Gwhise,
Welcome to oil burners!
The the members have already shared a common sign of a failing injection pump (IP). A truck that is not starting when hot but it will start when cold. So the water trick is to cool off your IP so you can test the IP. If hot = no start. Pour room temp water on the IP and re-test. Test = a start, likely a IP on its way out.
Now the white smoke often has to do with timing.
1) So check that the IP advance solenoid is not getting +12v power once the engine is say +130*. It is the electrical connector closest to the front window on the top area of the IP. It should only get power when the engine is under 120*. If it stays advanced it will cause some smoke issues.
2) If you find your truck does start fine when hot
, is still sluggish, and smokes white I suggest you check your timing. You will need to find a member near you on the “Timing Registry“ thread in the Tech Article section. They offer to help you time for free or a small fee. Or you can buy a meter and do it your self. If you change your IP or Injectors you need to re-time your truck.
You can learn more about timing equipment and how to time your truck in this tech article.
This thread is how to set timing on your NA or Turbo 6.9 or 7.3 F250 or F350 pick up. When should you check your timing? Every 30,000 miles. As your pump and injectors wear, they begin to open sooner generally. (Lower pop pressure.) This changes your engines timing. As an IP wears it may take...
www.oilburners.net
Let us know what you find out next.
Also attach some pics!