2015 F550 Intermittent crank no start... but in the stangest way that I have ever seen.

[Fire Chicken]

I have a 2015 F550 6.7L power stroke.

It intermittently fails to start, but in the strangest way that I have ever seen. Here is what it does:
Starter engages
No bad sounds. No grinding, clicking, popping or banging sounds
The engine cranks, but in a strange way

Probably the best way to illustrate this would be to compare how it acts when it starts properly, vs when it does not.

When it starts properly:
The starter engages
The engine cranks

It goes "rarararara vroom... clack clack clack clack" <- normal diesel idling sounds.

When it fails to start and does the strange thing. It goes like this:
The starter engages
The engine cranks

"rara ... rara" ( for about 2 seconds )
( 1 second pause )
"rara ... rara" ( for about 2 seconds )
( 1 second pause )
"rara ... rara" ( for about 2 seconds )

Then it just stops and gives up

Then once it gives up, you crank it again and it'll start up completely normally the next time you crank it over.
Some times you have to crank it multiple times to get it to start, sometimes it starts right up the next time.

Not sure what is causing this, but I've been thinking about it for a while. These are some possibilities / questions that I've thought of:

• Could it be a sensor issue? Maybe the automated starting system is monitoring the crank sensor and not seeing engine movement? Maybe not enough movement?
• Is the engine cranking over too hard? Do these modern diesels use decompression valves for cranking? Could it be one of those failing?
• Maybe a failing glow plug causing the automated starting system to give up and stop cranking?
• Maybe a failing starter or starter solenoid not delivering enough current?

I don't know how the automated start system works in detail yet. I'm hoping that someone can help me out.

• System voltage looks good. Both batteries are new and look good ( 12.6v at rest, 11-ish volts while cranking )
• Cleaned both + starter cables and crimped / soldered on new ends ( the original one was shot, but replacing it made no difference )
• No Codes or error messages on the dash ( using an aftermarket OBDII reader, not the special Ford scan tool )

What could this be?

[Fire Chicken]

Soon I will put an O-scope on it and measure starter current and voltage drop across the starter solenoid on a dual trace.

I'll get the normal conditions and also try to capture a failed start.

At least then we'll know if the starter is having to work harder during the failed start condition.

Also at the same time we'll learn if there is an intermittent solenoid issue dropping significant voltage across the high current terminals.