ECU ground to the battery

Could you just ground the ecu to the battery rather than to the block? Rather than the both to the block and to the battery.

Have you had a go at watching this webinar?

Star point earthing might have some answers you're looking for.

TLDR; grounding the ECU direct to the battery is bad. Watch the Star Point Earth module.

Short version.

The problem is that if you use different earth/ground points for the ECU and the sensors, etc, connected to it, there will be slightly different voltages at the earths/grounds and these can cause problems with electronics that use voltages, or currents, for sending data.

The slightly different voltage levels arise because there is current passing through the engine, chassis, and ground wires and if you consider V=IR, because there is a current, and there is a resistance (even if low), there is going to be a voltage difference.

With electronics, it's normal for 1/1000th of a volt to b3e significant and with some electronic circuits 1/1000000th of a volt may be significant, so you can see why a 1/10th of a volt can cause big problems. When you consider there could be close to a volt, if cold starting and/or a low battery, with an undersize or poorly connected battery-engine cable, or battery-chassis-engine ground path, it can be critical.

NOTE, some of the better text books, etc, will refer to a 'voltage potential', or 'potential voltage'. This is mostly used in reference to a datum "zero" point, which in a vehicle would normally be the battery negative.

You may also like to check out "ground loops" on Google, you should find some better explanations and examples there.

There is also a very good haltech article on how to star ground:

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

One thing that doesn't get mentioned enough:

Star Earth grounding does NOT require every wire to be grounded on the same physical bolt. It only requires that everything is grounded to the same component.

Eg: If you're using the block as your star point, then everything can ground anywhere on the block and that satisfies the Star Earth design. They don't all have to be grounded to the same bolt on the block. But everything is grounded to the block and something is grounded to the head or a manifold or bellhousing, that will break the star earth rule.