Matt,

to cool cylinders, why would someone introduce hot exhaust gasses rather than cold fresh ambient air? I can see the point of NOx being re-introduced to the system where the NOx compound is broken into N as one element which would cool the system but think cool air would be of more help. Anyhow, this is not important to me.

Certainly so, the boost control limits the turbo from overboosting. There is a boost switch near the accumulator. But what does it do? When it detects too much pressure it sends a signal somewhere. Then something is done to limit the boots. Now, there are only 2 parts on the turbo that can do something - the intake flap and the EGR.

This leads us back to my original question - does overboots sensor close the intake flap in the turbo? If this is the case where does the excess pressure escape? Through EGR? It is disabled.

If it doesn't control intake flap but opens EGR itself to let the pressure out then it's the same - EGR is disabled.

Any ideas?