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Old 21-10-2021, 00:30   #1
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A very different reason for engine overheating

A few days ago I continued sea trials on my refurbished Mayflower 48 which has a Perkins 4236 engine.

The engine has 10 hours on it since the water pump">raw water pump was rebuilt, the hoses and clamps checked and the engine panel was replaced.

Early morning I started the engine, checked exhaust water flow at idle and high revs. Then shut it down to wait for the guests. While waiting I checked the oil. Was between max and min as expected as it takes time for the oil to drain to the pan after the engine runs.

I checked the water strainer, clean. The strainer is above the water line so I did not have to close the seacock.

Went sailing, was magnificent but rocky with swells 0.7 m.

Its open water outside the marine breakwater, the fetch is to Italy or Gibraltar depending on wind direction.

We returned to the breakwater, started the engine and dropped the sails. The engine thermostat keeps the engine at 82 deg C but it quickly reached 91.

Unfurled the genny and headed away from the breakwater while I started the checks, engine in idle but temp climbing slowly.

The exhaust is now below water most of the time due to the swells and without climbing onto the swim platform, its difficult to tell if water is coming out the exhaust.

I went below and saw that the rubber exhaust pipe was not overly hot, the plastic muffler was not burned out and I could see water flowing in the clear wire reinforced pipe feeding water into the elbow.
The fresh water reservoir was 3/4 full due to the water expansion. So no loss in cooling water. The engine room was hot so not a sensor problem.

My guess was reduced water flow or a heat exchanger clogging problem, neither of which are easy to solve on a rocky boat.

We sailed back to the marina, furled the genoa and kept it ready to unfurl in case of engine problems at the

We headed slowly back to the marina and into the slip, temperature reached 99 deg C. Then shutdown the engine.

When the engine room cooled down I started the engine and saw there was NO water flow from the exhaust at idle and when revving. Shut down the engine. So there is a water flow problem but not severe enough to burn out the plastic muffler.
I checked the water strainer, the cap was loose. The scupper pipe runs alongside the strainer and I mistook its friction on the cap for the cap being tight.

I had checked the water flow before checking the strainer but not after!

I secures the cap and started the engine. After a minute water out of the exhaust.

Conclusion, the seawater pump was sucking air and water resulting in enough water to cool the exhaust but not enough to effectively cool the engine.

The takeaways:

1. Check water flow from exhaust EVERY time you start the engine.

2. At what height should the strainer be mounted. Above the water line is convenient because no need to close the seacock and the strainer may act as a siphon break? My thinking is that it should be mounted so the lid is just above the water line and the body is below. In my case I can just see the water in the pipe just below the strainer. Time to relocate the strainer about 5" lower?

3. I am lucky that the powerful raw water pump managed to pull enough water.

4. It's time to add a loud audible temperature alarm. My new panel has this capability but its not yet connected.
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Old 21-10-2021, 00:35   #2
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Re: A very different reason for engine overheating

You've posted this in "Forum Tech Support and Site Help".
You may like to contact a moderator and ask for it to be moved to a more appropriate area of CF
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Old 21-10-2021, 01:12   #3
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Re: A very different reason for engine overheating

Excellent, detailed report, Jack.
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Old 21-10-2021, 03:25   #4
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Re: A very different reason for engine overheating

Thanks for sharing. Have you considered checking / replacing the impeller (which may have suffered from running without sufficient water flow)?
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Old 21-10-2021, 05:45   #5
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Re: A very different reason for engine overheating

1. Check intake to see if you sucked up debris or a bag. Is the tru hull reduced diameter/restricted from marine growth?

2. Check strainer to see if clogged and strainer seals are good. Either could allow too much air into strainer. IMO the strainer should be below the waterline to help assist/make it easier for the impeller/salt water pump to do it's job and not cause a possible area for air to get trapped reducing water flow.

