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Old 19-12-2015, 22:33   #1
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Idle first or immediate shutdown if temp alarm goes off?

If a marine diesel (such as a 3GM30F) appears to be overheating (temp alarm goes off) should one reduce to idle to enable it to cool down before shutting down, or shut down immediately.

On one hand, it would seem prudent to idle for a few minutes before shutting down in an overheat situation, given that the Yanmar manual warns that one should idle for 5 min for normal shutdown to avoid increase in temperature of various parts which may result in engine trouble.

On the other hand, is not immediately shutting down more likely to cause damage if the overheating is due to cooling water obstruction or pump/impeller failure, which may be hard to immediately ascertain particularly on watch alone at night when it may be hard to quickly see if the exhaust is still spitting water?

What is the safest course of action?
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Old 19-12-2015, 23:32   #2
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Re: Idle first or immed shutdown if temp alarm goes off?

I don't have the expertise to answer your good question, but I bet it brings forth a LOT of diverse opinions, all of which are correct...

I'll be watching with interest!

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Old 19-12-2015, 23:41   #3
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Re: Idle first or immed shutdown if temp alarm goes off?

You may not see the water out of the exhaust but you can hear it gurgling in the water break, I would idle it back a bit, listen for the water or see it over the side then decide to shut her down. Look also for materials in the water, IE seaweed or as in Australia, jelly fish. In that event I would shut it down. In any event I would idle down to determine the best course of action.
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Old 20-12-2015, 00:22   #4
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Re: Idle first or immed shutdown if temp alarm goes off?

I used to shut mine down as soon as possible after the alarm went off then let it cool right down before starting it again to find the problem. Part of the reason was that I did not want to cook the short section of hose between the exhaust manifold and the waterfall device or the waterfall device itself which was plastic. Of course if you are about to run on the rocks it might be a better choice to wreck the exhaust system than the whole boat.
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Old 20-12-2015, 00:26   #5
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Re: Idle first or immed shutdown if temp alarm goes off?

I'd give the gauges a quick scan first before doing anything.If oil pressure is OK and if tachometer needle is steady (i.e. not bouncing in an abnormal way) and the motor "sounded" like it normally does then I'd idle down. However, if any of those 3 things or anything else seems abnormal or out of the ordinary (has the bilge pump been running non-stop for the last 10 minutes?)then I'd do an immediate shut down without a seconds thought. However, as mentioned already if you take the working load off an engine often they will cool down by themselves if all they have is a partial restriction.
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Old 20-12-2015, 00:35   #6
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Re: Idle first or immed shutdown if temp alarm goes off?

I would shut her down on the instant...

That said it is quite amazing how long you can run an overheated engine for.... don't ask me how I know this....
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Old 20-12-2015, 00:43   #7
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Re: Idle first or immed shutdown if temp alarm goes off?

Quote:
Originally Posted by rgesner View Post
If a marine diesel (such as a 3GM30F) appears to be overheating (temp alarm goes off) should one reduce to idle to enable it to cool down before shutting down, or shut down immediately.

On one hand, it would seem prudent to idle for a few minutes before shutting down in an overheat situation, given that the Yanmar manual warns that one should idle for 5 min for normal shutdown to avoid increase in temperature of various parts which may result in engine trouble.

On the other hand, is not immediately shutting down more likely to cause damage if the overheating is due to cooling water obstruction or pump/impeller failure, which may be hard to immediately ascertain particularly on watch alone at night when it may be hard to quickly see if the exhaust is still spitting water?

What is the safest course of action?
Hmm… you could be guided by Yanmar; I haven't got my operation manual handy but I'm pretty sure it tells you to shut the engine down immediately if either the overheat or low oil pressure alarm sounds.

Their advice to idle for 5 mins really only applies to a normally operating engine, not to an abnormally operating one.

To my way of thinking, something serious has happened already by the time the overheat alarm sounds and to idle back rather than stopping seems only to be making matters worse.
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Old 20-12-2015, 01:18   #8
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Re: Idle first or immed shutdown if temp alarm goes off?

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Hmm… you could be guided by Yanmar; I haven't got my operation manual handy but I'm pretty sure it tells you to shut the engine down immediately if either the overheat or low oil pressure alarm sounds.<snip>
I have searched the manual and find nothing on this.
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Old 20-12-2015, 01:21   #9
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Re: Idle first or immed shutdown if temp alarm goes off?

When the overheat alarm goes off your engine is above normal safe temperature, but not yet at a temperature that is causing damage. Immediately slowing the engine will give you a few moments to view your gauge and check the exhaust water flow. Slowing also lowers the heat made in the cylinders. The temp may go down with lower rpms. If the temp goes down you can start your problem solving. If not shut it down.
Older auto engines often overheated when climbing mountain passes and pulling heavy loads. Drivers let them cool and then went on. Rarely did the overheating lead to permanent damage. People as old as I am will remember the 2 lane highways and cars and trucks alongside the road cooling their engines on hot days.
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Old 20-12-2015, 02:18   #10
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Re: Idle first or immed shutdown if temp alarm goes off?

Quote:
People as old as I am will remember the 2 lane highways and cars and trucks alongside the road cooling their engines on hot days.
Hell, Lepke, I remember Conestoga wagons resting the oxen and looking out for redskins!

Well, kinda...

Jim
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Old 20-12-2015, 03:17   #11
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Re: Idle first or immed shutdown if temp alarm goes off?

I would shut it down so I could check the oil and see if anything is obviously amiss, such as debris in the water strainer. It's amazing how little it takes to affect the cooling. It's also amazing how impossible it can be to see the jellyfish that's blocking your strainer. Don't just look, turn it upside down and dump it.
Low oil can contribute to high temps, and low rpms will not help low oil pressure. Not every body has gauges.
Then I would start it and let it idle and check the exhaust water and the engine again.
If the water output seems low I would check the impeller as soon as convenient. If it overheats again convenient is now lol.
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Old 20-12-2015, 05:01   #12
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Re: Idle first or immed shutdown if temp alarm goes off?

It depends on whether the cooling system is working at all, or not.

If the cause is restricted flow of raw water (baggie in the raw water strainer?), for example, better to let it idle and cool off. Much better.

If there is total failure of water flow (disintegrated impeller, or on the fresh water side -- say you had a coolant leak and it all ran out), then shut it down IMMEDIATELY.

And how do you tell? Probably you can't tell instantly, so better shut it down.
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Old 20-12-2015, 05:10   #13
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Re: Idle first or immed shutdown if temp alarm goes off?

Shut it down!!! You are only adding more heat. Many people think that they are getting away with something, BUT there is accumulative damage that may not show up immediately. The only exception would be because the overheat was caused by too much load too long
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Old 20-12-2015, 09:48   #14
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Re: Idle first or immediate shutdown if temp alarm goes off?

Shut it off immediately if there's no cooling water idling won't help
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Old 20-12-2015, 10:11   #15
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Re: Idle first or immediate shutdown if temp alarm goes off?

And be very careful when looking for debris in your water strainer---- don't stick bare fingers in there! Put on a pair of vinyl gloves first- you never know what you might have caught in your strainer.
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