EVAP purge valve
Well I finally discovered the mysterious fault with my engine check light and the main reason why the previous owner sold it and how his so called Volvo specialist could not cure the turbo brick.
I was at my wits end replacing all the hoses, EVAP canister and double triple checking parts the P0315 code would come back on after ever 17 minutes of idle. This went on for months. I pressure tested everything and replaced the fuel cap and nothing helped.
I used a brand new EVAP purge valve but it was not the correct part but I figured they all do the same job. Wrong.
I ended up buying a similar Bosch purge valve but it was from an older Volvo, The connectors was designed not to fit the 95 850 Turbo so I just cut the offending plastic off and it clicked right in and the engine check light has been off for 4 weeks after 500 miles.