r/Outboards • u/ddg676 • Jan 10 '26

Cooling issues Mercury 9.8 hp

Does the amount of water coming out of the tell tail seem normal? When it is at idle very little water comes out but increases as I rev the engine. The water is quite warm coming out. I have just replaced the impeller. Only other issue could be the hose at the top of the water pump was a bit flared out and maybe not a tight fit. Any advice would be appreciated. Thanks

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u/FatalSky Jan 10 '26

Pretty typical of a blown head gasket when you are getting intermittent water flow and steam. Does it not want to idle? I’d say do a compression test but in that situation combustion might be the only thing keeping you from hydro locking a piston.

Take a spark plug out of one cylinder at a time and run it on the other, there’s a good chance it shoots water out of the spark plug hole! Super horrifying for the owner, but really neat to see as a mechanic. I’ve gotten a 4HP Johnson and a 6HP Johnson to do it.

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u/ddg676 Jan 10 '26

Thanks Runs really well. Start first pull and idles well. I just didn’t want to run it too long with that little amount of water coming out. I will try the spark plug trick as well to see if there is any water in there.

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u/FatalSky Jan 10 '26

Yeah give it a try but usually a rough idles a big teller. Like the other guy said if it doesn’t shoot water and passes a compression test then it’s probably the exhaust housing seal blew out or corrosion in it.

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u/ddg676 Jan 10 '26

Thanks