[C320-list] Water heater not getting as hot from the engine

[Leonard Hooks]

On my 2001, when water is heated by the engine, the temperature is 10 degrees lower than when heated electrically.  My heater is original, so it is 19 years old.  The manual says it should heat to 140 degrees, but my electric heater only gets the water to 103.  But when heated with the engine, it only gets to 93 even after running the engine 20 minutes (Yanmar 3GM30f).

I have checked my coolant and it is not low?

Any ideas on why the engine would not heat the water higher than the electric?

Leonard Hooks
SV Revelation #856