Mileage all over the place, O2 sensor?

[mdchaser]

Hi, I have a 99GA 2.4L and my mileage seems to jump around a lot. I'm very light on the throttle yet sometimes I will get 28mpg going to work and at others I can't get above 23 or so (same driving conditions, chevron fuel). I've done a tuneup and that seemed to help a bit. I was thinking about replacing the O2 sensors as the car has almost 90K miles on it. Will this be a good idea and where are those sensors located on the car? The sensors are pretty inexpensive so I'm not too worried about cost.

[mdchaser]

Can anyone describe the location for the O2 sensors?

[litdevil316]

one should be around the exhaust manifold, other after the cat. the one after the cat will be rather expevsive, but the first one in like 20bucks or so. not sure how hard it is to change on a stock manifold. replaced mine while back, but i have a custom header and the O2 is easy to get to.

[Wstangland]

i dont think an o2 sensor will help you. did you try an air filter? were the plugs black? you could have some carbon build up on the injectors not atomizing the fuel properly.

[mdchaser]

Thanks for the info guys. I just put new acdelco plugs in the thing. The tires are at 35psi, I've run fuel injector cleaner through it a number of times. It has a k&n filter which was cleaned about 10k miles ago and still looks clean. I use seafoam to clean the top end every 5k miles after which I change the oil (mobile1 synthetic). The only fuel it gets is chevron. I think I just have a very picky car. I managed to get 35mpg on my last long trip then just 24mpg driving to work... My only real complaint is that I have managed in the past to get 28.5mpg on the same trip to work with worn out plugs, a dirty air filter, low tire pressure, and old oil :).