'98 E300 engine start requirements

[torsionbar]

Trying to diagnose an intermittent no-start on '98 E300. It just began doing this earlier this week.

Background: The following stuff has been replaced in 2017 all are Genuine MB parts. Fuel is visible in the clear fuel lines, and no bubbles, even after being parked facing up hill for a week.

- all six clear fuel lines
- shut off valve and o-ring
- fuel filter and o-rings
- pre-filter and o-ring
- fuel thermostat and gasket

Current state: Yesterday engine cranks and cranks but no start, does not fire at all, not even a little bit. No fuel smell or smoke from the exhaust either. So sounds like it isn't getting fuel. Tried cranking some more, on and off for 15 seconds at a time until battery got real low.

Today went out and tried it again, engine cranked really slow from low battery, but it fired right up! Started immediately, less than 1 sec of cranking. Went for a spirited drive and it runs great, loads of power, everything feels really great.

I didn't touch anything between yesterday and today. No start yesterday. Start today. Weather and temperature outside same both days.

It almost sounds like the SOV wasn't opening, because yesterday, it wouldn't fire at all, not even a little stumble. But the SOV is a brand new Genuine MB part. Could it be at fault here?

I'm trying to figure out what else, if anything, is required for the car to start. Is there anything other than fuel supply and SOV that could prevent the engine from starting? I know the '98 E300 is heavy on the electronics and sensors and such, but I don't know how much of that is needed just to start the engine? The problem sounds like it almost has to be electrical. Either the SOV, or some other electronic component.