but it has a flaw in it. The larger tire also reduces the leverage of power between the axle and the road by increasing the loaded wheel radius. It takes more power to turn the wheel and move the vehicle with larger tires. This more than offsets any gains due to distance traveled per revolution differences.

Larger differences in the loaded wheel radius between tire sizes = larger changes in true fuel economy when corrected speedo readings are considered.

Don't think any changes in tire size were mentioned by the original posters but it is good to ask............

JD

[JDPascal]

Have to agree with Fred and tracyc.

The O2 sensor is a definite possibility. While you are at it, check the catyletic converter - could be causing excess back pressure if it is starting to go.

If other work was done in the recent past and the wrong RTV silicone was used on the engine, the O2 sensor could be fouled.

As for other possibilities, is the new computer the right one? Did they put in a performance computer?? That may change the fuel delivery patterns.

JD

[YtseJam454]

Why is everyone always so quick to start throwing parts at cars? There is no such machine you can put your car on to run a diagnostic to tell you if parts are good or bad. What type of diagnostic did you have done? For a vehicle with a customer complaint of poor fuel economy when I connect my scantool looking at your scan data the first thing I would look at are my fuel trims.

As far as the O2 sensor is concerned if it's biased lean sure the PCM will try to fatten up the injector pulse width but you would first need to see if it is biased lean and why it is. What does engine coolant temp read? If you can go to your mechanic and ask him to give you a printout of your datastream with the engine at operating temperature and then we can better determine what might be the cause of your excessive fuel consumption...

[hulkmn069]

Originally Posted by RCLowIndy

OK, Off topic, but LARGER tires would decrease gas milage? Go with me here.... if the contact patch, the part of the tire that is actually on the ground, was larger, then yes, due to friction, the gas milage would decrease.

However, if the sidewall height (or over all height of the tire if you will) is larger, then presumably gas milage would improve. Here is my line of thinking.

Let's say that 1 gallon of gas is enough to power a wheel to turn 10000 rotations. The distance traveled would be 10000 x circumfrence of the tire. For the sake of ease, lets assume that a 225 tire has a circumfrence of 3.5 feet, and that a 245 tire has the circumfrence of 4 feet. The 225 tire would allow the car to travel 225,000 feet burning one gallon of gas, while the 245 would allow the car to travel 245,000 feet. Of course the speedometer, odometer and info center would be off, but I could live with that for a little gas savings!

wow too technical but if you want to go that far you have to think of the extra added weight (moment of inertia) and the additional effort it takes to move the bigger tires in accel and decel or any change in direction. but either way i got better gas mileage on little bit smaller tires
(both narrow and shorter) so go figure

[jfk718]

Thank you all for the advice. I have been calculating the time and distance in which I am driving and so far in four days, Ihave driven 59 miles (mostly to and from work) an i have burned almost a half tank of gas. I have come to notice that in the morning, when i first start the car, the gas gage drops a lot. This leads me to believe there is a leak of some kind, even though I don't smell any gas. While I'm driving, I am burning gas pretty quickly, but the problem is very noticeable in the morning after the car has sat for 10-12 hours.

I don't know if that made sense to some of you who are more technical with this stuff.

*Note*

1. I don't have larger tires on my car.
2. My car's emmission computer was change because the previous was found not to be sending the correct signals, causing the car to cut off in the middle of me driving.
3. I do not keep a lot of stuff in my car besides the neccesities, some CDs, and my son's car seat.

I will get a real test today because i have to drive about 30 miles, and deal with NYC traffic... I will let you know the outcome.

[fredjacksonsan]

Originally Posted by YtseJam454

Why is everyone always so quick to start throwing parts at cars? There is no such machine you can put your car on to run a diagnostic to tell you if parts are good or bad. What type of diagnostic did you have done? For a vehicle with a customer complaint of poor fuel economy when I connect my scantool looking at your scan data the first thing I would look at are my fuel trims.

As far as the O2 sensor is concerned if it's biased lean sure the PCM will try to fatten up the injector pulse width but you would first need to see if it is biased lean and why it is. What does engine coolant temp read? If you can go to your mechanic and ask him to give you a printout of your datastream with the engine at operating temperature and then we can better determine what might be the cause of your excessive fuel consumption...

We probably all mentioned the O2 sensor due to experience. You're correct of course about getting a good technical scan, but most of us backyard mechanics may not have a scan tool or know what a fuel trim is.

[snackman]

All that technical info was interesting but even if true we would be talking maybe 1-2mpg at best what jfk is talking about is 6mpg or more. Bigger tires, weather conditions(also can affect mpg), fuel quality(not just the octane rating) all affect miliage. If it were me I would first go back over what was done during the tune up. What are your RPM's? Idiling,driving,when the trans. shifts(standard or auto?) all clues to what is going on. Are the plugs the right temp. range? etc. "FuelTrim?" Naw just a little of the top

[sneaky7980]

my 2000 GC has been starting really rough, usually takin 3 tries and my MPG has went down to 10.2. After i post this message i'm going outside and changing my cam sensor and both O2 sensors and taking it for a test drive. I'll let you know what happens later tonight.

Sat Mar 25th