You could try disconnecting a cooling system hose somewhere at the midpoint of the water flow inlet -> to final outlet them start at that point and work back to the inlet or forward to the outlet until you find the blockage. The alternative is to start at one end of the cooling system and work from that end to the other end until you find the problem.
Manifolds have a nasty habit of becoming blocked.
This would be a good time to plumb for the Thatch Modification if you don't have it already.
You're RWC I presume?
ex TRUE GRIT
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