Overboosting

[Michael]

I was reading the forum and a few people mentioned that overboosting may be caused by either a stuck actuator or a cracked turbo. Can someone please explain how a cracked turbo can result in excess boost.

Thanks.

[BOOSTBOY]

They crack right where the wastgate valve shaft comes out of the turbine housing.  When subjected to heat, this crack expands, causing the shaft to sieze, holding the wastegate shut, or at least close to shut.  This causes the overboosting problem on the city turbo.