The "plunger control" gizmo on the FJR raises the idle rpm when the coolant is below normal operating temperature (i.e., cold start). This gizmo is attached to the underside of throttle bodies #1 & 2. It has a spring-loaded linkage to each of the throttle bodies. The gizmo is filled with a waxy material which liquifies when warm. Coolant enters the gizmo via a small hose directly from the thermostat pipe assembly above the engine head. It bypasses the thermostat so it receives coolant flow at all times even when the thermostat is closed. The coolant transfers heat to the waxy material and as the wax liquifies the mechanism gradually pushes a plunger sideways in each throttle body. This causes the idle rpm to gradually drop to its normal 1K rpm. The coolant loop is completed as the coolant leaves the gizmo and travels via a hose to the radiator.
It would seem that the system raises engine rpm by letting additional combustion air bypass the throttle butterfly valves and throttle body air screws. With this additional source of combustion air the vacuum pressures that we observe when syncing the throttle bodies is reduced and the ECM injects more fuel commiserate with the reduced vaccum pressure. As the wax melts the bypass air is gradually reduced and finally reaches zero. Then the air screws and throttle valves control the steady-state idle rpm.
It would seem that the system raises engine rpm by letting additional combustion air bypass the throttle butterfly valves and throttle body air screws. With this additional source of combustion air the vacuum pressures that we observe when syncing the throttle bodies is reduced and the ECM injects more fuel commiserate with the reduced vaccum pressure. As the wax melts the bypass air is gradually reduced and finally reaches zero. Then the air screws and throttle valves control the steady-state idle rpm.
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