My 85 Westy automatic (150k miles, California emission) was running
poorly -- mechanic suggested a new AFM but noted that the main
problem was low oil pressure probably due to worn bearings.

Took it to the mechanic to get a new AFM and air filter, but after
getting the van back it ran worse than ever (stalling both warm and
cold).  I tried adjusting the timing and idle bypass screw with
little luck.

Finally today I did a more comprehensive check of the engine.  I also
took apart the air box to see if the new filter had been added (it
had).

Surprise -- what did I find?  With the air box removed I suddenly
realized that the throttle manifold is rattling around loose.  It
turns out the two screws that hold it to the intake manifold were not
even finger tight.    After tightening it up and putting the airbox
back on, the van now starts when cold, will now maintain a 950rpm
idle and no longer stalls at any temperature.

I'm still not sure WHY the throttle body was loose -- it could have
been when my  mechanic replaced the AFM, although removing those
screws are not part of the procedure so it also could have been from
the p.o.'s mechanic.    I'm guessing that as the manifold shifted
around, it would at various times leak or not leak air, explaining
the erratic behavior.

I also discovered a couple of vacuum lines that may have been
cracked, so I replaced them too.

Moral of the story?  If you have a funky idle problem, check all air
pathways.  Since you can't really tell if the throttle body is loose
when the airbox is on, you should remove the airbox.   This has the
added benefit of revealing more vacuum and air lines that are hard to
get to otherwise.

Now, I'm back to the original question: do I need a new engine?  I'll
save that for another email...