One test is to listen whether the air suction stops when the engine warms up from cold. Place a piece of hose next to the small air filter that sits on the side of the injection pump and hold the other end to your ear; as the engine reaches operating temperature, the air suction noise should slowly stop. That is your confirmation that the little thermostat inside the WRD has extended and shut off the airflow, and reduced also (if I am not mistaken) the fuel flow to the engine.
