Overheating at idle, cool as a cucumber at speed

[l008com]

Story time: To my knowledge, I was having no thermal problems before yesterday.
So I went for my last ride of the year yesterday since it was about 30° above normal outside. I went up a small river which was just full of leaves.

So after an hour or so, the engine starts smoking. It's a 2 cycle so that happens sometimes. But I noticed theres NO water coming out so I quickly shut it off. I lift the motor out expecting to find a huge glob of leaves and seaweed, but there was nothing. Very odd.

After a while I start it back up, still no water. But it's going to take me all night to paddle back so I put it in gear and idle with my fingers crossed. And suddenly the water is back! Flowing what looks like full speed.

At this point, I'm thinking everything is good. Engine seems to be running fine, cooling water seems to be flowing normal, everything is good. So I put it in gear and head back towards the ramp, maybe a mile or so away.

Well before long, the overheating alarm goes off. I take off the cowling and touch it and it sure is hot. So long story short here, I get into this cycle of driving for a few minutes at idle, shutting it off, splashing the motor with an oar for 5 minutes, starting it up again and driving for another few minutes.

Progress is slow but I'm getting there, until I round one of the corners and hit a very stiff headwind. No way I can make progress through that. I'm kind of running out of options. So I figure I'll splash it cool again, then just gun it and hope for the best.

To my surprise, no overheat alarm. I walk back and put my hand on the engine and its COLD. This is running just planed out so maybe 15 mph. Luckily it's November so all the boats are out of the yacht club so I just cruise on by and go all the way to the ramp. The whole area is a no-wake zone but I think my situation counts as an emergency.

At this point, I'm thinking I must have sucked up some gunk that clogged something in the motor. Even though that doesn't explain the water coming out normally. But maybe theres multiple parallel water paths or something. And going fast must have blasted out the clog?

But as soon as I got to the dock and unplugged the gas tank to let the motor run out of gas, it started getting super hot again. It was a battle to see which would happen first, the overheat alarm or running out of gas. I was splashing it with water while it idled waiting for it to run out and finally it did, probably seconds before it was going to start beeping!

So what the hell is going on here? Any ideas? The symptoms of the problem don't make sense to me. If my dad taught me anything, it's always make sure the motor is pissing water. And I've had many boat mechanics tell me that as long as water is coming out, the impeller is good to go. None of this adds up but I'm at a loss.

The boat is an old 80s thundercraft.
The motor is a 1989 Mercury 90 HP 3-cylinder 2-cycle Outboard

 

[saltchuckmatt]

Story time: To my knowledge, I was having no thermal problems before yesterday.
So I went for my last ride of the year yesterday since it was about 30° above normal outside. I went up a small river which was just full of leaves.

So after an hour or so, the engine starts smoking. It's a 2 cycle so that happens sometimes. But I noticed theres NO water coming out so I quickly shut it off. I lift the motor out expecting to find a huge glob of leaves and seaweed, but there was nothing. Very odd.

After a while I start it back up, still no water. But it's going to take me all night to paddle back so I put it in gear and idle with my fingers crossed. And suddenly the water is back! Flowing what looks like full speed.

At this point, I'm thinking everything is good. Engine seems to be running fine, cooling water seems to be flowing normal, everything is good. So I put it in gear and head back towards the ramp, maybe a mile or so away.

Well before long, the overheating alarm goes off. I take off the cowling and touch it and it sure is hot. So long story short here, I get into this cycle of driving for a few minutes at idle, shutting it off, splashing the motor with an oar for 5 minutes, starting it up again and driving for another few minutes.

Progress is slow but I'm getting there, until I round one of the corners and hit a very stiff headwind. No way I can make progress through that. I'm kind of running out of options. So I figure I'll splash it cool again, then just gun it and hope for the best.

To my surprise, no overheat alarm. I walk back and put my hand on the engine and its COLD. This is running just planed out so maybe 15 mph. Luckily it's November so all the boats are out of the yacht club so I just cruise on by and go all the way to the ramp. The whole area is a no-wake zone but I think my situation counts as an emergency.

At this point, I'm thinking I must have sucked up some gunk that clogged something in the motor. Even though that doesn't explain the water coming out normally. But maybe theres multiple parallel water paths or something. And going fast must have blasted out the clog?

