White smoke and engine pressure - 1998 Volvo V70

Engine pressure? you mean compression or oil Pressure? I suppose it's the oil pressure light blinking right?
Look at the oil level dipstick!(have your engine off, car parked horizontally-not on a slope, take out the dipstick, wipe it clean, put it back and take it out again) Is your oil low?
You might have worn out piston rings(not much of a chance) a crack in the motor block(is it a turbo?), worn out turbo-blower(if it is a turbo) or worn out valve guides. But the most probable(and HOPE that this is your case) is a burnt head gasket. Anyhow refrain from traveling with the car, or you could seriously damage the engine.

the colour of the smoke:
greyish-white smoke with heavy burnt smell - oil leaking into the combustion chamber(any case of the above mentioned probable)
bluish-white steamy thick smoke - coolant leaking into the combustion chamber(burnt head gasket seal or cracked block)-though looks easier, but actually is much much more dangerous than the most of the oil leakage causes and can be followed or "supported" by any of the above(though they still leak oil not coolant into the combustion chamber)

all of the problems are repairable, though a cracked block could be a pain in the ****.
If it's a burnt head gasket or any of the head related parts think about changing the timing belt and all the driven components(bearings, evtl. water pump - if you have already changed them, it's enough to change the belt) and do any set up jobs related to the head. the same goes for the cracked block, as it mostly requires a complete disassembly of the engine to the bare bone(or metal in this case:-).

I hope all is much more clear now

rate please

Sounds like this may be a blown head gasket. Take your oil cap or and look to see if the oil is a milky substance. Also pull out the dipstick and look for the same color the oil will be milky in color. If that is not the problem, then check your radiator

White smoke out of the exhaust; blown head gasket.