idle not steady

[joeranger]

strange issue on my V6 2011 Ranger. if I have the heater control knob turned to use both dash and floor vents I hear a clicking noise and idle will cycle up and down. Checked and not AC clutch engaging. Move knob to dash or floor vent only - idle steady. If I move knob to defrost AC does cycle on/off with corresponding increase in idle.
Too cold to fix - just an annoying problem and like most problems I get it doesn't happen consistently . Any suggestions?

 

[RonD]

Vent flaps are controlled by Vacuum Motors, engine vacuum is routed to these to pull open a flap and hold it open or vacuum is cut to close that flap.

So on the back of your Vent Selector is a Vacuum hose matrix.
If one of these hoses has a leak then it is a vacuum leak for the engine.

Now you may also have another issue because a vacuum leak in the cab Vent system should not effect the engine.

Set vents to Panel and fan on high, then accelerate and see if air starts to be diverted to Defrost Vent, put you hand at defrost vent
Defrost is the default setting for all the flaps, low or no vacuum = Defrost vent

The Vent system has a Vacuum reservoir in the engine bay, a black plastic ball, usually located low under passenger side Rad support.
If this reservoir is cracked or hoses to it are leaking then Vent system will lose vacuum when engine does, i.e. accelerating = low engine vacuum
Reservoir is there to prevent this loss of vacuum and also to prevent any engine issues when there is a leak in cab system.
The ball or the hose from the ball to engine intake manifold has a check valve, like brake power booster does, to prevent cab system from effecting engine