A failed O2 sensor is my best guess for a dramatic change in mileage. When it, or they, fail, you get a dramatic change in the fuel used. The first thing I'd do is have it changed. In fact, this is not hard to do, and O2 sensors are usually about $50 bucks, if that. They just screw into the exaust; use a wrench. Depending on how hard it's to get to, that's it.
A past g/f had a buick, when the mileage dropped from about 24mpg/avg over a tank to 15 the first thing I changed was the o2 sensor. Problem solved.
Quote:
Originally posted by redlemon
Is it running rough? I once lost a whole bunch of MPG, took in in to the dealer, and found out that the wire running the oxygen sensor back to the controller had come off, and since the controller couldn't figure out what fuel mix to use, it was running as fuel rich as possible.
Quick spot of solder, no charge, got all my MPG back. Best dealer experience ever.
|
Yes, he's right about that, and the explanation in why a failed sensor will shoot your mpg is correct too.
New tires would not hurt your mileage unless they were severely underinflated.
The AC can be responsible for 5 to 10% change in MPG, they are no where near as demanding as many people have convinced themsleves with little dubious connections made into causes in ther own minds. A smaller engine will get hit harder from AC as it will use a higher percentage of the engine's available power. But still, we're not talking the kind of change to take you from 400 miles / tank to 300.
The other guy is right about windows and AC, drag vs power draw.
I've read time and time again that around 55mph you might as well run the AC, because the having the windows open causes so much drag that you're burning just as much, if not more fuel to make up for it, you may as well run the AC. Think about it. With the windows down, your car is a big scoop running down the highway. At enough speed, scooping in all that air is going to eat more fuel than AC. Again, smaller cars take a bigger hit. If your AC compresser was seriously fucked up, then I could see incredible amounts of extra drag on the engine. But, I think at that point the compressor would be so jacked, you'd not have a functioning AC.
If you have made a huge change in the mix of driving you do, it MAY be responsible. When I do almost all freeway in my car, I get 32mpg/over a tank. When I end up doing city running around, I'll get 23-25. Over a tank, that'll add up to a big difference in total miles driven over a tank.
I'd have the o2 sensor checked/replaced. Beyond, that you need to be looking more at what kind of MPG your getting over a tank, considering in your type of driving, instead of miles per tank. While it's an easy figure to see on your trip odometer, it's not a good value to use when deciding if your car is running right.
FIll your tank. Reset the trip odometer.
Next time you have to fill up, devide the number of miles driven on the trip odometer by the number of gallons you just put in to fill the tank (which = the number gallons you used to drive those miles.) That will give you miles per gallon. If you're getting near what your car is rated at in city/hwy driving, then you're just experiancing shock over a change in expected mileage as a result of changing your driving mix.
But, if you're getting 13 mpg, in city driving, and the car is rated at 29mpg city, you've got a mechanical problem.
Generally, the actual per tank mpg is somewhere in between the factory rated city/highway mpg figures, leaning towards what the majority of your mix is.