Outboard Motor Cooling Question

Bill Burruss

Bill Burruss

Well-known member
Hi. I have a question for the outboard mechanics out there. On 2 recent hunting trips my 2004 Yamaha 25HP 2-stroke lost power on the way back and dropped RPMs. It died after letting off the throttle. I also noticed a huge puff of steam comming out of the rear of the engine (believe from tell-tale). First time it happened I was not sure if if it had stopped peeing (thought the problem was most likely a pinched fuel line), but this time I know there was no water from the tell-tale- only steam. I frequently check the tell-tale and know it was peeing before heading back this last time. When trying to restart, the engine would not hit at first. Both times, after sitting a couple minutes, the engine would start, act normal, and discharge water from the tell-tale. I pulled the water pump and it looked fine, no debris. I also pulled the thermostat, and it looked like it had a little corrosion. The aluminum passages under the thermostat had some spots with aluminum corrosion (thank you Puget Sound, but it was worth the 2 years of salt exposure!), but no noticible debris. I also did a compression check and have 100 psi top and 105 bottom cylinders. I have replaced the impeller a couple times since new (last one within 2-3 yrs), and this was the first time I have had the thermostat cover off, so it is the original. The engine has been getting a little more difficult to start, which I had attributed to the plugs being about a year old. Now I am concerned it may have been overheating and doing damage.

Any thoughts on the remedy or other concerns? Should I pull the cover off the front of the head and check behind it?

I bought the engine new in 2004 and it has a little over 200 hrs on it. New impellor and thermostat are on the way and will be installed before next trip. Also, since breakin I've always used 100:1 Yamalube along with a little Sea Foam.

Any help appreciated.

Thanks,
Bill
 
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Sounds to me more like you have clogged jets and/or junk in the carb/fuel filter/fuel system.
 
Bill, That is a puzzle and nothing jumped out at me until I got to the 100:1 oil mix. Is that motor oil injected or pre mix? That doesn't sound like enough oil to this ol school guy. Not really thinking that is your problem, just recalling OMC's trouble with the 100:1 oil. Good luck and keep us advised as it goes along. One tool I have found a big help in looking for overheating probems is an infared thermometer that we ussually call incorrectly a "heat gun". You can pin down a hot spot very easily with one of those. You could have an exhaust leak into the water jacket.
 
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Bill, I had a Mercury 15 hp and it began to overheat within minutes of operation in the local river. I noticed the discharge water was non existent and when I took the cover off of the discharge hole, I found a thermostat plugged up with leaves and everything else the river had floating in it. I left the thermostat out and NEVER had a problem with it again. It wouldn't hurt to remove the thermostat and see if that changes anything, maybe the thermostat is locking up and not opening properly forcing an overheat. Is the head crackling, peeling paint, or super hot after shutdown?

dc
 
Only issue I have with the jets being suspect is that the motor runs fine until it gets warm. Bad jets the motor would run like crap the whole time. Maybe even worse upon start up verses when the motor gets hot. Advice from a very shade tree mechanic so take it for what it is worth.

I had similar issues with our 15 hp (not Yami) and it turned out to be a blown head gasket. Compression check didn't show much compression loss but after taking off the head it was evident gas was blowing through the gasket in a real small area. Had the exact same symptoms except my motor never ran great after over heating. Ran sluggish until I replace the head gasket.

Mark W
 
Bill

I didn't realize you were running 100:1. Jeff Smith had that same motor except oil injected. His motor was diagnosed by a mechanic as having a bad main bearing and died an early death. There was a lot of speculation the 100:1 ratio was just not enough lubrication. I don't think your motor has suffered this fate but I'd err on the conservative side and run 50:1. I can handle a little blue smoke much better than premature failure.
 
if you run it again and it starts to stall choke it if it picks up rpms it's not getting fuel. dirty carbs, bad tank vent etc. good luck
 
Bill, I don't think you have a carb problem but have had a thought. Maybe Yamaha uses a ("fail soft") or as OMC calls it S.L.O.W., I forget what it stands for but what it does is reduces R.P.M.s if you overheat or run low on oil in the injector tank. Again, not sure about Yamaha but just some other ideaas to think about.
 
All,

Thank-you for the replies- all the advise has been very helpful. Nothing worse than an intermitant problem. I'm not 100% sure the engine is overheating (may need to get an IR thermometer), but after it quit I took off the hood and quickly touched the head. It sizzled , so I spashed a little water on it and it sizzled. Not sure if that means anything. Any idea how to check for leak between exhaust and cooling systems? I am not sure if the engine has the overheating shutdown feature, but have been going off that assumtion and will now try to verify. The steam coming our of the tell tale seems odd to me and is why I have been investigating potential overheating.

Right now the thermostat is out and the skeg off. I blew some compressed air through both sides of the thermostat port. Ones side clearly went to tell tail, other side felt like it was coming out bottom of the tail shaft housing (perhaps the exhaust port?). From the underside I blew up throught the water pump tube and air came out the thermostat housing, but I could not tell if air was missing.

Seems like there would still be at least 12.5 HP if it was a carb issues since the engine has 2 carbs, and except for the shutdown the boat ran fine, so I am doubful the carbs are the cause. The on-engine fuel filter is clear and appears to be fine. Guess I could clean that. The 6 gal gas tank recently started seaping a little around where the black plastic that contains the fuel gauge/fuel pickup mates to the red plastic tank. I may swap to my 3 gallon tank to prove the 6 gallon is not the issue ( I need new 6 gallon tank, no matter the engine prognosis). The power down really sounded a lot like when the fuel connector falls off the tank. Will try the suggested choke trick if it happens again.

As far as 100:1 premix, it is what that the owners manual calls for in that generation engine. I have both 15 and 25 HP mid -2000 Yamaha 2-strokes, and both take the same mix.



Thanks for the help!
-Bill
 
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Just a thought.If just one cylinder isn't getting any,or very little fuel,that cylinder isn't getting any lubrication.
No amount of cooling water will help that.I'd take a good look at the carbs.
 
Just a thought.If just one cylinder isn't getting any,or very little fuel,that cylinder isn't getting any lubrication.
No amount of cooling water will help that.I'd take a good look at the carbs.
Click to expand...

Exactly, had an issue with my old offshore boat and a bad fuel pump surging, lack of lube scorched a cylinder.

I dont know of anyone who runs the prescribed 100:1 mix in their Yamahas. Everyone I know runs 50:1.
 
Well, finally took a good look at he impeller and may have found the issue. Hopefully it is this easy. This is the 6th or so impeller I've replaced on an outboard, and it is the first one that showed obvious damage. Seems something sharp got caught between the rubber and steel casing, cut grooves in it from top to bottom, and took a few hunks of rubber out of the bottom of the impeller. Wonder if I sucked up a piece of oyster shell or something when I used to hunt the salt? I really wonder if whatever it was passed all the way thru.... Anyhow, new one is installed, and the engine pees at low RPM. Appreciate all the thoughtful advice and hope not to have to revisit this issue!

Take Care!

-Bill

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Bill,

Did you carefully inspect the leading edge of the impeller housing? Your new impeller will not last long if the steel liner of the housing has any, I repeat any, burrs in/on it. The edges and surface should be smooth as a baby's butt. If so, it sounds like you have fixed your cooling issue. Hopefully there was no damage to the engine itself during the times it overheated.
 
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