Steph Lakner of Annapolis, Maryland, asks:
"I have a Westerbeke engine (circa 1987) on a Cal 28. The engine overheats (to 210-215 F) after about 20 minutes running at full rpm. The impeller is fine, but water flow is reduced beyond the heat exchanger. I have found no obstructions (broken vanes, etc). I brought the heat exchanger home and cleaned it with vinegar. It was already pretty clean, but I found leaks in the “little tubes,” so raw water has been mixing with the antifreeze. Interestingly, the antifreeze level is always normal. Should I try to repair the heat exchanger or should I replace it? Could this be the cause of the overheating, or I should look elsewhere?"
Nigel Calder replies:
If raw-water flow is reduced, this most likely is the cause of your problem. Although you have replaced the pump impeller, the pump housing may be worn, and this can also cause a loss of output. If this is not the case, another place water flow can be obstructed is at the injection nipple into the exhaust system. You should also check the feed to the water pump to make sure the hose is not kinked or collapsed. Sometimes the hose gets soft with age and then collapses due to the suction pressure from the pump.
One way or another, you want to get the raw-water flow back to normal. You also need to replace the heat exchanger. If salt water is mixing with the fresh water, the salt will scale up your engine, which can itself lead to overheating. Descaling the engine will certainly be more work than sorting out obstructions in the raw water flow.
Lastly, is the hull seriously fouled or might there be something wrapped around your propeller? This would increase the load on the engine and could also cause it to overheat.