Buly,
Somewhere in this series the words 'thermostat' wire and 'thermocouple' wire were mentioned. It it is truly thermocouple wire, it is difficult to solder, and you may have a cold solder joint at the injectors. This could manifest as having continuity, but unable to pass a sharp pulse from the EC-2.

I.E. When you 'ground' it manually, you hear it click, but you don't know how fast it operated. When the EC-2 sends a fast signal, the rise time may be too slow through a cold solder joint to allow the injector to open before the EC-2 has shut it down.
Bill Schertz
KIS Cruiser # 4045