I’m new to inboard engine repair, so I will do my best to describe my problem. I have a 1969 Bertram 31’ SF with twin 1978 Crusader 454’s. Engines were rebuilt about 4 years ago and have run fine since I bought the boat 2 years ago. Starboard engine began overheating, 220+. No leaks in cooling system, hoses & clamps OK, no blockages. Changed impeller, backflushed system, no improvement. Disassembled exhaust manifolds/risers, found highly corroded, replaced both on that engine. Replaced thermostat, was a 180, replaced with a 160. No leaks. Gauges read 180-185 at idle. Shot with IR thermometer, read 162. OK, the sender is probably bad. Ran on plane at 2800. Inside riser 160, outside riser 180, thermostat housing 176. Shot IR temp of port engine (old manifolds & risers are probably corroded as well), risers 110-115, thermostat housing 182 (180 thermostat). Water is flowing nicely from transom exhaust, although much more exhaust/steam on starboard side with new components. Why are the new starboard risers running so much hotter than the old ones on the port side, and why is the thermostat housing running hotter than the thermostat rating. Thanks very much.