Mercury 2.5 more cooling questions

[jimbouy]

Having swapped the gear box and installed a new impeller I have had my little OB running in a bin of water.... yes I took the prop off!!

On start up there is plenty of water coming out of the piddler hose. As the engine gets hot the amount of water reduces .... but there is then a good steady amount of steam coming out of what I think is the exhaust relief valve...2 holes 2/3rds of the way up the rear of the leg.


Is this normal.....


TIA

Jim

 

[Gordonmc]

I have the same motor and the same symptoms.
My assumption has been that the impellor needs the impetus of a flow of water over the leg, which isn't there with the prop off. The water coming out of the head is the residue from last use.
I have stopped running the motor in a bucket sans-prop and take it to a tank instead as steam indicates overheating to me.
Of course my assumption could be completely off-course.

 

[paulrossall]

I have the same engine. I would never run an outboard without the propellor on it and the explanation given re water flow might be right. When I bought my engine second hand I was told the water flow was bad and I unbolted the leg off the engine which then lets you clean all the water chanels that get blocked up. I bought a new gasket but you might get away with useing the old one. Dead easy job provided the bolts come out easily. Put some thread lock or even grease on them when you put it back together so they will come out easily next time.

 

[jimbouy]

Ok.... interesting, I have been told many times to remove the prop to avoid cavitation and thus lack of cooling water.

The amount of water that comes through initially can not be down to water retained in the system as it goes on for several minutes before it eases off.

I'm not convinced that a flow of water over the pump intake holes is needed, BUT I'm no engineer by a long chalk so I will refit the prop and give it a try.

Intriguing.


Jim

 

[ditchcrawler]

Why not run an outboard without a prop on it ? My Tohatsu 3.5 runs quite happily in neutral which is no different to having no propeller isn't it.Is their something different in the design of an outboard with a simple gearbox to one without ?

 

[cliff]

I was about to ask the same question. My Merc 3.5 runs quite happily in neutral and my old Merc 2.5 ran quite happily without the prop. A flow of water across the pump inlet is not required for the pump to work on either type. I did notice, from time to time, on the old 2.5 that the the flow of water from the tell tale dried up but that was due to crud getting swilled about in the waterway and partially blocking the tell tale hole- a poke with a piece of brass wire would clear the tell tale (until the next time).
--------------------

"Artificial intelligence is no match for natural stupidity"

 

[paulrossall]

I guess I should not start posting when I am also trying to cut the lawns as fast as possible so I can drive down to Walton on the Naze and spend the rest of the week getting ready to move to my Deben mooring.
Most of my outboards don't have gears so I would leave the prop on to make sure I don't rev them too high. I accept that if you have gears you can , or indeed will, run them in neutral.
I think clearing the water outlet with a bit of wire might work but suspect you will have to take the leg off to really solve the problem.

 

[jimbouy]

The fact that I get a good flow of water for a few minutes would suggest to me that the water ways are not badly blocked.

I think prop back on and a change of water as it is now a bit mucky ...... poke the tell tale hole a little....

Further investigation ......


Jim