Hi John

If you are a subscriber to Autodata it is always worth giving them a ring. It's a long shot I know but they have provided me with wiring diagrams that are not listed before ( they really are surprisingly helpful)

Have you got a scope?

Can you put an amps clamp round one of the injectors?

Is there enough fuel pressure being generated?

Is this the Merc engine with duel point fuel pressure control?

Hth
James