When you run it with the thermostat cover off, a LOT of water should come out. In addition to the many good tips of BobW, I would add that, if someone “replaced the impeller” but didn’t notice a badly worn impeller housing, sometimes they will pump WORSE with a new impeller in a bad housing. This is apparently because over time, an old impeller sort of form-fits the worn housing to some extent. A new one won’t fit the wear patterns as well.
Long live American manufacturing!
