By a temp gun and then aim it straight down on the glow plug thats where you take it at. you do not want the temps to go over 250. But depanding on your engine it might be a little higher or lower.

If you are sure you are at factory and the truck will only run when its cold. Then it dies once it warms up and will not start again until it cools back down then your engine is broken. You will need a new piston and sleeve or you can pinch it for about $20 dollars. Here is a link to someone who will do it. I have used them myself and it works and they are for real.

http://cgi.ebay.com/sleeve-resizing-...QQcmdZViewItem