This could be a couple other items. Here is the test you need to go through to find out what the cause is.
It is a bit of a lengthy test but it will lead you to the correct issue. The basic idea with this system is it is in essence a sealed system. So in order to build the high fuel pressure that you need to build there can't be any leaks. This includes external leaks (uncommon) and internal leaks.
When you said you tested the injectors and they past, how did you test them?
Also, if you have a fuel pressure control valve stuck that will prevent the pressure from building and create a "leak" in the system.
Most of the time these issues are all cause by fuel contamination. Diesel engine are sensitive to dirty fuel. People often think that diesel fuel being less refined means it is dirty (dust, water, debris) but this is not true. Diesel fuel injectors cannot have any dirt run through them or they will leak and cause this issue. So it is imperative that they are tested to ensure they are not leaking or you will not make any progress on this issue.
Let me know what you find and we can go from there. Thanks
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Tuesday, September 10th, 2019 AT 3:33 PM