My Mercruiser 470 engine consumes a large amount of coolant. The last time that I ran the boat in the water it ran fine. It started quickly and the engine ran smoothly with normal power. The oil pressure and engine temperature were normal. There was no sign of engine overheating and the engine did not make any unusual sounds. No coolant was dripping into the bilge.
I discovered the problem when I drained the coolant since it was time to replace it. Prior to draining it, I noted that the coolant level in the recovery tank was at the “full” line so I expected to collect 10.5 quarts of coolant since the cooling system has a capacity of 10.5 quarts. But I collected only 7 quarts of coolant which surprised me since if coolant was being lost within the engine I expected that replacement coolant would be drawn from the recovery tank. This indicated to me that there was a significant leak in the coolant system both due to the loss of 3.5 quarts of coolant and the lack of suction to draw replacement coolant from the coolant recovery tank.
After draining the coolant, I filled the coolant system with water in order to run the engine and flush out the cooling system. It took 10.5 quarts of water to fill the system which is the coolant system capacity. I ran the engine for about 15 minutes at 2,000 rpm. The engine ran smoothly.
After the engine cooled I drained the cooling system and collected only 7 quarts of water. So I lost 3.5 quarts of water in just 15 minutes of engine running. There is a big leak somewhere internally. Nothing is dripping into the bilge.. I drained the oil to see if the oil looked “milky” but I could not tell. The oil was black as it always is when I drain it at the end of the boating season. I got approximately 5 quarts of oil when I drained it plus there was still some oil in the oil filter. The capacity is 5.5 quarts of oil so none of the missing coolant wound up in the oil.
I refilled the cooling system with water and did a pressure test. I pressurized the system to 10 psi. and the system held at that pressure. No sign of any leakage.
The only time that the coolant or water is lost is when the engine is running and hot.
Based on the tests described above, my non-expert opinion is that the head gasket is OK since:
1 There is no coolant in the engine oil.
2. The engine runs smoothly when passing large amounts of coolant. The engine consumed 3 quarts of water in just 15 minutes. It would probably not run at all with that amount of water being ingested. Also, since no coolant is winding up in the bilge, the loss of coolant must be through the exhaust.
I suspected that a leak in the heat exchanger was the cause of the large coolant loss since coolant only flows through it when the engine is running and hot. When the engine is cold, like when I did my pressure test, the thermostat bypasses the heat exchanger. I removed the heat exchanger from the engine and did a pressure test on the coolant leg using hot water at 75 psi. It held that pressure. No leaks.
What else could be causing my coolant loss problem?
I discovered the problem when I drained the coolant since it was time to replace it. Prior to draining it, I noted that the coolant level in the recovery tank was at the “full” line so I expected to collect 10.5 quarts of coolant since the cooling system has a capacity of 10.5 quarts. But I collected only 7 quarts of coolant which surprised me since if coolant was being lost within the engine I expected that replacement coolant would be drawn from the recovery tank. This indicated to me that there was a significant leak in the coolant system both due to the loss of 3.5 quarts of coolant and the lack of suction to draw replacement coolant from the coolant recovery tank.
After draining the coolant, I filled the coolant system with water in order to run the engine and flush out the cooling system. It took 10.5 quarts of water to fill the system which is the coolant system capacity. I ran the engine for about 15 minutes at 2,000 rpm. The engine ran smoothly.
After the engine cooled I drained the cooling system and collected only 7 quarts of water. So I lost 3.5 quarts of water in just 15 minutes of engine running. There is a big leak somewhere internally. Nothing is dripping into the bilge.. I drained the oil to see if the oil looked “milky” but I could not tell. The oil was black as it always is when I drain it at the end of the boating season. I got approximately 5 quarts of oil when I drained it plus there was still some oil in the oil filter. The capacity is 5.5 quarts of oil so none of the missing coolant wound up in the oil.
I refilled the cooling system with water and did a pressure test. I pressurized the system to 10 psi. and the system held at that pressure. No sign of any leakage.
The only time that the coolant or water is lost is when the engine is running and hot.
Based on the tests described above, my non-expert opinion is that the head gasket is OK since:
1 There is no coolant in the engine oil.
2. The engine runs smoothly when passing large amounts of coolant. The engine consumed 3 quarts of water in just 15 minutes. It would probably not run at all with that amount of water being ingested. Also, since no coolant is winding up in the bilge, the loss of coolant must be through the exhaust.
I suspected that a leak in the heat exchanger was the cause of the large coolant loss since coolant only flows through it when the engine is running and hot. When the engine is cold, like when I did my pressure test, the thermostat bypasses the heat exchanger. I removed the heat exchanger from the engine and did a pressure test on the coolant leg using hot water at 75 psi. It held that pressure. No leaks.
What else could be causing my coolant loss problem?