3. Check mixing exhaust elbow for rust/restriction reducing water flow.

We have a 4.236 and is a good motor once you get the bugs out.
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Old 21-10-2021, 05:56   #6
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Re: A very different reason for engine overheating

Quote:
Originally Posted by Jamme View Post
Thanks for sharing. Have you considered checking / replacing the impeller (which may have suffered from running without sufficient water flow)?
Yes I have but considering there was enough water to keep the exhaust muffler and rubber pipe from burning out, it has taken a lower priority.

Also the access to the impeller is difficult as I have no hatches on the port side.

But great suggestion. Thanks.
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Old 21-10-2021, 06:03   #7
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Re: A very different reason for engine overheating

Quote:
Originally Posted by Bill O View Post
1. Check intake to see if you sucked up debris or a bag. Is the tru hull reduced diameter/restricted from marine growth?

2. Check strainer to see if clogged and strainer seals are good. Either could allow too much air into strainer. IMO the strainer should be below the waterline to help assist/make it easier for the impeller/salt water pump to do it's job and not cause a possible area for air to get trapped reducing water flow.

3. Check mixing exhaust elbow for rust/restriction reducing water flow.

We have a 4.236 and is a good motor once you get the bugs out.
Bill great suggestions. I changed the order based on ease of doing. So I checked the strainer first which was the problem. I agree that it should be dropped lower and also away from the scupper pipe so that it is easy to open.

Checking the thru hull for blockage involves diving under the boat or poking down from the top through a pipe that is long enough so that the water is below the end of the pipe. My preferred method as diving is problematic here. Enough said.
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Old 21-10-2021, 09:15   #8
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Re: A very different reason for engine overheating

I've been very happy with my Engine Watchdog temp alarms:
ENGINE WATCHDOG Audible Engine Overheating Alarm, Gauge & Sensor


I now have three, one reading temp on the block itself, one on the exhaust elbow, and one on the transmission.


What I like most about them is they are completely independent of anything else (not reading coolant which might be gone, not reading ejected raw water which may not be there etc) so they should hopefully alarm when temps rise too high regardless of why.


As far as the raw water circuit I prefer having everything under the waterline... yes it means I have to close the seacock to check the strainer but it means no worrying about priming etc.



Nice recovery on your experience though, back in port in one piece means all good.


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Old 21-10-2021, 09:43   #9
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Re: A very different reason for engine overheating

Quote:
Originally Posted by Jack C View Post
A few days ago I continued sea trials on my refurbished Mayflower 48 which has a Perkins 4236 engine.

The engine has 10 hours on it since the raw water pump was rebuilt, the hoses and clamps checked and the engine panel was replaced.

Early morning I started the engine, checked exhaust water flow at idle and high revs. Then shut it down to wait for the guests. While waiting I checked the oil. Was between max and min as expected as it takes time for the oil to drain to the pan after the engine runs.

I checked the water strainer, clean. The strainer is above the water line so I did not have to close the seacock.

Went sailing, was magnificent but rocky with swells 0.7 m.

Its open water outside the marine breakwater, the fetch is to Italy or Gibraltar depending on wind direction.

We returned to the breakwater, started the engine and dropped the sails. The engine thermostat keeps the engine at 82 deg C but it quickly reached 91.

Unfurled the genny and headed away from the breakwater while I started the checks, engine in idle but temp climbing slowly.

The exhaust is now below water most of the time due to the swells and without climbing onto the swim platform, its difficult to tell if water is coming out the exhaust.

I went below and saw that the rubber exhaust pipe was not overly hot, the plastic muffler was not burned out and I could see water flowing in the clear wire reinforced pipe feeding water into the elbow.
The fresh water reservoir was 3/4 full due to the water expansion. So no loss in cooling water. The engine room was hot so not a sensor problem.

My guess was reduced water flow or a heat exchanger clogging problem, neither of which are easy to solve on a rocky boat.

We sailed back to the marina, furled the genoa and kept it ready to unfurl in case of engine problems at the

We headed slowly back to the marina and into the slip, temperature reached 99 deg C. Then shutdown the engine.

When the engine room cooled down I started the engine and saw there was NO water flow from the exhaust at idle and when revving. Shut down the engine. So there is a water flow problem but not severe enough to burn out the plastic muffler.
I checked the water strainer, the cap was loose. The scupper pipe runs alongside the strainer and I mistook its friction on the cap for the cap being tight.

