Hi mgb150 and welcome to FTE

The IMRC and IAC are different. The IAC just controls the idle speed. The IMRC works sorta like a 4 barrel carb. Your intake manifold has 2 ports for each cylinder. It opens the second set of intake ports when under a heavy load or high rpm.

Possible causes: (generic list)

Leaky vacuum reservoir.
Damaged vacuum lines.
Damaged vacuum solenoid.
Damaged vacuum actuator.
IMRC actuator cable/gears seized.
Cables improperly routed (binding) or seized.
Damaged/disconnected IMRC housing return springs.
Lever/shaft return stop obstructed/bent.
Lever/shaft wide open stop obstructed/bent.
IMRC lever/shaft stick/bind or disconnected.
Open IMRC control circuit.
Open VPWR circuit.
Shorted IMRC circuit.
Damaged PCM.