The M103 at its age has mostly leaking injectors.

They flood the motor making it hard to start when hot/warm.

Verify by trying next time with throttle depressed a little.

Replace old leaking injectors and don't worry much about pressure regulator.

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Kent Christensen
Albuquerque
'07 GL320CDI, '10 CL550. '01 Porsche Boxster
Two BMW motorcycles