Hello,

The black section of the gauge measures vacuum, the white part measures boost. In the middle, the gauge shows atmospheric pressure (hence why it goes to the middle when the engine is off). The further into the black at idle, the better. The closer to the middle it sits at idle, the less engine vacuum is present. Most 850 turbo gauges tend to read around half-way into the black at idle, depending on whether it is in gear, A/C is on, etc. The gauge in my 760 turbo is almost at the far edge of the black section, indicating very good vacuum. Anyway, if your gauge is reading half-way into the black you're probably okay. How far into the white it will go depends on many, many factors. The ECM regulates boost, so crappy gas (it will generally not allow as much boost with regular gas as it will with premium, for example), bad plugs, vacuum leaks, etc. will affect how far it boosts into the white section. Anyway, let me know if you need anything else.

I hope this helps...

Joe S. - Volvo Master Technician