My son has a diesel Walker, that is giving him fits in the morning. It will not start. He follows the starting instructions to a tee....but no go. That is until it warms up a bit, around 9 or 10 am. Then after that it will start with no problems until ther end of the day. Then next morning......will not start. I also have a diesel Walker, but mine will fire off first thing every morning just like clockwork. I really know nothing about diesel engines so am not able to help him. He has replaced the fuel pump, filters are clean, and he even drained the fuel tank and filled with fresh fuel. Any ideas of what could be causing this problem?