1989 760 turbo dismal fuel economy

Greetings.

I have a 1989 760 turbo that has been recently getting only around 15mpg
(mixed driving). I like in Pittsburgh, PA (USA). The best I have ever
gotten was around 21mpg on the highway. I drive fairly conservatively, use
mid-grade or higher gasoline, and recently had the car inspected (no safety
issues such as dragging brakes were found).

The car runs well, has plenty of pep when on boost, but stalls a few times
in the morning when started.

I'm wondering if my O2 sensor could be crapped out, thus causing a rich fuel
mixture. Does this seem likely? I have no problem getting a new one, but
the Bosch sensor runs about $150, so I'd prefer something better than a
"maybe" before I cough up the cash.

Thanks for any help,
--Dale

 

Our 85 turbo gets right around 20 mpg pretty much regardless of the way we
drive.

If the O2 sensor is the original, it is probably in sad shape. But that may
not be related to the fuel economy problem.

Start with a good cleaning of the throttle body and the Idle Air Control
(IAC) valve underneath it. They get *really* grody in the turbo versions
from inhaling crankcase vapors. Especially clean the idle passage in the
throttle body; count the number of turns and fractions as you screw the
plastic thumbwheel in, then remove it for cleaning the passage and reset it
to the original position when you are done. It isn't unusual to expend a
full can of carb cleaner on all that, but at least it should fix the cold
start stalling. Then you can recheck the fuel consumption.

BTW - if the temperature guage doesn't hang around 1/2 scale, replace the
thermostat with a genuine Volvo thermostat. I had nothing but bad
experiences with aftermarket ones, mostly fluctuations. Running cold will
kill the fuel economy.

Mike