Check the vacuum hose

WARNING: This page does not describe the selected car, but rather 8 other vehicles, including the 2006 Volvo S80, 2005 Volvo S80, 2004 Volvo S80, 2003 Volvo S80, and 2002 Volvo S80. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: Locating Vacuum Hose

Courtesy of VOLVO CARS OF NORTH AMERICA.

Check:

the vacuum hose connections at the manifold and the check valve
that the vacuum hose is not crimped or rubbing anywhere
that the vacuum hose inner weave has not broken and blocked the through flow.
Disconnect the hose. Start the engine. Check the vacuum.