2012 PCM 6.0 ZR409 Overheating at high RPM

We have a pair of 2012 ZR409s in a Tiara 31 Coronet. They both run fine up to 3,800 RPM then climb steadily and overheat. To date the impellers have been changed along with one new raw water pump, the thermostat(s) have been checked, removed and swapped, the heat exchangers boiled and the entire closed cooling system flushed, the port engine closed cooling circ pump was replaced, the seacocks and strainers have been increased to 1-1/2" from 1-1/4" and the port side sea cock relocated aft, the exhaust manifold has been inspected internally for blockage along with the gear oil and fuel coolers, the exhaust riser has been replaced and the solid block off plug on the port engine mounted thermostat housing was replaced with a plug with a hole in it. We have taken pitch out of the props. Running the boat with the computer hooked up to the engines the temperature on the port engine climbs slightly faster than the stbd engine but eventually both overheat on the block side while the exhaust manifolds remain within temperature range. Back off the throttle and the temp falls back to 160 degrees with a few minutes.

These engines have the second thermostat mounted directly on the heat exchanger and we have gone so far as to eliminate the port engine thermostat housing with absolutely no change.

We have spent hours bleeding these engines along with having a Crusader mechanic bleed them as well prior to running the boat with his computer hooked up to the engines. The results are always exactly the same - the engines can't run above mid-range RPMs without overheating.

Any suggestions?

Seems like you have covered about everything. Perhaps measuring raw water flow would be of some use. Could the head gaskets be slightly compromised?

nothing for nothing but on our 8.1s we found that we had a weak suction side hose on the raw water side that was collapsing at higher rpms due to internal coil wire having mostly rotted away.