With the water in the test kettle cold (about 60°F) and compressed air applied to the "from engine" connection air bubbles came through the "to cooler" connections only.

However with the finger restriction applied to largely but not completely blocking the "to cooler" connection a large number of bubbles came from the "to oil tank" connection but no bubbles from the "from cooler" connection.

The water heater was then switched on and the water heated.

With the same degree of finger restriction on the "to cooler" connection as the water temperature reached about 180°F bubbles stopped coming from the "to oil tank" connection and the "from cooler" connection.

With the water at boiling point and the same finger restriction on the "to cooler" correction there were no bubbles from the "to oil lank" or "from cooler" connections.

As before the water in the test kettle was allowed to cool naturally and the test repeated at lower temperatures.

At 185°F there were no bubbles from the "to oil tank" connection and just one or two bubbles from the "from cooler" connection.

At 170°F there were lots of bubbles from the "to oil tank" connection.

At 160ºF and 150ºF the results were the same as at 170ºF.

These results show that the DM thermostat operates at about the 185ºF design point and on cooling down returns to the "cold" condition somewhere between 170ºF and 185ºF.

This is basically what one would expect.