Hunting idle

[silviamaniac]

after putting all the turbo parts in, my car hunt at idle.

It idle at 2krpm and then hunt up and down between 2k and 1.5k, when it hunts down to 1.5k, there is a hiss sound come out from the turbo side, i guess it is the external wastegate.

vacuum reads at -15vac

My o2 sensor is not hook up at the downpipe, but connected to the wire. Does it matter?

What step should i try to counter the problem? thanks