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Old 02 August 2011, 07:46   #1
Country: UK - England
Length: 4m +
Join Date: Aug 2011
Posts: 1
Mariner 40hp pumps water then stops.

I have a 40hp mariner, the yamaha based one. The engine starts and runs fine but after 3-4 mins the tale tale stops and engine gets hot!
The impeller is fine and pumps plently of water while the engine is cold. The engine seems to stop pumping water when it gets hot. I'm am confused.

Any ideas? Many thanks.

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Old 02 August 2011, 08:16   #2
Country: UK - England
Town: worcester
Length: no boat
Join Date: Aug 2008
Posts: 40
Could the thermostat be faulty

Could the thermostat be faulty, think these are simple and cheap to replace

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Old 02 August 2011, 08:20   #3
Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,614
More likely the impeller - when cold, the water is "bypassed" to allow the block to remain (theoretically) at a constant temp. When the 'stat opens, suddenly there is a lot more volume needing filled with water, and if the pump is half gubbed......

Any idea when it was last changed?
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Old 02 August 2011, 15:25   #4
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Country: UK - England
Town: The wilds of Wiltshire
Boat name: WhiteNoise/Dominator
Make: Ballistic 7.8/SR5.4
Length: 7m +
Engine: Opti 225/Yam 85
MMSI: 235090687/235055163
Join Date: Jul 2005
Posts: 12,689
Does it choke up at the same time?
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Old 02 August 2011, 16:22   #5
Country: UK - Scotland
Boat name: No name yet
Make: Avon
Length: 4m +
Engine: Honda BF30
Join Date: Jun 2010
Posts: 153
Had a Yam 2.5hp that did exactly the same. When cold it would pump away fine, but as soon as it got hot, the flow would stop. Had the thermostat out, tested and back in 3 times, even stuck a new one in eventually to no avail. After a lot of head scratching came up with the theory that exhaust pressure in the leg was forcing the water back down through the head via the rubber bypass hose on top of the thermostat housing. Proved it by crimping the bypass hose, replaced the impellar (which looked perfect,) problem solved. Obviously it was able to create enough pressure to force the water around when cold, but as soon as the thermostat opened and there was a little exhaust pressure from the other side, flow stopped.

Not saying that yours has the same system, but certainly sounds like the same symptoms.

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