Losing prime in hpop
 

I have an 02 7.3 powerstroke. The hpop keeps losing its prime. For example last night I went out to start it before work. The block heater was plugged in from the time I got home in the a.m. The truck started and I feathered the throttle for a moment and the oil pressure never came up and the truck died. After that it would not start and the hpop reservoir had lost its prime. The truck has 110000 miles and the truck has been in the family since new. Also it will lose its prime at times after you drive it somewhere and shut it off and come back a few hours latter. This has happened no matter the temperature. It has happened in the summer and winter. Any info would be much appreciated. Thank you.