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Old 18-06-2009, 09:30   #1
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Urgent Diesel Engine Question

What would make a marine diesel engine, fresh water cooled, naturally aspirated, suddenly overheat when driven over 2,200 RPM?

At 2,200 and below it behaves perfectly normally. Cleaning out the heat exchanger didn't help.

Some ideas which you can laugh at if appropriate:

1. Sensor reading incorrectly? (But how can that be, if it reads normally at lower engine speeds -- why would it suddenly shoot up?)

2. Raw water flow impeded somehow? Impellers have been changed recently; strainer has been checked.

3. Heat exchanger shot? Engine is not that old, less than 3,000 hours, heat exchanger was recently cleaned (but I didn't see it done, so the job could have been botched).

4. Exhaust back pressure?

I was a pretty good gasoline engine mechanic at some point in my youth, but I am pretty ignorant about diesels (something I will be trying to change).

Your comments will be greatly appreciated.
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Old 18-06-2009, 10:15   #2
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Possibly the exhaust elbow is restricted. Are you sure it is propped right? Your engine should be able to reach within about 10% of it's maximum rated rpm. Sensors can be problematic. Maybe it always reads a little high, but at 2200 reached the alarm point...
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Old 18-06-2009, 10:15   #3
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I would guess you can see water coming out of the exhaust when running showing there IS really water running through the heat exchanger. The (rad)cap on the overflow may be bad and not allowing correct pressure (16lbs). The fluid may not be correct ratio 1/1 water and coolant is normal. How is the bottom perhaps it needs cleaning. I guess this is the usual rpm and you are making the usual speed? A water pump belt is slipping, a snail/crab/fish/baggie has sucked up out of sight into the strainer, a chunk of scale in the engine has dropped off and is blocking a water gallery or a thermostat is not opening properly. My bet is a partially blocked thermostat. I just rebuilt my 31yo Yanmar and I am, once again, a very happy sailor.Good luck............m
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Old 18-06-2009, 10:18   #4
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Is there any chance your cooling intake hose is not rigid enough? My old Volvo used to collapse the intake hose a higher revs. Took a while to figure it out.
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Old 18-06-2009, 10:21   #5
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Your thermostat may need replacing.

Check the flow of your raw water inlet by pulling the hose and letting it run into the bilge. How is the flow? Don't scuttle the boat of course.

Disconnect the raw water outlet that goes in to the riser with the engine running. How is the flow? You can get away with this for maybe 10 seconds or so....no more.

Your riser may be badly corroded from the inside causing a flow restriction.
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Old 18-06-2009, 10:26   #6
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Quote:
Originally Posted by Cheechako View Post
Possibly the exhaust elbow is restricted. Are you sure it is propped right? Your engine should be able to reach within about 10% of it's maximum rated rpm. Sensors can be problematic. Maybe it always reads a little high, but at 2200 reached the alarm point...
Thanks, the boat is propped right and will reach max rpm. The temp gauge reads normally up to 2,200 then shoots up beyond that (why the hell?).
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Old 18-06-2009, 10:27   #7
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Originally Posted by Rex Delay View Post
Is there any chance your cooling intake hose is not rigid enough? My old Volvo used to collapse the intake hose a higher revs. Took a while to figure it out.
That's a really interesting idea. I'll check that out; thanks a million.
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Old 18-06-2009, 10:29   #8
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Originally Posted by cantxsailor View Post
I would guess you can see water coming out of the exhaust when running showing there IS really water running through the heat exchanger. The (rad)cap on the overflow may be bad and not allowing correct pressure (16lbs). The fluid may not be correct ratio 1/1 water and coolant is normal. How is the bottom perhaps it needs cleaning. I guess this is the usual rpm and you are making the usual speed? A water pump belt is slipping, a snail/crab/fish/baggie has sucked up out of sight into the strainer, a chunk of scale in the engine has dropped off and is blocking a water gallery or a thermostat is not opening properly. My bet is a partially blocked thermostat. I just rebuilt my 31yo Yanmar and I am, once again, a very happy sailor.Good luck............m
Thanks very much; I'll check all that out. I guess a slipping water pump belt would explain the problem -- up to a certain force it holds, then gives way. It would screech, though, wouldn't it?

I'll check the strainer and theromstat again.

Cheers.
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Old 18-06-2009, 10:30   #9
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Originally Posted by David M View Post
Your thermostat may need replacing.

Check the flow of your raw water inlet by pulling the hose and letting it run into the bilge. How is the flow? Don't scuttle the boat of course.

Disconnect the raw water outlet that goes in to the riser with the engine running. How is the flow? You can get away with this for maybe 10 seconds or so....no more.
Thanks -- I'll check that. Thanks, everyone -- keep em coming!
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Old 18-06-2009, 10:36   #10
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Can't help with the immediate problem, but I can suggest some reading on diesels for ya...

Amazon.com: Marine Diesel Engines: Maintenance, Troubleshooting, and Repair: Nigel Calder: Books
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Old 18-06-2009, 11:03   #11
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Agree with Cheechako, it sounds like a mixing elbow problem. I would check this out first.
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Old 18-06-2009, 11:16   #12
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Agree with Cheechako, it sounds like a mixing elbow problem. I would check this out first.
Mixing elbow? Is that the same as the exhaust elbow? And that would account for sudden heating over a certain RPM, after behaving normally?

How do I check it?

Thanks a million.
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Old 18-06-2009, 11:18   #13
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Check the prop for barnacles. Too much fouling will overload the engine.
Actually any fouling will impact your fuel consumption and speed, but it can also cause overheating when it reaches a certain point.

Steve B.
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Old 18-06-2009, 11:37   #14
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good point on the prop/barnacles. IF your exhaust elbow is clogged up with rust or carbon, the exhaust is restricted. This effects the engine more when it is trying to push more exhaust out. It is amazing how exhaust size does effect diesel performance. When manufacturers build production boats and select an engine supplier, the engine supplier will come out to test the first engine installation and will monitor exhaust temperature, pressures etc. If they are not right, the engine manufacturer will require a different exhaust size prior to warranteeing any engines..... This is more common with power boats, but my Kubota wholesale supplier even requested it when I built my first 12V 6 HP generator years ago!
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Old 18-06-2009, 11:40   #15
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Have you had the boat long.... is this a new symptom? I built a 31 footer years ago with a brand new volvo 13 hp engine that did the same thing. Never did figure it out. In the end I figured the engine was just not big enough for the boat... (read too many traditionalist "blue water" books I guess!)
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