The D5 has a motorised version of this which indeed can have the cogs stripped off ...
Dirty exhaust gas from the EGR enters upstream of the throttle so doesnt actualy go through the throttle flap . The oily fumes from the crankcase ventilation pass through this throtlle so some residue is bound to build up , this is quite normal ... the throttle only works for a few seconds when you turn the engine off then it opens fully again ready for the next cold start . This is usualy enough to scrape off any residue . It is the distortion of the plastic housing which causes the sticking . To test , if you press the throttle flap shut against it's spring pressure with your thumb and there is any tendancy to stick , replace it ...
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 My comments are only based on my opinions and vast experience .
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