During the past year my 2003 Corolla LE (currently 68000 miles) has had some significant MPG drop: 30-40 miles less per tank of gas than previous years (comparisons with the same seasons). The car runs normal with no check engine light. I have recently replaced air filter and spark plugs (which were in pretty decent shape), and cleaned massive air flow sensor. Throttle body looks clean. I've also tried two bottles of Chevron Pro-Gard Fuel Injector Cleaner bought from Costco. All I've done so far seems to have no effect on improving MPG. What else could I try? Have a mechanic clean the fuel injection system?
The major mechanical items that affect gas mileage are the tire pressure, air filter, oil viscosity and spark plugs. A thicker oil can reduce mileage if you do a lot of short trip driving. As mikered30 pointed out, a weak or faulty front oxygen sensor (which partially controls the fuel mixture) can reduce gas mileage by 10-15% without triggering a Check Engine Light. Manufacturers recommended replacement of the oxygen sensor at 100K miles, but given the age of your Corolla (2003), it may be time now. The sensor costs $75-100 and is a pretty easy 30 minute repair.
Other non mechanical factors are the weather conditions and driving habits. MPG will be reduced by colder weather and if you are driving more short in-town trips in place of long highway trips. Finally, MPG will change if you have a different driver, especially a younger driver with more agressive driving habits.
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Thank you very much for the quick help, both mikered30 and aspen007!
There has been no change in driver, driving style, engine oil type (5W30) and tire pressure (I usually keep it around 35-36 psi year around). Also, the MPG drop with my 2003 Corrola during the past year has been consistent compared to the same seasons in the previous years. A mechanic I have talked to strongly suggested me to have the fuel injection system cleaned even though I asked him whether I should pay attention to the oxygen sensor. His reasoning of no concern about the oxygen sensor was that no CEL has been seen. So you think I should now focus on the oxygen sensor first before considering the fuel injection system?
Sounds like winter gas if it changes with the same seasons. Normal.
Or perhaps the gas station went to 10% (or more) ethanol in the gas. In an 05 Corolla I loose around 10% MPG per tank on ethanol gas compared to non ethanol gas.
All the stations in my area have been 10% ethanol for years. But I have filled up at station or two in other states while traveling that had non ethanol gas and the MPG difference is noticeable.
No, my MPG drop has nothing to do with season or gas station. I knew that weather temperature plays a big role in MPG, especially true for the 9th generation Corollas. And I pretty much pump gas at the same station. What I meant about the comparisons with same seasons with previous years was that the MPG drop happened during the past summer compared to the summers during the previous years. And such a drop is occurring now during this winter compared to previous winters.
My unprofessional opinion is that if your engine is idling properly and you aren't experiencing any hesitation when accelerating, I wouldn't spend the money on a fuel injector cleaning.
Regarding the replacement of the oxygen sensor. That's more a financial decision since it appears your car is running well, just using more gas than normal.
If you are doing the replacement yourself, I would invest the money to replace the sensor. Especially since the sensor will soon need (or already needs) replacement. The sensor costs $100 or less to replace (I just got a Denso sensor for my Corolla for $70 delivered from rockauto.com). Compare that cost to the likely fuel savings with gas at $3.00 a gallon. Worst cast is you installed the sensor a year or so earlier than you needed to.
If you are going to have to pay a dealer $250 or more to replace the sensor, then the decision is a little more difficult since there's the possibility that you will spend the money and not see any change in your gas mileage.
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My unprofessional opinion is that if your engine is idling properly and you aren't experiencing any hesitation when accelerating, I wouldn't spend the money on a fuel injector cleaning.
Thanks for your suggestions! My AT Corolla is idling properly, but it does have some hesitation around the speed around 40 MPH. Such hesitation started even when the car was relatively new, but it seems to have slightly more frequent hesitations recently. It especially feels hesitating lack of power when driving on an ascending road.
Does this sound more like a fuel injection system problem or a fouled oxygen sensor?
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If you are doing the replacement yourself, I would invest the money to replace the sensor. Especially since the sensor will soon need (or already needs) replacement. The sensor costs $100 or less to replace (I just got a Denso sensor for my Corolla for $70 delivered from rockauto.com). Compare that cost to the likely fuel savings with gas at $3.00 a gallon. Worst cast is you installed the sensor a year or so earlier than you needed to.
If you are going to have to pay a dealer $250 or more to replace the sensor, then the decision is a little more difficult since there's the possibility that you will spend the money and not see any change in your gas mileage.
How difficult is it to get the sensor off? Need a specialized open-ended wrench? If I manage to get it off, maybe I can test its condition with a multimeter and a torch before I buy a replacement?
Hesitation is a symptom of a worn sensor. However, it could also be caused by the fuel injectors -- or something else entirely. Sorry, but that's why the mechanics run the diagnostics before replacing parts.
Since the sensor currently isn't triggering the Check Engine Light, I don't think testing the sensor with a volt meter will tell you much about the sensor's condition. It should test good. The unknown is how good. You really need an oscilloscope to test the sensor's ability to respond to changes in exhaust oxygen content, as well as to measure its response time.
Replacing the oxygen sensor requires a 12mm wrench to remove the four bolts holding the heat shield surrounding the sensor. Then a 7/8 inch wrench or socket is needed to remove the sensor. No tools are needed to unclip the connector. Although a specialized oxygen sensor socket is preferred (and costs under $10), you should be able to get the old sensor off with an open end wrench. If you know you are going to replace the sensor and don't mind destroying the old one, you can cut the wires and use a long socket or the closed end of a hand wrench for a better grip on the sensor socket.
Just make sure to spray some penetrating oil on the sensor before removing it. Always apply some antiseize grease on the sensor threads when installing the new sensor. Some sensors include the grease with the sensor. Don't get too excited after installation if you get some smoke from around the sensor caused by the burning off of some of the antiseize grease. It'll go away in a few minutes.
Last edited by aspen007; 12-26-2010 at 11:41 AM.
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Not sure if this applies to an '03, but my '05 did the same thing, at about the same mileage. Also noticed that the idle would surge up and down a bit when warming up from extreme cold (-15 or more F.). Turned out the intake gasket was sucking air due to the different expansion rates of the aluminum and the plastic intake. Replaced the gasket (about $25 at napa) and no more problems, and mileage is back up where it belongs. I believe there was a service bulletin on this from Toyota, you can check with your dealer to find out if it applies to your car.
Hope this helps!
Steve
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