A few years ago when I had the exhaust off of my ac engine I cleaned out the
heat exchangers and had fairly clean smelling heat thereafter.

I recently cranked my heat back up only to smell oil smoke.  It turned out
that I had a leaky valve cover gasket and after fixing it I decided that I
couldn't tolerate the smoke smell.  I put the front of my van up on jack
stands (probably not necessary) pulled the heat pipe off of the air box
(behind the front grill) and stuck a running water hose in it.  I then
poured various cleaners and degreasers into the pipe.  I also removed the
plastic pipes that connect the heater fan to the heat exchangers and poured
more degreasers and cleaners into them with periodic pressure washing etc.
You can plug the drain holes on the bottom of the exchangers and let them
soak, or just bombard them periodically over the course of a few hours like
I did.  I used the stronger stuff first,, then flushed with lots of water
and milder soaps etc.  At this point my heat is working great, and smells
only lightly of soap.  Even if this was a dumb idea or had not been very
effective it was satisfying.  As for the cleaners that were used, I went to
the local auto place and the local hardware store and bought everything that
looked like it might work while not being too toxic from house cleaners, to
engine degreasers etc.

If you decide to try this be careful with the front heater pipe because the
paper layers get soft when wet.  Also, you may want to plug the heater
outlets between the front seats that service the rear, passenger area of the
van in order to keep from staining the carpet.

Jonathan Poole
'82 AC Westy