The door that controls the dash/defrost moves to defrost with higher rmps. i am pretty sure its a vacuum leak somewhere. but what i dont know is what line controls that and where that door is exactly or what to look for. any help would be appreciated. thanks

 

Try checking the vacuum hose that attaches to the vacuum motor by the right hood hinge and blower motor. On later F150s this hose is white.

THis is very common on later f150s.

 

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