Gas Millage!

I have a 1988 Delta 88 Olds w/3.8L V6 VIN C. When ever I clean out the throtle body and Air Idle Control valve, change the fuel filter,air filter, and the O2 sensor I get primo gas millage ~28-30Mpg. After around ~700-850 miles I am right back to 19-22Mpg. This is really fustrating that I can't keep a consistant millage. Would the O2 sensor have something to do with it? The O2 sensor is bosch, which I know the rep in this forum of bosch is shit, so that might be the cause. Anyone with opinions, statements ect...? Oh, and I don't have any codes stored in the computer.

Check the plugs to see if they are fouled. If you are using bosch plugs go with AC or Autolite. As for the O2 sensor replace it with an OEM from AC and that problem should go away. I think the fuel issue is due to the O2 sensor not holding up. It seems that the Bosch sensors are only good for 6 months.

Gotta think, the car is almost 20 yrs old. Gas milage tends to suffer with age.

Gotta think, the car is almost 20 yrs old. Gas milage tends to suffer with age.

I wish I had his problem. Mine is a '93 and I'm only getting 14mpg. Once the weather warms up I'm going to try to figure out what the problem is.

It shouldn't suffer with age, because the car is in near mint condision 78000 orginial miles. So age is not its problem. I am thinking it is something mechanical. I have ac delco plugs, and they can't be fouled because the engine runs fine when I did the above. So it may very well be the bosch 02 sensor. Hell, I don't think they are designed to last 6 months but 1000 miles. But would it be worth it to buy an ac delco O2 sensor as they are about $60-70 dollars vs. $20 for the bosch? But as I once heard "you get what you pay for."

Well I put the new AC Delco O2 sensor and it seems like it fixed the problem. I only paid $38 for it. Not bad. How long is the AC Delco sensor going to approx. last vs. the bosch unit????
Thanks!