What would cause smoke to come off the oil dipstick on a Volvo 850?

Answer 1

Blow by yes it is , however the cause which is what you require is due to a blockage in the crankcase ventilation system. The Volvo 850 has a reservoir located on the block below the inlet manifold, several pipes come of it and lead to the air intake hose at the throttle housing, usually this system gets blocked and when this happens the crankase is unable to vent through the correct system and therefore appears as pressure/smoke coming from the dipstick tube or the oil filler cap, oil often leaks fron here too when its blocked.

You need top check all pipes for cleanliness and ability to vent ptressure, failing that a kit is available on eBay to completely service the the PCV system. hope this answers your question and opoints you inthe right direction