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Old 20-12-2015, 10:26   #16
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Re: Idle first or immediate shutdown if temp alarm goes off?

Quickly check which light is on, and then shut down.

I have had three alarm situations involving the cooling system.

1) was probably a fouled raw water thru-hull. Let the engine cool down, restarted, all was fine.

2) clogged sea strainer.

3) split hoses on both side of raw water impeller. That one still makes no sense to me.
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Old 20-12-2015, 10:33   #17
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Re: Idle first or immediate shutdown if temp alarm goes off?

Sometimes, its running the engine at too high an rpm that overheats the engine in which case, slowing down will cool things down. However, caution dictates turn engine off, let it cool down on its own, then restart to see what might be the problem. Sometimes just an impeller blade stuck in the cooling system.
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Old 20-12-2015, 12:12   #18
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Re: Idle first or immed shutdown if temp alarm goes off?

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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.
Thats exactly what happened to the previous owner of my last boat. Thats why my boat had a whole new exhaust when I bought it. It happens.
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Old 20-12-2015, 12:17   #19
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Re: Idle first or immediate shutdown if temp alarm goes off?

If you have a Vetus waterlock muffler, shut it down immediately. Those are terrific pieces of equipment- but made of plastic- will melt!
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Old 20-12-2015, 12:35   #20
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Re: Idle first or immediate shutdown if temp alarm goes off?

[QUOTE=rgesner;1992456]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 one should have been watching the gauges before hand. If the temperature has been creeping and rising, checks should have been done during this time. Every time my engines do heat up or overheat... there was a warning before hand.

If you were not watching and the alarm does come on, you do have a minute or two to do quick checks. One, the gauges, and if the alarm is still on, observe the gauge for change and reduce engine rpm;s. If the gauge is at the lower end of overheat, you have some time. If the gauge is at the high end shut it down immediately. Assess your situation as far as navigation. You would almost never do damage to a diesel engine but prematurely shutting it down.... Roger
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Old 20-12-2015, 12:42   #21
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Re: Idle first or immediate shutdown if temp alarm goes off?

Of course if you are in a congested channel with 3+ knots of current with a bridge down current 40 yards off, then don't shut down. Overheating will most likely not damage your engine for a short period of time, only minutes to be safe, which should be enough to get to a safe spot to get an anchor out. Then hopefully you have one of those Non-Contact Infrared Thermometers with Laser Targeting which you can point at the block or better, the thermostat housing to verify if you are actually overheating, of course make a quick visual check of your intake strainer for any obvious blockage. Then verify water in your heat exchanger and hoses at engine. Oil pressure is the time to shut down immediately because you will not go any further under power anyways and will invariably destroy your motor if you try. Have a bow anchor ready at all times inform all vessels close by that you have lost power etc..
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Old 20-12-2015, 13:03   #22
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Re: Idle first or immediate shutdown if temp alarm goes off?

If the alarm just went off and the gauge confirms I would instantly reduce RPM's while looking to anchor and watching the temp. If the temp drops as will frequently happen you can run slower and watch things. If the temp does not drop I would worry about a blocked sea water intake and shut down as soon as possible. I just installed an exhaust temp gauge which give the earliest warning of this situation. A stray plastic bag can more then ruin your day.
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Old 20-12-2015, 13:10   #23
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Re: Idle first or immediate shutdown if temp alarm goes off?

I'm thinking...If water still coming out the back idle it. If not, shut it down.
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Old 20-12-2015, 13:35   #24
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Re: Idle first or immediate shutdown if temp alarm goes off?

Small raw water cooled engines are engineered to run with exhaust temperatures less than about 130F (and commensurate alarm settings) because long term operation above that temperature can result in plating of salt water deposits on heat exchange surfaces. Such deposits will impede heart transfer and over the long term result in loss of cooling, requiring chemical or mechanical descaling.

Short term operation of a raw water cooled engine above the alarm at 130F will not cause damage, but temperatures in a loaded engine will rapidly increase if the high temperature alarm has been caused by loss of or a severe reduction in cooling water flow. With such a reduction in cooling flow, the engine should be immediately shut down if possible, and the problem corrected.

If an engine has been operated for a prolonged period due to imminent navigation needs with a high temperature alarm sounding, the engine's oil should be drained and the old oil and plug checked for metal in case bearing damage has occurred.
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Old 21-12-2015, 03:04   #25
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Re: Idle first or immed shutdown if temp alarm goes off?

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Originally Posted by rgesner View Post
I have searched the manual and find nothing on this.
Hmm… maybe you have a different revision of the Yanmar manual than me!

I had a look at my GM series manual to day and found two references to your question.

First was in section 3.3.4 (Cautions During Operation), para 4 (Alarm buzzer sounds during operation) which reads " If the alarm buzzer sounds during operation, lower the engine speed immediately, check the warning lamps, and stop the engine for repairs".

The second was in section 5, (Trouble and Troubeshooting), "if the alarm sounds during operation, NOTICE, shift to low speed operation immediately, and check which lamp has come on. Stop the engine for inspection. If no abnormality is identified and there is no problem with operation, return to port at your lowest speed and request repairs".

Pretty straightforward .
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Old 21-12-2015, 10:01   #26
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Re: Idle first or immediate shutdown if temp alarm goes off?

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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?
If the engine is being operated normally, that usually never causes an overheat alarm, and suddenly this happens, the most likely culprit is a sudden fault on the cooling system.

When one has a faulty cooling system, there is not likely any chance that idling will help cool down the engine.

When one has a faulty cooling system, running the engine any longer may very well be doing damage to the engine.

Therefore, if under normal operating conditions, your overheat alarm goes off (or gauge goes into the red) the wisest move is to immediately shut down, when safe to do so.

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