But as soon as I got to the dock and unplugged the gas tank to let the motor run out of gas, it started getting super hot again. It was a battle to see which would happen first, the overheat alarm or running out of gas. I was splashing it with water while it idled waiting for it to run out and finally it did, probably seconds before it was going to start beeping!

So what the hell is going on here? Any ideas? The symptoms of the problem don't make sense to me. If my dad taught me anything, it's always make sure the motor is pissing water. And I've had many boat mechanics tell me that as long as water is coming out, the impeller is good to go. None of this adds up but I'm at a loss.

The boat is an old 80s thundercraft.
The motor is a 1989 Mercury 90 HP 3-cylinder 2-cycle Outboard

I'm thinking you plugged the intake screen and possibly burned up part of the impeller and the rubber bits are moving around causing havoc.

Pull the lower end and look at the water pump. Should tell the story.

 

[Chris1956]

Mercury impellers can get a bit of damage on their bottoms. This will cause them to overheat at idle and still run cool at speed.

You might check it for that. If so, replace impeller, wearplate and 2 gaskets.

 

[racerone]

Motor needs a new impeller about every 3 years.----And if you buy a used motor the first thing needed is a new impeller.-----Still lots of people ( sellers and buyers ) who think a boat motor can be run dry.

 

[l008com]

Ok so I guess a new impeller is on the list for next year.

Question: Is there some OTHER problem that could be causing this behavior or is this a situation where it is more or less definitely the pump?

Also, since at the very least, I probably need a new impeller anyway, where do you find those? I'm looking up parts lists for my motor and I'm not finding anything water pump related in there. Is it stuck in another category?

https://www.boats.net/catalog/mercury/outboard/90hp/90hp-0a996142-thru-0b240450

 

[racerone]

???----Gearhousing ( driveshaft ) -----Looks like the impeller is listed there.-----Item #28.

 

[l008com]

LOL I never would have found it there!

Also daaaaang the repair kit (8M0113799) is shockingly expensive! I mean it's all relative but I was expecting more like $40, not $180. Oh well beats $20,000 for a new motor.

Ok next question: I watched a video of how to do this yourself and it looks fairly straight forward. Is this a good job to D.I.Y. for someone who's fairly handy but has no real engine repair experience, besides replacing a mountain of starter solenoids

Or is this one best left to professionals?

 

[racerone]

Impeller is $30 and 99 times out of 100 that is all you need.-----Dealers / mechanics install kits to improve the bottom line.----Simple job.

 

[l008com]

It's been a solid 10 years since a mechanic has worked on this engine. He may have replaced the impeller, maybe not. If not, it's been 15+ years on this impeller. I think it's safe to assume I need the full kit. Granted I don't go out very many times these last few years. But from 2009 through 2013, I was going out for 3+ hours every weekend. So this impeller definitely has a lot of hours on it.

 

[racerone]

These have a stainless housing and stainless plate..-----Take a look first.----Unless it runs with lots of silt / sand you will only need an impeller !!!!

 

[racerone]

post picture of old impeller.

 

[l008com]

post picture of old impeller.

For sure. But I won't be doing this until Spring. And realistically, Spring will probably turn into Summer. That said, I don't even have to uncover the boat to pop the lower unit off so who knows.

 

[l008com]

Ok getting back to this.
First question: Does anyone know if it's just four bolts to pop the lower unit off on my older unit? It's a 1989 90HP.
I guess that's the only question. I'm watching a video for a newer mercury engine and it looks extremely straight forward. I like to buy the parts first but in this case, I'll probably disassemble first and leave the lower unit in the basement while I order parts. The housing for the impeller might have 20 years on it. But I generally boat in very clean and very deep water, so theres also a chance its all perfectly fine (the housing, not the impeller).

I do have one more question actually. The fact that I was overheating very fast at idle, but running very cold at speed, has me a little concerned. Is there a chance there is some kind of seaweed clog up in the passageways that this cooling water travels in? If so, should I borrow my friend's air compressor and have it nearby to clean out those passageways? Or is this particular symptom really common and VERY likely just a junk impeller?

