[Shadeyman]

My XC90 had/has a slight boost delay too, I reduced it to almost nothing, doesn't bother me so will do something about it eventually, no rush. I've serviced the whole vaccum system replacing some of the parts so I think there can be only 2 things causing it.

I think the problem is the vanes in the turbo are not closing enough at low engine RPM to maintain a high turbo shaft RPM. If the turbo shaft is spinning to slowly when engine RPM is low, boost takes longer to increase. Variable Vane Turbos are supposed to limit turbo lag to almost nothing by keeping the turbo shaft spinning at high rpm.

So the causes:

1. Low vacuum caused by the vacuum pump itself. If its too low the actuator wont close the vanes to increase the velocity of the exhaust gases hiting the turbo impeller. I've monitored it and it does drop quite low sometimes for no reason. I'm guessing worn/faulty valve in the the pump itself could be the cause.

2. The vanes in the turbo are sticking momentarily, probabely due to carbon deposits.