"I've had this problem sin
"I've had this problem since late last summer, (the boat is always in the water, it is 8 tons)the first thing we replaced was the raw water pump, and cleaned all the debris out of the system. Then the exhaust manifolds and elbows.
Yesterday I removed the first cooler in line with the raw water pump, the transmission cooler, and I found a 1/2" sized, what looks like part of the raw water impeller. Other wise the cooler holes looked fairly clear, I then moved to the Mercruiser "Cool Fuel" cooler and there is definitely not a blockage there it is just a straight through pipe, I next pulled the oil cooler tube and found a bit more, what looked like sea weed in it, not much, but looked to be enough to imped flow a little. That's as far as I got at this point. After seeing the impeller piece in the trans cooler I will again pull the raw water pump later this week and inspect it again. I'm waiting on seals for the heat exchanger, in the mean time I'll clean the cooler tubes, trans, oil, and exhanger. I'm going to also pull the raw water pump and verify it is still in good shape.
The above cooling system drawing shows the fuel cooler before the pump, it is actually between the transmission cooler and the oil cooler, very tough to get at on the lower port side under the motor mount and exhaust manifold and very near the port fuel tank.
I don't really see these minor blockages as the problem, but I'll find out as soon as I clean them all and put it back together again. Any suggestions on how to clean these cooling tubes?
I have some small engine cleaning brushes but they are a bit too tight to push through the small holes.
Thanks for your suggestions, work will continue, as it's another overcast weekend here in Tampabay area, no boating in this weather."