I had checked the water flow before checking the strainer but not after!

I secures the cap and started the engine. After a minute water out of the exhaust.

Conclusion, the seawater pump was sucking air and water resulting in enough water to cool the exhaust but not enough to effectively cool the engine.

The takeaways:

1. Check water flow from exhaust EVERY time you start the engine.

2. At what height should the strainer be mounted. Above the water line is convenient because no need to close the seacock and the strainer may act as a siphon break? My thinking is that it should be mounted so the lid is just above the water line and the body is below. In my case I can just see the water in the pipe just below the strainer. Time to relocate the strainer about 5" lower?

3. I am lucky that the powerful raw water pump managed to pull enough water.

4. It's time to add a loud audible temperature alarm. My new panel has this capability but its not yet connected.
Thanks for sharing this; we all have a lot to learn, particularly me. We've just bought a new (to us) boat in Mallorca and I'm very happy that it has a sea-chest engine intake (two engines) and two strainers on the side of the sea-chest that the tops of are about half an inch above water line. I can check them and not create a potential air block. Somebody that built this vessel knew what thay were doing!
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Old 21-10-2021, 09:44   #10
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Re: A very different reason for engine overheating

Also: siliconmarine.com exhaust temperature probe.
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Old 21-10-2021, 10:01   #11
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Re: A very different reason for engine overheating

If it's overheating because of not enough water flow the first thing to check would be your raw water intake filter, second would be the impeller and next would be to check the heat exchanger for clogged ports. It's not always easy getting to the impeller housing but youtube always has good videos on how to get to it the easiest way on your boat.
cheers, and cooler water
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Old 21-10-2021, 11:08   #12
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Re: A very different reason for engine overheating

The fresh water cooled Volvo MD11c on our boat suffered from intermittent over heating even with enough raw water flow. Finally figured out that idiot who designed the system ran the raw water into the TOP of the heat exchanger and out the bottom. There was no air bleed, so any air sucked into it, from a leaking gasket or heavy chop, stayed in it and caused an air lock. Just got some copper sweat fittings and reversed the flow to IN at the bottom and OUT at the top, so any trapped air just dumped into the exhaust. Problem solved.
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Old 21-10-2021, 11:26   #13
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Re: A very different reason for engine overheating

Jack, thanks for your post, very complete and well written. A potential problem I had not been aware of.

Regarding height of strainer, our engine's strainer is mounted so the cap is very close to waterline level and that seems to have worked out fine. We also have two other strainers, one for generator and one for reefer cooling, which are mounted 1 foot below waterline. Strainer inlet is at the top of the strainer, so to avoid air suction it might make sense to be a couple inches below waterline level. If cap was loose it would be easier to detect than a suction leak since water would constantly leak to the outside.
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Old 21-10-2021, 11:45   #14
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Re: A very different reason for engine overheating

Quote:
Originally Posted by Jack C View Post
. . . I checked the water strainer, clean. The strainer is above the water line so I did not have to close the seacock.
. . .

Quote:
Originally Posted by Bill Seal View Post
The fresh water cooled Volvo MD11c on our boat suffered from intermittent over heating even with enough raw water flow. Finally figured out that idiot who designed the system ran the raw water into the TOP of the heat exchanger and out the bottom. There was no air bleed, so any air sucked into it, from a leaking gasket or heavy chop, stayed in it and caused an air lock. Just got some copper sweat fittings and reversed the flow to IN at the bottom and OUT at the top, so any trapped air just dumped into the exhaust. Problem solved.

I was thinking you may have admitted air into the cooling system when you checked your strainer with the seacock open. Water level in the strainer will drop in this case.

Bill Seal's post reinforces this thought.

Perhaps you can see if there's air trapped in the heat X? You might be able to vent air from your pencil zinc port.
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Old 21-10-2021, 12:19   #15
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Re: A very different reason for engine overheating

Jack posted a very comprehensive account and finding of the cause. It was an FYI.
Why am I reading what he should check.
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