worlock
New member
I have a 1983 Nissan 3.3 V6 engine which gives about 100HP which has been marinised in the 25.5 ft bayliner motor cruiser boat I now own (this engine is from the old Nissan Patrol jeeps). She is moored in Christchurch, Dorset. She is overheating when operated at speeds of 7 knots or more.
The system has sea water and a fresh water cooling system.
The boat has an OMC leg which has an internal pump which pushes sea water through a tube heat exhanger to cool the fresh water part of the cooling system. The sea water then exits through the exhaust outlet.
On the pressurised fresh water side, there is a small header tank which feeds the closed fresh water side. The original automotive engine water pump pushes the fresh water around the engine.
The boat works fine at low speeds (e.g. 5-6 knots or less), with the water gauge reading about 70-80 degrees and i can cruise up and down the river for hours with everything working fine. However, when taking the boat out to sea and pushing upto 7 knots or more, the temperature slowly rises and it will overheat unless action is taken. The previous owner tells me that he was able to cruise at 10 knots without over heating problems and i have no reason to disbelieve him. The top speed is about 12 knots according to him with this combination of engine and boat.
I am not loosing any fresh water from the cooling system as the header tank water level remain contant. I do not believe i have a head gasket problem. There is no excess pressure when i remove the oil refill cap and no milky oil which can often be seen if you are loosing water via the head gasket.
With the engine on idle, I have removed the sea water outlet pipe that exists the heat exchanger into the exhaust outlet and there is plenty of sea water coming out (it fills a large bucket in about 20 seconds). From this i believe the sea water side of the cooling system seems to be working fine. The only exception to this is that the heat exchanger could be partly blocked in the sea water part ? I have not taken the heat exchanger off yet to examine it as it is quite bulky at about 3 feet long and about 6 inches in diameter. The heat exchanger is mounted as low as possible inside the hull and is at the same hieght as the lowest pat of the engine. This suggests that any air blocks would be unlikely.
If i am not loosing water and if the engine has no obvious problem causing it to overheat, then this is either a problem with :-
1) The fresh water pump (perhaps not pushing enough water around).
2) A faulty thermostat.
3) A partly blocked heat exchanger.
4) A heat exhanger which is not rated highly enough for the job it needs to do.
There is no obvious noises from the fresh water pump.
I tried to find the thermostat, but it is NOT inside the engine thermostat housing where it should be ..... I did some research and when marinising automotive engines and installing heat exchangers , some heat exchangers have a thermostat built into them and the installations instructions can tell the installer to remove the original engine thermostat. I can't see whether the heat exhanger has a thermostat built into it - so i dont know if i am operating without a thermostat or not ...
In general I think it would be ok to operate without a thermostat except that the engine would run colder than expected - however this is clearly not the case here as it overheats !! In fact, if the thermostat was in place, i would have probably removed it as a test and run the engine at a higher speed. A faulty thermostat can cause over heating of course.
I will remove the heat exchanger to see if there is a thermostat inside of it and to see if there is any obvious blockage or build up.
Any help or assistance would be greatly appreciated. I have fixed 4 problems with this boat already that have interrupted the season (NOT related to overheating - 2 different steering problems, 2 electrical problems) and as a person new to boating (this is my first season), I am almost at that point of giving up (my wife was on board when the engine overheated out at sea.... you can imagine her disapproving reaction). It would be a real shame to quit now considering how enjoyable boating can be when the things actually work !
My thanks in advance,
Mark.
The system has sea water and a fresh water cooling system.
The boat has an OMC leg which has an internal pump which pushes sea water through a tube heat exhanger to cool the fresh water part of the cooling system. The sea water then exits through the exhaust outlet.
On the pressurised fresh water side, there is a small header tank which feeds the closed fresh water side. The original automotive engine water pump pushes the fresh water around the engine.
The boat works fine at low speeds (e.g. 5-6 knots or less), with the water gauge reading about 70-80 degrees and i can cruise up and down the river for hours with everything working fine. However, when taking the boat out to sea and pushing upto 7 knots or more, the temperature slowly rises and it will overheat unless action is taken. The previous owner tells me that he was able to cruise at 10 knots without over heating problems and i have no reason to disbelieve him. The top speed is about 12 knots according to him with this combination of engine and boat.
I am not loosing any fresh water from the cooling system as the header tank water level remain contant. I do not believe i have a head gasket problem. There is no excess pressure when i remove the oil refill cap and no milky oil which can often be seen if you are loosing water via the head gasket.
With the engine on idle, I have removed the sea water outlet pipe that exists the heat exchanger into the exhaust outlet and there is plenty of sea water coming out (it fills a large bucket in about 20 seconds). From this i believe the sea water side of the cooling system seems to be working fine. The only exception to this is that the heat exchanger could be partly blocked in the sea water part ? I have not taken the heat exchanger off yet to examine it as it is quite bulky at about 3 feet long and about 6 inches in diameter. The heat exchanger is mounted as low as possible inside the hull and is at the same hieght as the lowest pat of the engine. This suggests that any air blocks would be unlikely.
If i am not loosing water and if the engine has no obvious problem causing it to overheat, then this is either a problem with :-
1) The fresh water pump (perhaps not pushing enough water around).
2) A faulty thermostat.
3) A partly blocked heat exchanger.
4) A heat exhanger which is not rated highly enough for the job it needs to do.
There is no obvious noises from the fresh water pump.
I tried to find the thermostat, but it is NOT inside the engine thermostat housing where it should be ..... I did some research and when marinising automotive engines and installing heat exchangers , some heat exchangers have a thermostat built into them and the installations instructions can tell the installer to remove the original engine thermostat. I can't see whether the heat exhanger has a thermostat built into it - so i dont know if i am operating without a thermostat or not ...
In general I think it would be ok to operate without a thermostat except that the engine would run colder than expected - however this is clearly not the case here as it overheats !! In fact, if the thermostat was in place, i would have probably removed it as a test and run the engine at a higher speed. A faulty thermostat can cause over heating of course.
I will remove the heat exchanger to see if there is a thermostat inside of it and to see if there is any obvious blockage or build up.
Any help or assistance would be greatly appreciated. I have fixed 4 problems with this boat already that have interrupted the season (NOT related to overheating - 2 different steering problems, 2 electrical problems) and as a person new to boating (this is my first season), I am almost at that point of giving up (my wife was on board when the engine overheated out at sea.... you can imagine her disapproving reaction). It would be a real shame to quit now considering how enjoyable boating can be when the things actually work !
My thanks in advance,
Mark.
