Hate to say it but MB lacks in control logic. Few lines of code and pressure sensor would pick up that fault right away.

BMW control logic. Starter terminal 50 activated> engine speed 0 (zero). Resulting fault "Starter activation"

 

Pressure sensors on the "low side" of fuel delivery systems are very rare across the entire industry... I don't think I've ever heard of an OEM sensing PFI or lift pump pressure.
"High side" pressure for both gasoline and diesel direct injection? Of course... but never low side.

Are you saying BMW has a low side pressure sensor?

 

I agree. However, Y94 error is essentially a finger pointed at a lack of fuel being delivered to the HPFP. It does makes some sense. In my case seeing as this was so intermittent and, usually, not a lasting condition, the LPFP wasn't really showing up as a suspect. It really comes down to the age-old saying that if you make things fool-proof a bigger fool will come along and prove otherwise; in my case, and it seems it's the second instance of such (first being the original engine and the destroyed GP hole), a bigger fool in the form of a bigger monkey-with-a-wrench! I'm sure that I'm not the first person to ever discover something like this but it seems it's pretty rare. Coding in conditionals also requires documentation and that can start to add up; not an excuse but a point could be made for why no such condition was included for monitoring.

In previous episodes I didn't have my Foxwell connected to get a better look. Well, it wasn't until later on in all of this that I could be able to know what I was needing to look at. This latest episode I was able to see a synchronization error and it had to do with cam+crank signals not syncing: engine unable to fire due to no fuel; this ends up as just another observed functional (or non-functional as the case may be) point.

IF this is now finally sorted then I figure I've got about $1,500 into "solving" this issue. That would include the towing charge. I now have a new fuel rail and Y94 and that gives me a pretty new fueling system (with the original install having new injectors). I also have a fully functional (and proven) ECM as a spare. AND I have a lot of attained knowledge (hat tip to everyone here).

 

Intermittent conditions can be fiendishly difficult to troubleshoot. Don't underestimate the value of the knowledge and skills acquired.

 

OK, folks, I'm signing off on having this issue RESOLVED. No issues after logging over 1,800 miles. Many thanks to all for contributing. I would hope that there are usable bits of information in this thread for others who come along.