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The turbo is such a simple device, attached to the exhaust it spins a turbine on one end and the other end pushes air towards the intake manifold. The faster a turbine spins, the more air is forced into the manifold. The additional air can carry more fuel to be burned in the cylinder, giving more power. When the turbo is not being used, the waste gate is activated to let the exhaust gases bypass the turbine and reduce the spin. Remember, the turbo can spin in excess of 100,000 rpm.
Controlling this process is a turbo boost controller solenoid. This is vacuum dependent. The more vacuum generated by the engine, the less boost. Vacuum at idle is usually around -17psi. As you give the engine more gas/air when you accellerate, the vacuum pressure drops. The lack of vacuum switches the boost controller to close the waste gate and spins the turbo. So, when you have your pedal flat on the floor, the vacuum in the manifold is turned into positive pressure, the turbine kicks in supplying even more pressure, and your car takes off!
That is a nutshell answer.
Your problem could be just a loose vacuum line at the boost controller or at the turbo. I have heard of people who swap the vacuum lines at the controller to essentially 'turn' off the turbo when they let their children start driving the car. No harm to the engine and it keeps the child from becoming a 'lead' foot.
Klaus
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Volvos should be saved from the junk yards, not donated.
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