grounding

when you said ( Any part of the EFI system that requires a power ground is either grounded to the body of the vehicle or the engine block but never both at the same time )

i thought we ground all sensors and components that require a power ground to a single point, that is the engine block , so how come we come we can connect to wither the engine block of the vehicle body ?

We ground sensors to the ECU, that's important.

While this is a complex topic, I'm going to try and keep it simple. We don't ground EFI systems to both the block and the chassis because if the main ground path for the starter is lost, or compromised the grounding in the EFI system becomes the only ground and its usually undersized for the starter current.

Usually the block is chosen as the star point of a grounding system to ensure a small ground loop for the ignition system. This keeps EMI down to a miniumum.

I will link to an article from Haltech, as I have many times before lol, they do a great job with visuals to explain this thinking

https://www.haltech.com/news-events/ecu-grounding-the-dos-and-donts/

That's a good article/video. A separate star off the head makes a lot of sense.