Interesting problem. I am always trying to make my idle smoother. I wonder if when you tap it the diaphram in it lets some vacuum loose and sends a signal that the car is warmed up, or is running too cool, changing the fuel air mix. MAybe it is affecting the cold start injector, but that is a swing in the dark.

Sold Truck: 89 DLX extra cab 4x4 22re Automatic/ New truck 97 T100 4x4 auto Sr5