As it cools off this fall our d6 isn't starting good, 30s and cooler. Just had it serviced at cat, they say fuel system is good and pre heater is working. 7,000 hours, C-9. Could intake heater be tired? Runs good no loss of power when working
What grade (viscosity) of oil does it have in it..? For example a change from 15W/40 to something like 5W/30 or 10W/30 would improve cold starting immensely. Also consider checking battery condition (voltage & output) and maybe engine cranking speed when cold. Come to think of it maybe a check of the cold engine cranking RPM would show if the batteries or something else in the starting circuit was not up to scratch.
When you figure this out let me know cuz mine is exactly the same. I always figured it was oil viscosity combined with HEUI injection system. Its not the oil age or bats. I've been watching this for several seasons. The ether injection system helps to keep it running while it is working to get to idle. But yeah, I know where you are coming from. The engine will sit there for a minute at about 400rpm trying to decide if it wants to run or quit. Then, as if told to idle, it pops up to 850 rpm or so and sits there warming up. I've gotten used to it.