My gas mileage on my 230K 850 turbo dropped to 12.5 miles per gallon with black smoke out the exhaust. With 3$ gas prices and with what our dollars fund when we buy gas (terrorists), I absolutely loathe stopping at the gas pump, so this problem was driving me crazy.

The ODBII code I got was 172 (system too rich, bank 1). I figured that if the upstream oxygen sensor was the problem I would get a code indicating an oxygen sensor was the problem, right - WRONG!!!

First, I replaced all the pcv hoses and made sure that nipple on the turbo intake was squeaky clean. Then I checked the fuel pressure regulator with a fuel pressure gauge hooked up to the shraeder valve on the end of the fuel rail and then removing the little white vacuum hose to the regulator, while the car was running at idle (make sure the connection to the pressure gauge is tight so you don't get fuel around the running engine) - the pressure shot up 5 pounds so it passed the test. Then I checked the fuel injectors with a mechanic's stethoscope. Each of the five clicked away just fine. Ok. Then I checked the fuel injectors for leaks by removing the injection rail and injectors from their ports and turning the ignition key to postion II, with a cool engine (BUT DO NOT TURN ANY FURTHER OR YOU MAY CAUSE AN EXPLOSION AND FIRE). No drips, no leaks. Still 12.5 miles per gallon.

Today, partly as a result of reading posts on Brickboard I installed a new upstream oxygen sensor. WHALA! Now getting 28 miles per gallon on highway as opposed to 13.5. Better than a 100% increase in gas mileage. In a month of driving I will cover the cost of the new o2 sensor 100$ (free shipping and discount from fcp groton).

Also, I am thinking that odbII will pinpoint the o2 sensor for some conditions, but not necessarily when the o2 sensor allows for the burning of too much fuel - by signaling the injectors to stay open too long. I am wondering if anyone knows more about this?

Anyway, thanks for the posts by ClausC and others and the Brickboard for all the helpful info. And still find it hard to believe that the little o2 sensor means so much to mileage - at least in my case.

VolvoWalt