Yachtwork question-
Q. Yachtwork-My diesel yacht engine overheats when I give full throttle. If I drive along slow all seems well. I have changed the impeller and cleaned the sea strainer but when I try to drive hard the engine temperature rises. How do I fix this problem?
A. Good job on starting your repair with two of the most common reasons marine engines overheat, worn out impellors and clogged sea strainers. Now it’s time to put on our “Marine Engineer” hats and begin some real trouble shooting. The purpose of this trouble shooting is to find the exact cause of the overheating engine by performing some simple tests. This way we locate and fix the exact problem without changing extra parts.
It should be noted even if your engine is in perfect condition you can run this same series of tests noting the results in your log. At any time in the future if your engine were to begin to overheat simply compare the old number to the new to find the source of the problem.
To perform these tests we are going to use a laser temperature gauge to check if each component in the cooling circuit is performing to standard. Before we begin we must first ensure the salt water pump is moving enough water. A clogged salt water system will distort our test results.
With the engine in neutral run the engine at 80% of it’s maximum RPM’s. You might set typical 3600 RPM yacht engine at 3000 rpm’s. Take a bucket to the exhaust outlet and time how long it takes to fill with exhaust cooling water. Use this measurement to figure how many liters per minute the cooling system is passing. Compare this number with your manual.
If you don’t have a manual you might be in the ballpark if your 12 hp engine is pumping about 15 liters per minute and a 50 hp engine might move something like 30 liters per minute.
If your engine has flow restriction you can locate this by breaking the salt water circuit in various sections of the rout and doing a quick flow check till you find the clog. Remember the exhaust side of the loop may want to blow back so put a plug in the downstream side of the break and only run the engine for a few seconds jut to check flow rate.
Now the real testing can begin. We want to know if the engine is overheating because it is producing too much heat (IE-bad head gasket) or if a cooling component has failed (IE-clogged fresh water heat exchanger).
Ensure you boat is well tied to a strong dock and pilings. Double your stern lines if necessary because were going to pull against them while trying to make the engine overheat.
Put the engine in gear and slowly increase the rpm’s till your at about 80% load. Check all your dock lines, cleats, and chafe points one more time. If everything looks secure take your laser temp and head to the engine room.
Wait till the engine begins to overheat and test. Start at the sea water inlet and follow the salt water through the cooling circuit testing and noting temperature as you go.
The salt water will probably pass the sea strainer, and on to the gearbox heat exchanger. You can expect a one degree rise in sea water temperature. A larger temperature rise indicates the gearbox is producing too much heat. (IE-slipping clutch), the seawater should pass through the salt water pump and on to the main engine heat exchanger. Expect about a five degree rise in sea water temperature as the salt water passes through the exchanger . More indicates the engine is producing too much heat.
At this time also check the fresh water in and out at the heat exchanger. The temperature change should be about ten to fifteen degrees between the cooling water leaving the engine and the cooled water returning to the engine. If the water re-entering the engine is still hot the problem is in the heat exchanger. (IE-clogged with heat insulating gunk), but if the water re-entering the engine has been properly cooled the problem is in the engine (IE-no cooling water flow, or blown head gasket).
Continue this test till the salt water leaves the boat. You might come across a couple more heat exchangers like maybe a hydraulic system cooler or HVAC or refrigeration system. Test them the same way. A one degree rise in sea water temperature should be about maximum for any of these additional cooling systems.
If you found the engine is producing too much heat continue testing with the laser temperature gauge. Begin at the head and look for hot spots. Follow the cooling water circuit looking for the sudden increase in temperature. Check the oil temperature and oil heat exchanger the same way looking for oil heating due to thinning (IE-fuel pump with leaking seals bleeding diesel into the lubricating oil thus diluting it).
With this series of tests and a little reasoning you should find the problem and only have to fix that single component. If you get completely stuck find another boat with the same engine and run this same series of tests then compare the numbers.
This will save time and expense in the long run.
Marine Engineer
MEC3 and RYA Yachtmaster Ocean