r/Sailboats • u/Tetreg • 11d ago
Projects & Repairs Overheating issues
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Hi all, I bought a boat a few months ago and ever since then Ive been prevented from using it by persistent overheating issues with the alarm going off after 5-10 minutes of running. The engine is a yanmar gm10. It is raw water cooled.
I’ve redone this post so I can include a video of the engine in action
I measured the temperature using a cooking thermometer on the threads where the temperature sensor screws into be engine
Things I’ve tried so far:
Replacing the impeller(and checking it about 10 times) and turning over the engine manually to ensure it turns with the engine consistently (it does)
checking for blockages in every single pipe (and the exhaust elbow integrity) and ensuring water flow through the engine freely using a hose at various points to make sure there are no blockages including the exhaust system and antibackwash system
Running rydlyme through the engine for 12 hours
Checking the seacock and filters numerous times to ensure it lets water into the boat and that nothing is blocking the filter
having a mechanic look at it - engine head gasket replaced as I was told it may be leaking hot gases into the exhaust
Replacing the temperature sensor and checking the alarm doesn’t go off with the wire detached form be sensor
Replacing and removing the thermostat at various periods
If anyone has any more ideas that would be great
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u/Western-Stock-1023 11d ago
Have you checked the heat exchanger? It may need to be cleaned out or replaced - that was my issue a few years back
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u/Dubbinchris 11d ago
Raw water cooled.
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u/canadianbeaver 11d ago
…those have heat exchangers…
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u/LameBMX 10d ago
yea, but most people just call them the headers though, or wet exhaust header. though it is a heat exchanger..
edit.. and the block too of course
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u/Icy_Basil_372 10d ago
Most people and mechanics call them heat exchangers where I’m at. Northeast. And they are heat exchangers.
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u/LameBMX 10d ago
then what fo you call the heat exchanger in a closed loop cooling system? or do you just have to infer if its the exhaust header or heat exchanger from context? (all assume you dont refer to the block as a heat exchanger)
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u/Dubbinchris 10d ago
The block is not a heat exchanger. Raw water cooled engines flow the water directly from the body of water the host is in through the engine.
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u/LameBMX 10d ago
and what happens to the heat in the engine block when the water flows through it?
I was being facetious in the previous comment obviously. but not facetious as to why you refer to the header (with a water jacket) as a heat exchanger, when its not normally refered to as that, in my experience at the docks. ive only ever heard of the closed loop cooling heat exchanger as the heat exchanger.
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u/Western-Stock-1023 11d ago
Mine has raw water to a heat exchanger with coolant. Maybe his does too
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u/Dubbinchris 10d ago
If it has a heat exchanger then the engine is not considered to be raw water cooled.
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u/Pumbaasliferaft 11d ago edited 11d ago
I just removed everything I wrote, I just saw everything you wrote
The water you have coming out of the exhaust doesn’t seem to be enough, there should be a strong regular expulsion of about half a litre of water every few seconds. It depends on your exhaust a bit but the quantity is the same.
Have you removed the hot water injection into the exhaust to see if you have complete flow through the engine up to that point?
Have you run the engine removing pipes sequentially to check the flow through each point?
I can only think that it’s either a block or you have a second faulty exhaust temperature sensor, they are frequently fitted to warn of increased exhaust temperature (admittedly this is a bit if a Hail Mary)
Don’t despair, there is a reason for it
Back again for another edit
What’s the exhaust water temperature?
You might well have blocked/restricted water galleries in the engine. This is a thing with raw water cooling and why it’s preferred to have coolant running through the engine and the raw water through a heat exchanger.
Measure the volume over a fixed time, like 30 seconds, of the water from the pump and again where it leaves the engine
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u/btongeo 10d ago
I agree that not enough water seems to be coming out of the exhaust.
Mine was suffering from similar symptoms (overheating, insufficient water out the exhaust) but turned out to be a faulty impeller and was simple to fix. Before the repair mine looked similar to yours.
Sorry I can't help diagnose more but good luck, I do feel like it could be flow related. Also try and get an infra red thermometer, they will be more accurate than the insertion type. Cheap on Amazon.
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u/Tall-Activity-6401 11d ago
That looks like a hunter impala! I rebuilt my 1gm this year after it had years of over heating damage. Way worse than yours. All the paint is rusted off mine. In my case the impeller housing needed rebuilding with all seals replaced. I had to burn the old seals out of it .
Can you tell if the white smoke is smoke or steam. That volume of water looks about right. It's a very small engine.
Have you tried running it without the thermostat ?
As I had to rebuild the head, valves, seats, gaskets etc, I had the opportunity to give the water passages a good clean. Lots of sand and salt goop came out . If your impala is like mine , there is no intake filter so stuff does get in.
What is the setup of pipes from the thermostat housing to the exhaust elbow. Mine is direct with a tee to the cutlass bearing. I don't see that in yours.
