Yamaha Malta overheating

[David Mawer]

I have a Malta with a tell tale hole, and it is overheating.

Water comes out of the tell tale and the engine runs well at idle and starting revs.

The water fluctuates between being cold then warm.

When I rev up out of gear or without the prop on, the water from the tell tale starts to splutter and reduces to an intermittent spray and the water heats up and the cylinder head gets hot (water flicked onto it sizzles).

The impellor is new.

I have stripped it right down to inspect all of the galleries and poke wire through to check they are al clear.

I have run the water pump using an electric drill on the drive shaft and there seems to be a reasonable flow and pressure of water (I did this with the head off so I could see water coming out of the delivery tube just before the tell tale hole.

have run the outboard with a mains water pipe connected to the delivery tube (with the leg off) and it ran fine with no overheating.

I have tried running without the thermostat but this makes no difference.

When running without the thermostat cover water bubbles and sprays out until I increase revs and flow slows/stops.

If I pour water into either side of the thermostat housing it quickly drains out of the exhaust port in the bottom of the leg

My feelings are that it must be the water pump, but I can see no reason for it not to be working correctly.

Any suggestions would be very welcome, please.

Thanks

[DennisG01]

Are you running it on a hose or in a bucket of water? If on a hose, try a bucket.

[David Mawer]

It over heated at sea, I ran it with a hose and it was fine, in a bucket it runs as described then overheats

[99yam40]

I have a feeling it is getting exhaust gases into the cooling water channels from what you wrote.
but then I have no idea what a Malta is

[DennisG01]

When you replaced the impeller, was there any scoring, at all, on the housing or the wear plate? You didn't mention if you replaced those parts, as well.

[David Mawer]

I have a feeling it is getting exhaust gases into the cooling water channels from what you wrote.
but then I have no idea what a Malta is

That sounds likely, I thought I could see exhaust fumes coming out of the tell tale. Is this a gasket failure or something else? A Malta is a Yamaha 3.5 hp

[David Mawer]

When you replaced the impeller, was there any scoring, at all, on the housing or the wear plate? You didn't mention if you replaced those parts, as well.

There was no scoring but the plastic part of the housing had melted and sealed over the exit hole. I drilled this out and have not replaced the housing or wear plate, just the impeller

[DennisG01]

There was no scoring but the plastic part of the housing had melted and sealed over the exit hole. I drilled this out and have not replaced the housing or wear plate, just the impeller

It's "possible" that that is the issue. Of course, I'm not saying it is, but if the housing was melted, it may not be making as tight of a seal as it should. When you're on the hose, the hose pressure is helping things out - but when the little impeller has to do the work all by itself, it may not be able to do the job as well.

[99yam40]

yep if housing melted then it needs replacing along with all the other parts in the pump. especially the cup and wear plate.
usually best bang for the buck is to buy the water pump kit and the housing.

but if you are getting combustion chamber gases in the cooling system, I would adress that 1st.
could be a hole in metal or a gasket.
a leak down test with compressed air may indicate if it is in the head, gasket, or block.