Detroit Diesel 638 engine problem, please help
 

I have a Hyndai HLD 150/Bering LD-15 truck with a Detroit 638 that has a strange problem. The engine starts and runs fine, but after few mieles when driving the engine develops a hesitation and looses power. It gets progressively worse to the point where u can only drive about 20mph after a few miles the engine shuts off, and it wonts to start, unless you whait few hours then the engine starts without a problem,
When the problem started you could drive normal for 100 miles normal, now its maybe 10 miles then its starts to losses power.
If u shut the engine off and let it sit a bit the problem goes away for next few miles.
We removed the fuel tank cleaned it and cleaned all the fuel lines and changed the filters and this did not help at all. We bought
a new injection pump and it alsow did not help. I'm not sure what to do next and was hoping someone may have seen this problem in the past and may have a solution.
Please Help. Thanks.

were the fuel injectors tested?

were they cleaned??

Your on the right track, it's defiantly a fuel problem. Are you sure you changed ALL the fuel filters? Does it have an electric in tank fuel/lift pump? If so that could be weak and heating up and then slowing creating a low or no pressure situation and force the injector pump to suck or pull the fuel into it and it's not designed to do that resulting in loss of power and stalling.

Just thought about another thing, Check the fuel tank vent! If the tanks not vented then it will create a vacuum that the fuel pump will have a hard time to overcome.