Mercury 115 telltale problem / overheating

[SeaStu]

Hi all,

Any suggestions for what may be happening here?

Mercury 115 (2018), approx 150 hours, last service 11 months ago, used in salt water.

The telltale when in neutral and at low revs is fine and as would be expected. When the revs are increased to over 5000 rpm the telltale begins to fade away to a trickle after a few minutes and the overheat alarm sounds. I reduce the revs and go into neutral and shortly afterwards the telltale returns back to normal. After increasing back to 5000 rpm the same thing happens over again. Last time out I had no chance back to come back at low revs.

It's not entirely new this issue but has become steadily worse, previously flushing the engine with fresh water has assisted. More recently after an episode I flushed the engine using Blu Thru through the engine flush port (engine not running) and this caused a noticeably better telltale and allowed a prolonged trip at over 5000 rpm with no problems. However the next time time out I experienced the issues described above.

The impeller was replaced last service and remains in good condition, the thermostat is opening up when the engine is warm.

Thanks.

 

[Mr Googler]

I would flush with blu thru again with the engine up to temp and running. It ensures you reach every part.

maybe do it 2 or 3 times

 

[jfm]

I have had dozens of outboards for decades and never experienced this. Really weird symptoms. Very hard to guess what the problem could be.

As a long shot, and presuming that this has only been a problem since the last service, I would pull the leg and check that the service folks did not forget to fit the woodruff key into the new impeller. Long shot, but that's would I would check next.

IMHO this will not be fixed by blu thru flushing.

 

[QBhoy]

Hi. I have the same engine. This can be caused by one of two things, outwith the certainty that the impeller is in good health and order;
1. There is a water strainer on the top of the engine. It’s before the cooking goes into the fuel pump cooling, from memory
2. Your flushing fitting (where the hose attaches to) is compromised and leaking water at higher rpm.
Both can and are known to happen. The latter of which is likely most common.
Of course, this only applies if you are sure the impeller and thermostat is good and your engine height on the transom is correct for the type of boat. As in, it’s not drawing air into the intakes on the lower unit. You should be running a temperature in the 50’s oC and the 120’s oF from memory of mine. Do you have the water pressure indication option fitted to your set up ? Mine has this through the digital conversion unit that sends data to the smartcraft gauges and VesselView mobile device.

 

[SeaStu]

Thanks for the input on this, good suggestions and plenty to consider. I'm going to work through each of them, hopefully get to a solution. I will let you know.

 

[QBhoy]

Thanks for the input on this, good suggestions and plenty to consider. I'm going to work through each of them, hopefully get to a solution. I will let you know.

Yeah. Definitely check that flushing port connection. Very well known not to seal properly. Fine at lower rpm. But can leak and lose pressure when up the rpms. The wee strainer is on top of the engine. Wee hose comes off it for the fuel pump cooling. Go easy on the thing though. Quite fragile thing.
Failing that, your engine might be too high mounted perhaps. There was a Mercury bulletin about shaft lengths being slightly shorter than the usual dimensions. What kind of boat is the engine on ? Any pics of it on the boat ?

 

[SeaStu]

Yeah. Definitely check that flushing port connection. Very well known not to seal properly. Fine at lower rpm. But can leak and lose pressure when up the rpms. The wee strainer is on top of the engine. Wee hose comes off it for the fuel pump cooling. Go easy on the thing though. Quite fragile thing.
Failing that, your engine might be too high mounted perhaps. There was a Mercury bulletin about shaft lengths being slightly shorter than the usual dimensions. What kind of boat is the engine on ? Any pics of it on the boat ?

I'm going down to the boat this weekend , I will take a look at the strainers as well. To be honest I didn't know about them until now but found a YouTube vid showing the locations. Engine is on Oceanmaster 630, I will get a pic and take a measurement or two.

 

[QBhoy]

I'm going down to the boat this weekend , I will take a look at the strainers as well. To be honest I didn't know about them until now but found a YouTube vid showing the locations. Engine is on Oceanmaster 630, I will get a pic and take a measurement or two.

No worries. You’d assume the engine is set up correctly perhaos. Nothing unusual about that hull that might be an influence I’d imagine. Just for my own compatible interest, can I ask if it’s a CT gearbox model of 115 she has ? And what propeller pitch you are running ? I have the CT model and run a couple of different props on her. Usually a 19” tempest plus for all round ability. Occasionally a 20” enertia for top end silliness. She will run around 43 mph most given days with the 19”. Juat occasionally touching the limiter at a little over 6000 rpm. The 20” can see 45 mph gps at around 6000 or just below. Boat is a 6m cuddy. Great going for the apparent 115hp on the back. But I think these things are well known by now to put out a fair bit more than the claimed. Likely to be around 130hp or a little over most suspect.

 

[Rappey]

When the revs are increased to over 5000 rpm the telltale begins to fade away to a trickle

With much higher rpm the tell tale is under a lot more pressure so the water can turn into a fine spray and trickle. Possibly same volume of water but looks totally different.
I have an earlier generation which overheated at higher rpm. Took out thermostast which was very gunged up. New one did not fix the problem.
Impellor was fine. Put a cordless drill on drive shaft to gearbox and spun it up in water. Wow , that is one powerfull pump !
The copper water pipe goes through a stainless steel gasket into an aluminium block. The result was that the grommet the water pipe goes into was totally constricted and needed replacing. The down side was that the whole powerhead had to come off to get to it.
Relaunched and Yay, no more overheating at wot. But now it overheats on tickover as water stops ?????
Turned out it was the waterpipe coupling in the leg not connecting properly.