Offshoreonly.com - (Another) 496 Overheat Puzzle

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Removed transom-PS cooler hose. It looks fine and is very stiff so probably wont collapse. Was clean on the inside. Bought a new one so will put it in.
https://cimg2.ibsrv.net/gimg/www.off...99d3efe19.jpeg
https://cimg3.ibsrv.net/gimg/www.off...c2219be30.jpeg

When I had the stern drive off and transom to PS cooler hose off I ran an electrical cord through the transom tube. It when through without any obstruction and was clean. No powdered corrosion or dirt/grit.
https://cimg4.ibsrv.net/gimg/www.off...47e95ec8d.jpeg
https://cimg5.ibsrv.net/gimg/www.off...f2f15ae5c.jpeg
https://cimg6.ibsrv.net/gimg/www.off...1467cdfbb.jpeg

Update - new intake hoses installed, new temp sender to ECU installed, manifolds and risers flushed through thermostat housing hoses (all clean and free flowing), oil cooler tubes cleaned with long brush and pipe cleaners (no surprises). Installed 0-30 water pressure gauges on engine block drain hose and exhaust riser.

started boat on the hose muffs, inside a water filled tank. Idled and water discharging until temp rose to 150. (140 thermostat). Zero water pressure on block and riser. Removed hose muffs. No change in pressure. Increased rpm to 2000, still zero on gauges. Water flowing from discharges. Temp never got above 150.

IR temp gun confirmed no temps over 145 anywhere on system. Oddly the port riser and manifold are 20 degrees warmer than starboard. Hose feeding manifold from thermostat housing also about 15 degrees warmer than hose from same housing feeding starboard manifold. Go figure. Check thermostat housing nipples - all clear.

Zero pressure? Maybe no restrictions means no pressure?

Headed to the lake this morning.

It is normal fro the 20* temp difference from risers.
Idle water pressure is usually only about 2psi.
Pic of how water pressure gauge is installed??

Well f-ck. Temp gauge quit working half way through test. Gauge matched IR gun up to that point. Half throttle no temp increase above 150 at the sender. Full throttle five minutes then engine alarms and derates. IR gun temp at sender was 227! Had to be steam since it’s lake water. Again took several minutes to drop. Attempted to pull thermostat but hose removal wasn’t possible due to high temps. Gonna pull and test (new) thermostat and replace (new last year) impeller. Then back to the lake. Still no pressure readings on block or risers, even under WOT.

Crossover tube seems to be the hottest section of the cooling system. Goes from thermostat housing to front of both heads. Also has the bung for temp gauge sender.

2 psi probably wont even register on a 0-100 or even 0-30 gauge. Didn’t know what to expect so started with a high range.

Could crossover tube (between heads and temp housing) be clogged? It’s the highest temp anywhere on the engine/exhaust.

One gauge on riser and one on block drain hose. Not ideal.