Or could be that the return spring of the throttle(located on the inside of injection pump when you look the pump from the head side you will see it) has lost its strength and can not return the throttle.But 99% you will solve a problem by lubricating joints.
P.S.You have bad one or more glow plugs that is why glow plug light come ON when the engine is running.And your idle is good when it should be when hot 550-650rpm,when cold should be 150-200rpm higher.

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Mercedes W140, S350TD, 95.
Mercedes W140, S350, 92.
Porsche 944, 86 N/A.
Porsche 944, 85 N/A automatic.
Porsche 944, 84 N/A.
Porsche 944, 83 N/A.
Opel GT, 73.
Toyota Supra MA61,84.