A Diesel engine ignites its fuel by compression. The temperature increase in the cylinder due to friction of compression and will need to reach a temp of about 1,000 deg f. During cold ambient...
if 7.3 IDI disconect connector and remove with deep I think 10mm socket might be 11mm dont rember. if 7.3 DI (powerstroke) remove valve covers and use about the same size socket.