Well for as long as i can remember my 87 t2 has had a slight idle problem. When i start it up cold it revs to 2000 rpm and slowly drops as it warms up. The problem is that when it gets to around 1500 it bounces around and hunts for the idle. But the other weird thing is that when the hood is up, if u listen to the intake you can hear a noticeably louder sucking noise when the rpm drops to 1500 and it begins to hunt. It almost sounds like the bypass valve is opening and closing. But even crazier is when the rpm finally does level off around 900 or so the sucking sound is constant and the motor spits and pops a little bit. I am thinking that the bypass valve is not working properly cause the vacuum is hooked up to the manifold.

 

well the ECU changes the timing when you are idleing around those RPMs, kinda creates a pulsing idle. Test your bypass though if you like, or get rid of it!