that is what it does, but not the reason why it does it.
as Kev's say diesels are fuel regulated.
Throttle flaps on diesels can be fitted for 3 reasons.
1) to strangle the air intake on stopping the engine, i.e to make it come to stop smoothly, without shaking the vehicle.
2) to create a depression to increase EGR flow. Mainly (but odd eceptions) modern ECU controlled vehicles.
3) to create a depression ( vacuum ) to operate brake servo (only used on old land rovers from memory )
without specific knowledge of this engine I would say only number 1 applies to this vehicle.
This means that the flap should be Fully open at all times, except a short period when ignition is turned off to stop the engine.
Armed with that system under standing you will hopefully be able to form some diagnostic strategy.
Cheers Aidan