Ok now it's my turn to have some troubles.

Car starts and runs great when cold.
Car runs great when warm. Never hesitates, stumbles, or shows lack of power once it is started and running.

If you shut off the car when it's super hot (like today, when it was 96 here) and then come back to it in a few minutes, it is very difficult to get it restarted. You have to crank it for about 30 seconds (so you have to hold the key in start or it will time out) and hold the pedal down about half way to get it running. It will cough and stumble to life the same way it did when I changed the fuel lines.

However, like today, I got to my office at around 8 am. It was about 70 degrees. Shut the car down (after driving for about 25 miles), went inside and went to work. It baked in the hot sun all day. I got out at 4 pm and the car started right up. Drove to the store and went inside for about 25 minutes to pick up some things on the way home. Got back in the car and it was difficult to start. Once it was started it ran perfectly and I got home with no troubles. And my drive has the gamut from 70 mph on the expressway to crawling in stop-n-go traffic.

Then when I got to the house, I parked on my fairly steep driveway nose up (where I usually park). Came back in about 45 minutes to run an errand. Again it was very difficult to start. Yet this morning, after the car sat all night, parked in the same location, it started with no problem.

The engine throws P0400 if it fails to start by the end of the cranking cycle, either by the timer, or if I let go of the key. I have the "diode - n - resistor" egr bypass and I think this is a byproduct of that. The engine will go into limp mode once it gets started, but if I break out the handy OBDII reader it will clear and the engine will run normally after being reset.

It acts like the fuel system has lost its prime. I can see air bubbles in the line from the fuel preheater to the pre-filter inlet.

I have changed all 6 of the plastic fuel hoses in the last year.
I have replaced the pre-filter and its o-ring.
I have replaced the o-ring behind the shut off valve.
One thing I have not done is change the main filter. It is probably due for one. (edit: filter has 30k miles on it, according to PO's records, so it's time)
All parts genuine MB from the forum or the dealer.

Ok what's next?

Change the main filter??
Crank position sensor - not getting any other OBDII codes though?
Lift pump?
Leak in the lines from the tank to the fuel heater?
Leak in the fuel heater?
Clogged screen in the tank?

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech