Cold weather - no start

2000 6.0 4x4
250,000 km

2 winters of no trouble but now have cold start problem.

-20 deg C it won't start on first crank over.

-25 C it takes a few tries.

-28 C will not start.

I plug in the block heater when temp. drops below -10 C.
The engine cranks no problem at all no matter how cold it gets.

Same thing happened last year. Towed it to the Goodwrench wizards. Of course they took it inside and warmed it up before checking anything and it fired right up.
No fault codes showed up but they changed the fuel pump. "Seemed a little weak"
Left it there 2 more nights but temp warmed up to -20 C and still hesitated a bit as usual

I haven't checked spark etc. yet.
Not too pleasant working outside at -30 C.
Thought I'd try this warmer route first.

Any ideas??