Just joined. I have browsed this forum for many years. A problem has come up many times on this forum that has happened to me twice and I thought I'd share my fix. When all of a sudden my 7.3 wouldn't run even when mildly cold without prewarming.
1st time was after letting Walmart change my oil (never again) they didn't get the oil filter (cheap Fram) tightened properly and after 2 days I realized I was a GALLON low and going up hill it would just die. Easy fix. Changed it at home. Had to use impact wrench on drain plug and crush washer was missing. Truck had never used any oil between changes.
2nd time was when truck was 20 years old and I realized HPOP oil isn't circulated and filtered. Sucked the oil out (nasty! looked like rustoleum oil based gloss black paint mixed with honey) and replaced with new which made a huge improvement to what I had thought was a good running engine.
Can't guarantee this will fix anybody else's problem but it worked for me.