Just went through this on mine. It’s been an ongoing issue for a while in the colder weather after the truck sat overnight. Never and issue when the truck sat at work for longer periods of time. I also had a tapper cover leak that I repaired. Truck was good for 2 weeks...... then.......
I replaced:
straight cut orings on the t fitting
o rings on the return line at the vp44
new return relief valve on vp44
new fuel line and fittings from lift pump forward- 15 psi idle
new filters on the fass
still hard a hard start that was worsening, noticed fuel in oil.
replaced:
injector o rings- were VERY flat and came out easy, real easy
new crossover o rings
seemed to solve the fuel in oil issue
then:
new #4 injector line- was leaking at head after initial injector o ring change
new crossover tubes from BD diesel and orings- figured it’s was best as they had 240k on them and been through 4 injector swaps
so far so good! Truck runs a lot better all around. Power is back to where it’s supposed to be. Happy till the next issue arises. I’m 99% positive the injector o rings were my issue. Only time will tell if the oil level rises again, then it’s time to dig a big hole and burry it.