Dies when hot and driving slow
 

I have a 2001 GMC 2500HD Duramax. Lately when I am pulling a hay hauler with 6 rolls of hay the truck dies after about 2 loads when weather is about 100 degrees and driving slow. Did die once is rush hour traffic driving slow when temp was 112 degrees. After I let it cool some, I can remove screw at fuel filter and prime it. At first it takes a few pumps to get fuel coming out then it looks like foam coming out as there is some much air in the fuel. Once I get the air out it runs fine just driving down the road. Had it is GMC house twice and they put pressure on the line and couldn't find a leak. No codes to give them any indication of a problem. They drive it down the road and it doesn't die as they don't have a load. Engine never gets hot, but appears something outside the engine is getting hot and malfunctioning when weather is hot and pulling a load. Talked to a cousin who has a newer diesel Chevy and he has the same problem. Currently can't use truck to pull heavy loads on farm and that is the whole purpose of the heavy duty truck. Any thoughts?

It sounds like there is air getting into the fuel system.
what line did they put the pressure tester on? There could be air getting in from another line.
maybe a damaged fuel line, or cracks in a fuel line. and it starts sucking air when the weather is very hot and the line is very hot.:humm:

Try looking for leaks or bad gaskets on your fuel line. It could be heating up and expanding those gaskets causing a leak when hot, then when it cools off the gaskets re seal.