 

[Lpgc]

My dad owned a small 2 stroke outboard he used on a tender, he told me "If you ever use that engine be aware it overheats after a few minutes, I've changed the impellor but it still overheats and I don't know what's wrong with it". Shortly after his death I took that engine and an inflatable to the river, water came out of the telltale so I thought it was going to be OK but it overheated just like he said it would. A few weeks later I stripped the engine down and found debris blocking the exhaust ports water channel. Removed the debris, put the engine back together, fixed the problem. Next time I tried the engine it ran fine, no overheating.

Edit - I'd take change the impellor first (as other people have advised above), see if that fixes it, if that doesn't fix it then look for debris clogging waterways in the powerblock.

 

[l008com]

Hmmmm ok, given that, I guess it can't hurt to have the compressor on hand. Is it going to be obvious by looking where the water pathways are once the lower unit is removed?

Also, I was driving in some notoriously weedy river water at the time this started, so it's entirely possible something did get sucked in. Hopefully a new rubber wheel and some compressed air and I'll be good to go.

 

[Lpgc]

I forget what make/model the little 2 stroke my dad had but it's just a little 1 or 2 cylinder engine around 5hp, it's still at the back of the garage, I might dig it out to use as an emergency/pusher engine on my boat.

I have since (still around 10 years ago) rebuilt an early 1980's Mercury 70hp 3 cylinder 2 stroke, fitted new pistons in it etc... I will never rebuild a 3 cylinder 2 stroke with none-removable cylinder heads again! I seem to remember the waterway at the exhaust port side of the block being a similar design on 70hp Merc as the little engine that had the cooling problem, waterways on both engines could be accessed by removing a couple of plates (maybe after removing a few other minor components such as ignition advance linkage). But I'm sure others will be able to give more accurate advice on how to access the exhaust side water channels on your engine if changing the impellor doesn't fix it.

 

[Chris1956]

I would replace the thermostat as well as the impeller, wearplate and 2 gaskets. Check the inside of the pump "cup" for wear and scratches. Replace if you find some.

The thermostat controls temp at idle up to about 2000RPM. After that the poppit controls the motor temp. Since she runs cool at speed, the poppit is likely working.

As for unbolting the gearcase, my Merc V6 has 5 nuts and a bolt hidden under the trim tab anode, as did my inline 4 and 6 motors. Not sure what you have.

Gearcases fit real tightly, so it is hard to tell if you have all the nuts/bolts off and she is just tight or what. Make sure to look under the trim tab for a hidden bolt.

Make sure you get all the bolts/nuts off, and try to wiggle it off. You can use a block of wood and a hammer to tap on the gearcase. The antiventilation plate is not the strongest so be careful. Use penetrating oil on the stud ends to help it slide out.

 

[l008com]

So today was interesting. I removed the lower unit for the first time ever. That wasn't bad at all. I mean it's the first time *I* removed it, not the first time it's BEEN removed.

So it came off nice and easy, I took it into my basement and took apart the pump cover and MUCH TO MY SURPRISE, everything looked perfect. The impeller looked basically perfect. The stainless steel looked like it had seen lots of use but didn't look worn, or like it needed to be replaced at all.
I was expecting the impeller to be nothing but a nub

I'll probably replace the rubber anyway since it's out. BUT. What's going on here? Where exactly does the water GO after it leaves the pump? Is there likely just clogs in the lines?
When this overheating issue was happening repeatedly, I eventually gunned it to get home and it ran ice cold. But then at the dock, with the throttle down at idle, it almost immediately started overheating again. So if it's a clog, you'd think I would have flushed it out but that doesn't seem to be the case.

 

[l008com]

Oops I forgot to attach the pictures.

Pic #1 Does this plastic washer thing go on the little geared spline to the front of the main shaft? Looks like it does but I didn't see where it actually fell from

Pic #2 Honestly that looks basically perfect to me. I know on here, people often say you should replace them once every year or two. But I've always been told, as long as the boat is peeing strong, the impeller is good to go.

Pic #3 & #4 It looks like theres grooves but it's all very smooth to the touch.

Pic #5 Is the water from the pump being sent up one of those holes? So if I were to try to blow one out with compressed air, that's where I'd direct it?

 

[Faztbullet]

I dont see the water pump housing seal thats on top of S/S housing. This will let exhaust to enter water pump housing. Also needs a impeller as has a set curl to it.