By way of follow-up for those souls yet to encounter this issue:

The Problem: Upon changing out the hardened rubber impeller in the water
pump of the Yanmar 2GM inline diesel on the 1989 S2 (no vanes broken,
just maintenance), the engine began to overheat. Subsequent investigation
showed zero flow through the water pump, even though manually turning the
pump pulley would rotate the impeller and the pulley was being driven by
the belt under direct observation.

The Cause: Apparently the fresh rubber on the new impeller provided
sufficient additional friction against the cam to cause the (probably
already failing) keyed pulley slot to fail. More succinctly, the pulley
is not secured via any sort of friction fastening; it simply slides onto
the shaft, which seats into a kind of "keyed" opening in the pulley and
the mounting bolt snugs to the shaft moreso than the pulley itself.

Upon replacing the impeller, this keyed open failed and was augered out
by the shaft, but not completely. Turning the pulley by hand did not
generate sufficient force for the pulley to shear and stop rotating the
shaft, but at a couple hundred rpms, it would immediately shear and the
pulley would spin freely on the pump shaft.

Removal of the pulley revealed the slightly routed pulley opening (not
particularly obvious, I had to resort to a caliper just to assure myself)
and metal shavings on the shaft bearing casing (obvious).

The Solution: Replacement of the pulley cured the issue completely. Thank
you, Mr. Biv, for diagnosing this issue for me!

===

Just for laughs, because my life is a party: Of course, upon replacing
the pulley and starting the engine it began to overheat after several
minutes of operation. Predictably. Troubleshooting the pump issue as
before, flow through the pump was observed to be nominal. In fact, better
pressure than ever. Following the flow downstream of the pump, the
thermostat (more accurately, the remaining metal that was thermostat-
shaped) was seen to be frozen in a partially opened position that allowed
some flow. Until the impeller was replaced and the stronger flow
dislodged whatever solids were maintaining there position with the old
impeller and lodged securely in the thermostat outlet, clogging it
completely.

Nota bene: While there theoretically shouldn't be an issue with a Yanmar
2GM in a salt-water environment provided the cathodic protection is
properly maintained, you probably want to check your raw-water cooling
system through-hull to exhaust more frequently than never. Incidentally,
for informational purposes, the Yanmar 2GMF is the fresh-water cooled
version of this engine that is normally found in marine applications. The
S2 was built and launched on the Great Lakes, hence raw-water cooling.

Regards,