Strange Outboard Overheating Problem

[thomm225]

After preparing all week to sail and finally heading out the creek midday on Friday, my outboard overheated.

The oil light was on and there was no tell tale water coming out which is usually consistent with a failed impeller.

So since the damage was done and I would have to somehow dock without the engine being 100%, I sailed North 25 miles so anyway

The plan was to replace the impeller in a day or so on a calm morning

This am, it was flat calm almost so I was up at 5 am and preparing to remove the engine and replace the impeller but decided to start the engine first just so I'd know it's full status.

It cranked up normally then spit a few drops of water out the telltale tube then started pumping water as usual.

After about 10 minutes, I pulled the anchor and stared to head back across the bay toward home while getting the sails up

The lower unit of the motor had been going in and out of the water for a few days due to my rough anchorages so the only thing I can think of is that the intake must have been blocked causing the over heating.

This is the first time that has ever happen to me with the 8 or so outboards I've owned over the years

Anyone else ever had this particular problem?

[atoll]

quite often tell tale tubes get blocked by salt if not in use this can dissolve once the outboard is run for a bit but sometimes will need a wire pushing up it to clear it

[thomm225]

Originally Posted by atoll

quite often tell tale tubes get blocked by salt if not in use this can dissolve once the outboard is run for a bit but sometimes will need a wire pushing up it to clear it

Yeah, I can see that happening but in this case the oil light was on fully, solid red and the only time I've seen it like that is the 2 times last year when I had an impeller fail. Plus there was quite a bit of smoke coming out of the exhaust when I shut the engine down which I took to be the overheating. Btw, I had just topped off the oil a week or so ago

I tried to get too many years out of an old impeller and it failed last Spring. So I ordered a new one but went ahead and replaced the bad one with one I had removed after a year or so of use that I still had for backup……. then that one failed 3 months later so I replaced it with one of the new ones

So when it overheated on Friday, it seemed strange with the impeller being only a few months old

[Pete7]

Did you get all the impeller bits out or were any of the fins missing when you changed them? Watched a friend strip a V6 powerhead down twice before he found the broken off impeller fin that had caused the problem.

Pete

[thomm225]

Originally Posted by Pete7

Did you get all the impeller bits out or were any of the fins missing when you changed them? Watched a friend strip a V6 powerhead down twice before he found the broken off impeller fin that had caused the problem.

Pete

I didn't change the impeller this morning.

On the first one that failed last year, the center section became disconnected from the outer section and it was spinning but the fin sections were not. (and they were still intact)

On the second one I replaced last year (the used one) , I just didn't like the way the tell tale flow looked so when I removed the impeller it just had bent fins but looked okay. Thinking about it now, the engine didn't overheat on that one and there was no oil light on

I motored/motorsailed 5 hours after start up this am so all I can figure is a blocked intake and btw, I haven't flushed this engine with fresh water in maybe a year and a half

I hate removing the bolt below the powerhead to flush it ……. if I were to drop it, it would be a pain to replace. Plus I start the engine so often it probably wouldn't help much except in this case it probably would have prevented the problem

You flush this engine with it off and the hose connected with an adapter directly to the power head

[HopCar]

I once had an Outboard over heat after it picked up a plastic bag.

[belizesailor]

Check the thermostat. Just had one fail after being laid up for a while. I initially assumed it was the impeller too, but it was not.

[thomm225]

Originally Posted by belizesailor

Check the thermostat. Just had one fail after being laid up for a while. I initially assumed it was the impeller too, but it was not.

That could definitely cause the problem if this little 5 hp outboard has a thermostat. Plus I rarely flush this engine.

I'm trying to find my shop manual now to see if it has one.

I did motor 5 hours yesterday though with no problem. Not sure if it was a clogged intake port or if it has a thermostat, a stuck thermostat that became unstuck

[thomm225]

Originally Posted by belizesailor

Check the thermostat. Just had one fail after being laid up for a while. I initially assumed it was the impeller too, but it was not.

Wow, it does have a little thermostat! 126 degrees F.

Thanks.

And it looks like it's much easier to get to and replace (or remove to make it back in) than the impeller would be if it were to fail

I think I'll at least flush it for a while for starters.......

[belizesailor]

Originally Posted by thomm225

Wow, it does have a little thermostat! 126 degrees F.

Thanks.

And it looks like it's much easier to get to and replace (or remove to make it back in) than the impeller would be if it were to fail

I think I'll at least flush it for a while for starters.......

Yes, Ive been surprised by the small motors that have them too.

Here in the tropics, I just remove them.

[john2445]

As I understand it, flushing from the port on the block does not do anything with/to the impeller. Ear muffs are a pain but at least the whole system gets flushed. I run the dinghy motor in a bucket.

[Squanderbucks]

Regardless of whether the pump is pumping water now, you said it ran long enough to overheat on the first day the problem was noted. Whenever an impeller is run "dry" for even a minute or so it is pretty well cooked. The blade tips even though they may not be broken off will be burnt and scoured.
My recommendation is to go ahead and replace the impeller and if it looks "ok" then put it in the spare parts box and save it for a real emergency.

Outboard impellers should be replaced every couple of years - maybe get an extra year if low hours. But hours can be deceiving as once they are installed and used even a little the rubber's time clock starts ticking and just sitting unused the rubber gets brittle.

[sailjumanji]

Since the oil light was on, did you check the oil?

[thomm225]

Originally Posted by john2445

As I understand it, flushing from the port on the block does not do anything with/to the impeller. Ear muffs are a pain but at least the whole system gets flushed. I run the dinghy motor in a bucket.

The impeller definitely gets flushed doing it by the book. (with the adapter to port setup)

These little motors come with a special adapter and you connected it to that port on the bottom of the block after removing the screw.

It is also done with the engine off.

Looking at the parts list for the 5 hp 4 stroke, I don't see any water intake holes where the ear muffs would normally supply the engine with water......guess that is why they have the port/adapter setup


https://www.mercurymarine.com/en/us/.../parts-catalog

[thomm225]

Originally Posted by Squanderbucks

Regardless of whether the pump is pumping water now, you said it ran long enough to overheat on the first day the problem was noted. Whenever an impeller is run "dry" for even a minute or so it is pretty well cooked. The blade tips even though they may not be broken off will be burnt and scoured.
My recommendation is to go ahead and replace the impeller and if it looks "ok" then put it in the spare parts box and save it for a real emergency.

Outboard impellers should be replaced every couple of years - maybe get an extra year if low hours. But hours can be deceiving as once they are installed and used even a little the rubber's time clock starts ticking and just sitting unused the rubber gets brittle.

You're probably right I should replace it, but I did motor and motorsail 5 hours yesterday am getting back across the bay in light to no wind and it worked fine with a strong telltale flow.

Maybe it was the thermostat and in that case the impeller would have stayed wet!

If the wind had remained strong (and I still had the problem) I would have sailed back and then sailed in as close to my slip as possible. Then I would have started the motor with the cover off and motored to the slip.

That is what I did the last time this happened.