blackwolf wrote:

When on tickover with the EGR open, the engine will be drawing in a certain volume of intake gasses, some of them from the air intake and some via the EGR. The engine will then be exhausting a significantly greater volume of exhaust gasses, some of which will be diverted from the exhaust manifold back to the inlet via the EGR, the remainder being passed though the turbine side of the turbocharger to exhaust to atmosphere.

If the EGR is shut with a map, even though there is no design change to the operation of the turbo at idle, assuming the same tickover speed the engine will be breathing the same volume intake gas and exhausting the same volume of exhaust gas. Now however all this intake gas will be coming via the air intake, hence the potential increase in induction noise as explained so well by Pete above, and significantly *all* the exhaust gasses (rather than just a proportion of them) will be passing through the turbine. A result of the gas laws will be that the velocity of the exhaust gas under these curcumstances will be greater than with the EGR open, hence the turbo will be spinning faster than it would be with the EGR open. Consequently if you shut down the engine the inertia of the of the rotating components will result in a longer spin down time. Inevitable. Either live with it or don't ness with the EGR! If you have changed the exhaust system (eg a de-cat or silencer removal) to reduce back pressure the effect will be exacerbated.