I would say find a mechanic who can figure out why the check engine
light is on, it's doing harm to your wallet if nothing else, you should
be getting more like 25 mpg.

First suggestion:  GET A NEW MECHANIC!  There is a reason the light is
on and a competent mechanic will read out the error codes and diagnose it.

Find a real mechanic who is familiar with European cars, particularly
Volvos, and get it fixed right.

Most likely cause is a failed O2 sensor, but lots of things could be
wrong.  By driving with a rich mixture you are not only wasting gas but
also putting the catalytic converter at risk as well as the rings.  That
said, don't shotgun and guess in the dark.  Get the codes read by
someone who knows what the **** they are doing.

John

Presume that the engine has no power lack?

Here are some suggestions:
1. When was the air filter changed?
2. Is the timing correct - sometimes the timing belt can jump one - two
teeth?
3. Check the oxygen sensor(volvo garage work or a Bosch garage), the EGR
   valve and for vaccum leaks
4. Check the flame trap.
5. (Check if the brake calipers are hanging)

Cheers Per