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Ok, I drive a 1990 325i with 212,000 miles on it and my gas mileage has dropped from about 22mpg to about 16mpg. This wasn't a gradual decline, it happened pretty much from one tank of gas to the next. I went through a tank and got my normal mileage, then the next time I had to fill it up I noticed that the mileage had dropped significantly. I'm thinking it might possibly be the Oxygen Sensor. Is there anything else that could cause this? :dunno:
 

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My car did the same before my other symptons arose. My fuel pressure regulator was dying on me. The pressure rose to much and was forcing fuel through the injectors. Check air filter, leaks around the injector rail, catalytic convertor, exhaust. Can't be to much. Are the plugs fouling?
 

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WHat Exactly does the O2 sensor do, (im a noOOB) does it control the amount of o2/fuel mix happening, related to the automatic choke ?
 

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Alluvium said:
WHat Exactly does the O2 sensor do, (im a noOOB) does it control the amount of o2/fuel mix happening, related to the automatic choke ?
it tells the ECU when the mix is too lean or too rich so that the ECU can correct it. a bad O2 sensor can screw up your mileage. four of the five BMWs I've owned still had their original sensors after 100K+ miles. they should be replaced every 50-60K. not only will this usually restore your mileage, but the car should run better. unless you know for sure that the sensor has more than 50K on it, I would check it first to make sure it is functioning. they aren't cheap and you don't want to spend money on this item unless you need to. Bentley has the procedure for checking the sensor.

Mike
 

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i have found the o2 sensor for @ 70 other places i seen them as high as 150. i dont think 70 is that expensive IMO
 

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bratmeister
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When you have a six digit salary to support your fleet of cars, you are correct. I don't even think that $150 for the sensor is excessive. However, not knowing his financial situation, I won't assume he'll be happy spending that kind of money.
 

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I think $70 for any BMW part is not expensive.
From what I know, the O2 sensors are a DYI since you can access them from under the car.
 
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