850 won't restart once hot, but restarts after cool down.

 

I'd start by checking the fuel pump relay and the injector relay (its on the cowling ). If you find bypassing the fuel pump relay gets the car to start, then its time to replace the pump as well as the relay. (a worn pump will draw too much current which can overheat the relay)

 

It starts again once it has cooled down. I've read somewhere about a purge valve???

 

Yes there is a purge valve that dumps fumes from the charcoal cannister back into the intake under certain conditions - during starting/idling the valve is closed. It's not your purge valve. \

If it won't start when hot - do you have fuel pressure? Spark? (fuel pump relays are a common problem - cracks form in solder joints on the circuit board, as current flows, the resistance in the crack creates heat and eventually current does not flow. Once the relay cools off - current flows and fuel pump runs)

They usually crack where the windings for the electromagnet are soldered to the board - It takes a magnifying glass to see them.

 

The culprit was the cam position sensor.