At idle it will create a vacuum in the manifold, just like a naturally aspirated engine. This because the Turbine doesn't spool up yet.
So if there is a leak at idle it will run on the lean side.
When the Turbo kicks in it will create a boost pressure above atmospheric in the manifold. A leak will then create a too rich mixture.
So if you have a manifold leak with the Turbo the fuel mixture will be all over the place.