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the filter is after the lift pump but before the injection pump.It’s odd because when I first got the truck, I drained a bunch of old gunk out of there through it with the truck running, didn’t shut off. This was before swapping the filter and IP.
This time when I did it, bone dry and not even a drip.
Does the drop in pressure going into the IP when sumping it usually kill the engine? Do you ever touch the water separator at all or just leave it?
Injection pump has enough suction to pull fuel from the tank, so you can still drive even with a failed lift pump.
As soon as you open the drain valve you let air into the system, and the injection pump doesn't have enough pumping ability to clear out the air, so truck won't run until the lift pump is replaced.
If the lift pump is working properly, you will get a steady flow of fuel out of the drain.
It's just a basic warning of "don't touch the drain if you aren't in a place to deal with a failed lift pump" because if you drive normally you don't always know when the lift pump has failed.