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Old 21 July 2008, 06:00   #1
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Inboard diesel overheating

Hi All,

I am having some trouble with my Mercruiser Cummins D-Tronic 4.2l 250hp inboard (on a bravo 3). It runns perfectly fine, but if I put the throttle down the engine overheats within about 5 minutes. This does not happen at cruising speeds...

The mechanic has checked the impeller, which is fine apparently...

Any ideas?

Thanks,
Ferdi
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Old 21 July 2008, 06:29   #2
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There's a blockage somewhere else I'd suggest. I'm not sure whether those diesels are raw cooled or 'closed circuit'. If raw, i could be the thermostat.
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Old 21 July 2008, 09:18   #3
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I had something similar to this, the raw water intake from the transom shield to the leg had become disconnected.
Thus is picked up water when slow but when the hull lifted out of the water the pipe lifted out. If it had still been connected it would have been pulling water from lower down on the leg and hence have a good supply.
Lift her out and see if it has come off.
good luck
James
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Old 22 July 2008, 00:22   #4
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it sounds like a clogged exhaust mixing elbow to me . how old is the engine?
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Old 22 July 2008, 05:05   #5
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Hi guys - first and foremost thank you for your posts!

Mollers - its a closed circuit and the thermostat is working as far as I can tell because when I get it too deep into the "red zone" of the thermostat some of the coolant boils out into the engine compartment...

Jelly - The boat doesn't overhead when it comes on plane - I really need to put the throttle right down so the engine revs close to or above 3,000 rpm to get it to overheat. My boat planes at below 2,000 so I am not sure it it really could be the pipe. Nevertheless, to exclude all options, is there any way to tell without lifting the boat out of the water if this pipe is loos?

mickyspec - The engine is about 7 years old, just over 500 hrs. What is an exhaust mixing elbow? How can i check if this is clogged?

Cheers,
Ferdi
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Old 22 July 2008, 08:31   #6
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Heat exchangers can also become blocked ..............
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Originally Posted by Zippy
When a boat looks that good who needs tubes!!!
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Old 24 July 2008, 18:26   #7
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hi ferdi
the exhaust mixing elbow is where as the name implies raw cooling water is mixed with hot exhaust gases before exiting through the exhaust.the elbow where the mixing takes place is
typicaly made of cast iron,and i dont need to tell you what happens when cast iron and salt water come into contact with each other,when the cast iron corrodes it flakes away blocking the sea water "galleries", impairing coling water flow and thus overheating.
i suggest you google the subject where you will find out more. its a common issue.
hope this helps
mike
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