1995 850 Turbo - Guzzling Gas and RPMs drop slowly

[tpettyrox]

I recently rolled 190k on the odo. I was getting pretty poor gas mileage and the performance was pretty bad, so decided to change my plugs, distributor cap, rotor and wire set. While I was at it, I threw in a new fuel filter. Of course, all of this is part of an ongoing quest to get rid of a P0171 ODBII error (System too lean - bank 1). I've noticed slower acceleration and some hesitation around 30-35 mph (if I'm not on the floor)

After the change, the performance has been great. The car responds a lot better, but there's still the occasional bit of hesitation. Also, the gas mileage seems to have gotten worse (last 30 miles dropped the gas gauge from 18 to 14-ish, computer is reading 16.6 mpg average and dropping quickly), and now there's a new problem: Stick the car in Park or Neutral and rev it up, let off the gas, and the revs drop slowly. It takes 2-3 seconds to drop from 4000rpm to the idle just under 1000. It used to drop like a brick (relatively).

What is going on with my car? Any advice? My fuel pump is pretty new as well, so I don't figure that's the problem.

[MadeInJapan]

Steadily dropping mpg and a bank-1 code is usually indication that an 02 sensor has gone or is going bad. I would change out the front 02 sensor, if nothing else, for your miles on the car.

[tpettyrox]

That's what I figured, but I'm no mechanic (and not at all experienced in troubleshooting cars), so I figured I should ask. Would someone please be so kind as to tell me how an O2 sensor works, and what about its function can cause MPG to drop like that.

Thanks!

[MadeInJapan]

It reads the fuel-air (oxygen) mixture and sends a code to the ECU (computer) to make adjustments. When adjustments are wrong, your car burns more fuel than needed to compensate.

[tpettyrox]

Thanks for the help MadeInJapan, and (I noticed you're in Knoxville) GO VOLS!!! Beat the Gators!

[MadeInJapan]

Beat the Gators!?!
Now, wouldn't that be nice!