Where is the cold start valve located on a 1984 300zx?

There is none all most cold start problems are either from the crank angle sensor in the distributor ( have to greplace distributor for this ) or the Cylinder head temperature sensor. Both send signals to the ECU. Run a diagnostic on the ECU to see if you have these Codes you can find how to do this by going to one of the z car chat sites. or do an internet search for 300ZX ECU Diagnostic check.