1987 560SL will not start cold
 

560SL will not start cold, if you remove air cleaner and keep priming with a little fuel it will start and run great. It will continue to start and run until it sits for several hours and then I have to prime it again. The fuel pumps seem to be working fine. There is a component called a "start valve" or "cold start valve" in diagram I was reading. It has its own fuel line as well as the 8 that go to the injectors. Could this part be causing the malfunction and how do I check it?

The over-voltage relay powers-up the fuel system computer & that computer controls cold start.

replace the Overvoltage Relay, as mentioned these relays fail with time ,ageing fatigue and developing cold solder joints . same for the fuel pump relay.
mak