The cold start valve is found on early 8V GTI up to and inc 1987 and all 16v GTI, it is part of the K-Jet fuel injection system (https://www.vwgolfmk2.co.uk/clubforum/index.php?topic=13.msg16#msg16).
The cold start valve is mounted in the inlet manifold and sprays additional fuel into the manifold during cold starting. The valve is solenoid operated and is controlled by a thermotime switch in the engine cooling system. The thermotime switch is actuated for a period which depends upon coolant temperature, the period decreasing with rise in coolant temperature. If the coolant temperature is high enough for the engine not to need additional fuel for starting, the switch does not operate.
(http://i69.photobucket.com/albums/i61/eddypeck/Mk2OC-KnowledgeBase/cold-start-valve_zps2gwwr1n9.jpg)
(http://i69.photobucket.com/albums/i61/eddypeck/Mk2OC-KnowledgeBase/cold-start-valve-in-inlet_zpsidffhmum.jpg)