Overheating Mariner

[lazydawg]

I have a 1989 40ELO Mariner, serial 6E9L451969. It is one of the Yamaha powerhead models. I have been fighting an overheat problem for a long time now. I have taken it to two different mechanics several times. Each time it runs great the first time out but after that it will overheat if run at WOT for more then a minute or two. You can back it down a little and it will run, but each time out you have to back it a little further The overheating is always accompanied with a reduction in power. Over the course of a couple of years it has had two new water pumps, three poppet valves and springs , new thermostat, de-carbed several times, and pugs re-gapped. Each time same story. It is not a false alarm because when you slow down and water washes up on the motor it creates a cloud of steam. Water flow is good from the tell-tale. Any suggestions or ideas? I am tired of throwing money down a hole.

 

[Laddies]

Re: Overheating Mariner

Once in a while a leaking head gasket will cause a steam dome in the top of a block and cause engine overheating

 

[lazydawg]

Re: Overheating Mariner

Is there any way to check for a leaking head gasket other than a teardown?

 

[CharlieB]

Re: Overheating Mariner

You could try doing a cylinder leak down test, i.e., turn the engine to TDC #1 Cyl, plumb an air line into the spark plug hole, add a 100 psi regulated air supply, remove the thermostat and listen very carefully for any leak into the cooling passage.

I would also like to see your spark plugs after a 'clean shut off', run WOT for a minute, yank to neutral and immediatly shut down, watch out for the wave coming over the transom, pull your plugs and read them.

A couple of the older engines are running a bit lean on the oxygenated fuels, more alcohol = less gas and combustion temps rise requiring rejetting, some not all, engines are suseptable to heating, spark plugs and piston dome coloration are a very good indication of this condition.

Lastly, you could have an ignition box starting to fail after it gets to temp, dropping a cylinder, the rest are working harder and causing a temp rise. here a clean shut off could show a wet plug on a dead cyl.

I had a time checking a customers boat, ran great until I put the cover on and left. Seems the ignition module would run cooler without the cover and alls well, but with the cover on it would start dropping a cyl. I finally proved it and replaced it. Now alls good all the time.

 

[lazydawg]

Re: Overheating Mariner

Thanks for the responses. Like you both mentioned I think it is time that things other than water flow are looked at. I don't think it is dropping a cylinder, its only a two cylinder motor and it doesn't sound like it. The loss of power that goes along with the overheat is what really concerns me. With two people and gear in the boat it will plane off no problem the first time off the trailer. After that the overheating begins and the passenger has to move forward to get the boat to plane off. Any other suggestions?

 

[CharlieB]

Re: Overheating Mariner

Is this run in salt water or in VERY hard water?

I've had a few 'salted' motors that consistantly ran hot, and a 9.9 that had an accumulation of mineral (freshwater) that had to be cleaned out.

Look inside any opening,(thermostat, poppet) can you see any sort of crusty accumulation?

 

[lazydawg]

Re: Overheating Mariner

To my knowledge it has never been in salt water. I haven't pulled the thermostat recently but the last time I did there was no buildup to speak of.