Priming the 906 LA fuel system - Part 1 of 3 

Glad it was helpful. That’s exactly why I posted it. I typically spend as much time trying to find the needed info on this motor, as I do making the repairs. ✌️

In a perfect world, you prime it any time you opened the fuel system. But, if you’re just changing the screen pre-filter, located on the engine, and the system’s only open for a minute, you should be fine to not prime. If the fuel system’s opened up for too long though, fuel will make its way out of your head, etc, and back down to the fuel tanks, making it difficult or impossible to re-start the motor. So, just have everything at the ready, so that you can get the filter changed out rather quickly, and you should have no trouble re-starting the motor.

should have a fuel screen filter, fuel filter both on the right side and an external fuel water separator none of them require repriming the system

The fitting is referred to as a “Test Port Quick Coupler”. They are commonly used in the hydraulics industry, as test ports for pressure testing.

What engine? Makes a big difference in my answer.

But, the simple answer is, if you have diesel fuel in your coolant, the likely culprit is leaking injector sleeves or injector o-ring seals, depending on what engine. The seals keep fuel and coolant separated in the head, but when they fail, the fuel is able to mix with coolant.