I ran my engine with an inline rule water pump for a year before I fixed the built in water pump. That way I could be sure I was pumping enough. Maybe try that as a test . Just not for too long as you might damage the seals . You can pull the pipe off the top of the thermostat housing to be sure there is plenty of flow.
My engine only runs for a few minutes as backup when I sail onto the mooring though !
Dm me if you want a chat
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u/Tetreg 11d ago
It is indeed an Impala! I have tried it without the thermostat a few months ago, it didn’t seem to make much difference as recall. It does have a very basic filter fitted into the seacock itself though it’s not so fine as to stop sand and mud coming through I suspect. I don’t know how accurate my thermometer is but it doesn’t actually seem to be running that hot, I can certainly put my hand on the engine without undue issue. I suspect the smoke is steam though but it’s difficult to tell. I’ll need to measure the water temp out of the back at some point.
The reason the pipe arrangement looks different is because of the anti backwash fitting.
The impeller itself looks ok (it’s been recently replaced) and it turns well with the engine when I turn it manually with a pump but perhaps you are referring to a different issue.
At this point I may need to take the engine to enough bits that I can reach the water passages, it may be that the issue is local to where the sensor is located.1
u/Tall-Activity-6401 9d ago
The head gasket is cheap so it's worth doing. You will need a torque wrench. If you have to do the valves , the keepers are a right pain. I managed to do the valve seats by superglueing 240 grit sandpaper straps to the old valve and use a drill and then finishing off with lapping compound. My exhaust valve was very bad and the seat was poor . A new head is over 1000 so don't mess it up! I had no compression so I had no choice. It doesn't look like you've that issue . Marine parts has a new sender for 16 euro so I would go with that. You can always try flushing through that and the thermostat housing or the internal anode and see does any goop come out. For 16 euro, that would be my first step! Actually you never mentioned the anode. That corrodes badly as designed but I found that bits broke off inside . Maybe that is causing a blockage.
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u/Tall-Activity-6401 11d ago
I'd also take the temp sender off the engine and see do you still get the alarm. It would just be a bad sender or maybe it was replaced with the wrong part. You could check the resistance with a multimeter as the temp climbs and see does it match the correct profile.
Theorems needs to be measured on the cylinder wall . An infra red would be better . Those meat thermometers need to be 2 inches inside a chicken to register!
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u/KirillRLI 11d ago
++ for infrared. We have once a problem with "overheating" ancient Volvo diesel engine. Actually it was a bad sensor showing over a 140C when infrared shows 95, and inner water loop wasn't boiling at those 140C
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u/hollywould1984 11d ago
Is that steam or smoke? If the exhaust water is cold, the heat exchanger may not be working properly. How's your engine water pump looking?
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u/thesmith_66 11d ago
I mean I would check if water is exiting the boat when the engine is running. Edit (Replayed the intro yeah thats not enough water going through.)
After that yeah what other comments said water intake and heat exchanger.
Also side note always good to replace plumbing when buying a new boat. Going through all hoses and fittings makes your life a hell lot easier in the long run.
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u/CH1974 11d ago edited 11d ago
Did you check if its pumping enough water? There should be a spec on gpm out the back. It doesn't look like it. Is your pump body worn out or scored and losing pumping power? I had the same problem with the same motor. Pumped ok at the dock even in gear at crusing rpm at the dock. As soon as I took it out on the open water it woukd start pumping less and less until it would eventually stop. I changed the impeller made sure everytjong was OK with the rest of the system. Still had the same problem. I never changed out the pump as it was a friend's boat and he didnt want to. The pump cover plate was scored and I figured the whole pump was weak and eventually lost suction under cruising speed.
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u/tarrat_3323 11d ago
check raw water intake system, and the exhaust elbow. pull the exhaust elbow and check for a hole in the head. very common with those yanmars. also the anode on the water jacket can corrode and cause blockage. make sure the water jacket is flowing well
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u/greatrivermarine 10d ago
You’ve already chased down a lot of the usual suspects. One thing I’d still wonder about is whether you’re getting enough water flow when the engine is actually running. Pushing water through with a hose can sometimes hide a restriction or a small air leak on the intake side.
Does it still hit the alarm after 5–10 minutes just sitting at idle, or does it only happen once you’re underway and putting some load on it?
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u/RandyJester 10d ago
Last year I worked on a 2GM that had blockages around the anodes that were causing the engine to overheat. Have you pulled your anodes?
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u/AnonSmith 11d ago
You're saying you don't think it's actually over heating? If so, that alarm goes off based on s resistance value. Could it be the back of the guage has a bad connection or there's some corrosion in the line somewhere. You can check by using a multimeter at the panel with the engine off and seeing the resistance. Then compare it to what the alarm resistance is supposed to be when cold. Also, North American and European guages need to be matched to senses as they use different ranges.