High idle issue
 

so let me start out, last week my heat control switch started hissing so so it wouldn't change vents and such I changed out the switch and everything was fine or so I thought. I turn the car on and switch through the vents everything works fine cept that the truck is idling around 1k instead of 500 rpms. When I turn the heat or air off it goes back down to normal idle it seems like its pulling too much vacuum ? is that even possible any help will be appreciated