My friend has an 03 Duramax and it has a starting problem. When the truck is cold it will start right up, but after the engine is at running temp it will take along time to start. It cranks but wont start for up to 30 sec of cranking. My other buddy works at a chevy dealer and he put 8 new glow plugs and a glow plug control module (not sure why, cause it starts when its cold, but ok), still the same problem. It has a new fuel filter and it runs fine once it starts. Any ideas? Thanks in advance.