Sound like the injectors are bad. High return rate can cause a no start condition. The Duramax waits for about 1500 P.S.I. of fuel pressure before it will even turn on the injectors. So if the injectors are sending to much fuel back the return line the the injection pump may not be able to build enough pressure when cranking the engine. By adding starting fluid to it you make the engine start which also spins the injection pump faster and makes enough pressure for the computer to fire the injectors. They also set fuel related codes especially when towing...p1093 is the most common.