Mariner 40hp pumps water then stops.

[Bbturbo2]

Hi,
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.

[ben tye]

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

[9D280]

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?

[Nos4r2]

Does it choke up at the same time?

[free]

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.

free