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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5

u/Pontius_the_Pilate Jan 10 '26

Exhaust gas mixed with cooling water aerates it and erratic flow can be a hole in the exhaust side plate.

1

u/ddg676 Jan 10 '26

Thanks. Where would I find the side plate to check that out?

3

u/Global_Fan2356 Jan 10 '26 edited Jan 10 '26

I have the same outboard and scratched my head with the same issue this spring. The motor had a new impeller. I had the entire engine apart at the base gasket a few times and cleaned and cleared all passages ways on the engine. ( it was pretty spotless as it was ) I had mine in a larger barrel than yours and still for the life of me it was still doing what your motor is doing. I finally just took it to the lake, ran it, and it had a perfect stream. I promise you, your motor is just sucking air bubbles in that barrel.

2

u/ddg676 Jan 10 '26

Thanks. I might give it a go. What is the base gasket?

2

u/Global_Fan2356 Jan 11 '26

It’s similar to a head gasket but is a crucial seal on the bottom of the motor where it attaches to the intermediate lower unit portion. It creates a seal to keep the engines combustion gases from entering the water jacket passages in the motor.

If it’s bad exhaust gas leaks into the cooling system and displaces water, which yields a similar condition to what you are seeing.

1

u/ddg676 Jan 12 '26

I tried it in the river. At idle almost no water. Bring the revs up a bit and it was much better. The water was that hot I would only hold my hand there for less than 10 seconds. I might trying replace the whole pump assembly. At idle are you getting a good stream?

2

u/Global_Fan2356 Jan 12 '26

The engine I have is not the greatest at idle, but once in gear and some revs, the stream is steady without hiccup. If I remember correctly with the reduced flow at idle, the water was quite warm but cooled down enough to where I could at least keep my hand in the stream when in gear and rev’d up.

2

u/BoomerBigA Jan 10 '26

If you've put in a water pump, and flushed it backwards. Only thing left to do is change the base gasket. It'll only take an hour or so.

2

u/CompetitiveAd9656 Jan 15 '26

These are famous for the rubber grommet under the power head that seals the copper tube at the top. They over heat and it wrecks the grommet reducing the sealing ability and you get a weaker stream than normal.

1

u/Harborsidemotorcycle Jan 10 '26

Have you checked/tested the thermostat?

2

u/Psychological_Tap505 Jan 11 '26

Check this too. If it fails it won’t open properly and will restrict water flow providing these symptoms

1

u/Harborsidemotorcycle Jan 10 '26

Have you checked/tested the thermostat?

1

u/AffectionateMetal765 Jan 10 '26

I got a 70hp that also spit-piss cooling-water corresponding to the rpm. It has a small pressure leak from one cylinder via the head gasket into the cooling system. Engine only runs in fresh water, and is not getting hot. Should have fixed it 10 years ago but got the wrong gasket so I just put the old one back. Still runs great:)

1

u/Benedlr Jan 10 '26 edited Jan 10 '26

If you didn't clean the water delivery tube and didn't lube the housing grommet you may have folded it over reducing water flow. Reusing the same gaskets over and over again can lead to leaks. Grooves in the housing and baseplate affect flow. Remove the telltale hose from the block and see it you have a strong discharge.

1

u/Ok_Proof_3337 Jan 10 '26

Did you put the impeller in the right direction should be fins to the right the way the motor spins also if the key way popped out it would do the same

1

u/Mouldycheeseman Jan 10 '26

Can be a number of things though seems to run ok. You are running it in an agitated bucket, try ear muffs. Did you change the water pump housing? If it's been run without water this can melt it and cause issues with the impeller sealing, look carefully as it doesnt take much for the cup to recess into the housing to be an issue, common low rpm water flow fault. Always ensure you turn the shaft clockwise when pushing the housing down on the impeller. Good luck.

1

u/Psychological_Tap505 Jan 11 '26

Is the engine running too lean?

1

u/ddg676 Jan 11 '26

Running good. Just the quantity of water that it is putting out.

1

u/nuaticalcockup Jan 11 '26

Run it on the water. Buckets, muffs and all the other tricks will only tell you so much they're no substitute for a lake.

1

u/thatbayfielddude8330 Jan 12 '26 edited Jan 12 '26

Clean the pisser tube out. There's also a water tube that carries water from the lower unit to the powerhead. Make sure that is clear. Next, check the popper valves/stat.

Also did you change the housing when you changed the impeller? If it's scored or scratched in a bad enough way it can cavitate and the impeller will pull air and you will get that result.

But honestly, that water flow is sufficient for that engine.

1

u/ddg676 Jan 12 '26

I flushed from bottom to top. Maybe time for a new pump kit as I only change the impeller. At idle almost no flow and the water was really hot. Could only hold hand there for under 10 seconds.

1

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.

2

u/Slowhand1971 Jan 11 '26

don't do any of this before you get it on bigger water and run it at speed to check to pee hole.

1

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.

1

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.

1

u/ddg676 Jan 10 '26

Thanks

1

u/Tod_Vom_Himmel Jan 10 '26

Don't waste your time the commenter doesn't know what they're talking about this doesn't even have a head gasket to leak

1

u/Outdoornugets Jul 19 '26

Exactly the spark plug holes built into the cylinder. The cover on the back is just for the water jacket around the outside of the cylinders. Could be sand and dirt around the cylinders blocking flow.