CO mystery - any ideas?

Andrew Gideon wrote:
Our Cessna R182 is getting CO in the cockpit. We've now had three
different
shops look at it multiple times, all to no (complete) effect.
They've done
things like resealing the firewall, the gear-wells, the exhaust, and
the
like. But still we get CO.

We've swapped CO detectors around between planes, so the detectors'
results
are trusted.

One interesting oddity: turning cabin heat on does seem to get rid of
the
CO.

We've noticed no difference made between type of flight. I
personally have
had readings both while XC and while maneuvering (practicing
commercial
maneuvers), and at various altitudes. Others have reported the same.

At this point, we're at something of a loss. The next thing we're
trying is
to extend the exhaust pipe. The presumption is that this would get
the CO
into the slipstream and away. But that "presumption" is really more
of a
"guess" or even a "hope".

Anyone have any suggestions or ideas or experiences that might help
us?

Thanks...

Andrew