I have a Cooper Yachts Prowler 28' with a Volvo Penta TAMD-41B turbo diesel & 290DP leg. I had just done a bunch of work on her, put her back in the water, and went out for a short test run. She ran great!! Except that when I got the revs up a bit, the temp shot up, steam out of the engine house, alarms going all over. Nursed her back to the slip, temperature cooling to normal. Got the revs back up a bit (1500 RPM), and the temp shot back up again.
Had her hauled out, and started tearing apart the entire cooling system (very complex in this engine!!), one piece at a time, and took them home for cleaning & rebuilding, returning each piece to try it again (these parts are VERY expensive to rebuild, so I did it one piece at a time, hoping each time to find the issue).
I went through the raw water pump, heat exchanger, oil cooler, aftercooler, (in that order, as they were said to be the most likely culprits, in that order), each time finding no improvement.
Frustrating!! Not to mention, just the O rings to reassemble those parts, alone, cost over $1200 CDN.
Finally, I was poking around inside the leg mechanism (boat was on the hard....again) and I saw something that looked like a piece of aluminum casting I'd never seen before. I started prodding it with a long screwdriver, and discovered it was....a huge oyster. It had seeded on the side of the rubber "S-hose" that feeds raw water from the leg through the transom to the cooling system and, as it grew, it forced the "S" into a tight "V" - enough to let only a trickle of water through. In other words, enough for cooling at idle, but no more.
I've got to say, I felt like an idiot when I found that!!
Not a single mechanic I spoke to (and there were many!) familiar with this engine had ever seen anything like it. Murphy is my co-pilot....
Anyway, since the aluminum fitting on the leg that the S-hose feeds was getting corroded, I decided to bypass it entirely. I drilled a 1 1/4" hole through the hull ahead of the transom & installed a bronze through-hull (with a grilled bronze scoop pickup on the bottom of the hull) & a stainless steel ball valve on top. Brand new hose to the aluminum cooling pipe that feeds the raw water pump, and problem cured. It now runs at a constant 175*F, no matter what the load or speed.
*Turns out this exact setup was an "optional extra at additional cost" for this power unit. Guess the original owner should have ponied up the extra $$!