This is my manuals list. ;-)
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As for the water flow, you are correct, it's just a pipe from the water pump to the HX. First thing I would do, pull the drive, stick a hose into the water inlet in the bell housing and see you have water flowing freely out of the exhaust reliefs and the exhaust cavity in the bell housing (engine does not need to be running for this). If you do, that means the problem is in the drive. If not, you have a restriction between the bell housing and the outlet of the HX, so remove the HX inlet hose and see if you have water... Divide and conquer.
If it's the drive, I would split the drive and check the water pocket cover (pressure test it. I did a long post on this many years ago, before the change to xenforo, and the link no longer works, and the search doesn't go back past 2018
) I suspect as the impeller was so bad the WPC is leaking like the houses of parliament.
Chris......