Leave the U pipe blocked or water will bypass the engine completely. Unfortunately this looks like a good place for an airlock so a compromise might be to do what I used to do and drill a 1/8th inch hole through whatever you are using to block the pipe.
On my injection engines I just plug the pump cover with a household radiator end plug which happens to be the correct size, but I don't have to worry about the inlet manifold end as I have done away with the standard inlet manifold.
I don't know if any air is actually trapped in the pump cover, but I am not getting a big change in the level of the header tank so I suspect that any air probably gets blown through by the flow of water.
Neil
On my injection engines I just plug the pump cover with a household radiator end plug which happens to be the correct size, but I don't have to worry about the inlet manifold end as I have done away with the standard inlet manifold.
I don't know if any air is actually trapped in the pump cover, but I am not getting a big change in the level of the header tank so I suspect that any air probably gets blown through by the flow of water.
Neil
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