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Catalina/Capri 25/250 Sailor's Forums

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Yamaha four stroke warning.

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Gary B.
Admiral

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USA
969 Posts

Initially Posted - 07/21/2006 : 11:37:20

Oops! I posted first on the C25 forum, but thought it should go here, too. If you read this, ignore the other one; I would delete it, but Larry's post in response would seem weird.....

Posted - 07/21/2006 : 10:16:18
------------------------------------------------------------------------
First off....let me say that I LOVE my Yamaha 9.9 four stroke high thrust, long shaft electric start. I think my current one is a 2000 model, but I earlier bought a 1985 model when they were quite new to the market.
HOWEVER, my first one "burnt" up/froze and had to be rebuilt. I never knew why. Then, about 2-3 years later, it did it again....I then realized that the crankcase was full of oil and gasoline MIXED. I was told the choke had stuck and that gas had gone down through the rings into the crankcase.
About the same time, this happened to a friend's boat, just as we were getting ready for a 130 mile coastal trip down the WA coast and over the Columbia River bar. We got it "unfroze" finally, changed oil twice (with bearing material in the oil/gas mixture). Somehow, the engine ran enough for us to get home before the needed rebuild.
THEN, just last week, I checked the oil in my Yammy. It was OVERFULL on the dipstick. I smelled it and, sure enough, there was GAS in the oil.
I changed it immediately and visited the dealer. He told me it was most certainly a "thermostat" problem. The engine was not heating up enough to expand the piston and rings, so the gas was going down the cylinder wall "making oil".
Luckily, I caught it in time this time. I seriously suggest that you Yamaha owners check oil levels often and watch for this.
Also, I think replacing the thermostat should become (as I have NOT done) a regular "maintenance" item. The dealer told me it should be done once a season, according to the book.
I LOVE these engines for their power, quiet, efficiency, etc. in spite of their weight. But THIS issue is a serious one; I know from too much experience over the last 20 years. Good luck!
Gary B.
s/v Encore! #685 SK/SR

Edited by - Gary B. on 07/21/2006 11:39:41

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 07/21/2006 : 21:17:58

New thermostat every year, new impellor every two or three--that's my rule. The thermostat is very subject to salt build-up that can cause it to stick open, or worse, closed. However, I'm a little dubious over that diagnosis. Keep an eye on it.

Edited by - Dave Bristle on 07/21/2006 21:19:03

lcharlot
Master Marine Consultant

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Antigua and Barbuda
1301 Posts

Response Posted - 07/21/2006 : 22:07:08

Is this recommendation for annual thermostat replacement only needed for salt-water use, or do us inland lake dwellers need to do it also?

Edited by - on

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 07/21/2006 : 23:13:12

"However, I'm a little dubious over that diagnosis. "

I share that opinon. IMHO: In order to get enough fuel into the oil to cause serious contamination, there has to be a carburation issue. During normal operation, there is no liquid fuel in the combustion chamber... just vapor.

Edited by - on

Gary B.
Admiral

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USA
969 Posts

Response Posted - 07/22/2006 : 02:59:50

It is just fine to be "dubious" folks. I am just reporting what I KNOW from experience. 3 times now a yamaha four stroke of mine has ended up with gas in the oil, twice with major rebuilds. Add to that the episode with my friend's Yamaha 9.9

My caution was to check oil levels often. I can't completely say that it was a thermostat problem, but I have NOT yet replaced the thermostat since the dealer gave me the wrong one. He said it could be a carburetor problem, or even a fuel pump issue. However, the "making oil" issue has NOT resurfaced after an oil change, making me think a stuck thermostat MIGHT well have been the problem. I doubt that a carburetion or fuel pump issue would have corrected itself......

Why is it not logical to think that a COOL running engine would not expand the rings/piston and allow gas to run down the cylinder wall???

Anyway......I am not making this up. Ignore this caution as you will.....

BTW: This boat is a FRESHWATER boat; some use in the San Juan Islands most years, but she lives in the Columbia River.

Gary B,
s/v Encore!

Edited by - on

Dave Bristle
Master Marine Consultant

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Djibouti
10005 Posts

Response Posted - 07/22/2006 : 07:39:59

I certainly wasn't suggesting that anyone ignore the problem, but I've known mechanics who have a rather vague understanding of the physics of an engine (and in one case, one who had no idea how a car air conditioner cooled air)...

I googled "Yamaha outboard gas oil" and found that this is an issue in larger Yammies (fuel injected), with a factory advisory. They suggest the injectors are leaking gas into the cylinders when the engine is shut down--but of course, you don't have EFI. I found no such discussion on carburated models... (See http://www.yamahaoutboardparts.com/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=1;t=000328#000001) But I suppose something like that could happen with a carburator, particularly in the lower cylinder if the carb is positioned midway between the two. That would be liquid fuel if the engine is not running...

On the other hand, if vapors are getting into the crank-case, they will condense after the engine is turned off and cools... Who knows...

I'm gonna check my Honda!

Edited by - Dave Bristle on 07/22/2006 07:52:19

ClamBeach
Master Marine Consultant

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3072 Posts

Response Posted - 07/22/2006 : 09:56:26

>"Why is it not logical to think that a COOL running engine would not expand the rings/piston and allow gas to run down the cylinder wall?"

1)Because there should be no liquid gasoline in the cylinder to begin with. Liquid gas in the cylinder means there is another problem. (aka flooding)

2)Because the cylinders are mounted horizontally and there's no reason for anything to 'run down the walls'.

3)Because the engine does not need to get warm to expand the piston rings. There is plenty of compression even when the engine is cold. Otherwise it wouldn't start and there would be huge amounts of blow-by.

The piston rings are designed to:
a)Provide Compression (top rings)
b)Keep crankcase oil from reaching the combustion chamber. (bottom rings)

They are NOT put there to keep liquid gas from reaching the crankcase.

I'm not attacking any of your advice. My problem is with the dealer's explanation.

Indeed, oil levels/quality should be checked and thermostats replaced periodically. Engines do run better and last longer when operated at proper temperature.

The only way you could be getting significant raw gas in the cylinders are:
1) Carb is not operating properly.
2) Fuel tank is pressurizing and forcing fuel past float valve.
3) Choke is stuck
a) Mechanical/Design Problem
b) An automatic choke is not opening due to cold engine.

Does your engine have an automatic choke or is it manually operated?

Edited by - on

tinob
Master Marine Consultant

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USA
1883 Posts

Response Posted - 07/22/2006 : 11:16:59

I find myself inline with Clam Beach's analysis of the problem and wonder if the problem might be cut off at the pass by doing one or two things. Run your engin dry after each use and by being sure that the fuel tank is vented.

Val on Calista # 3936, Patchogue, N.Y.

Edited by - on

Leon Sisson
Master Marine Consultant

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USA
1893 Posts

Response Posted - 07/22/2006 : 13:05:02

<center>Further muddying the water...</center>
I've heard of crankcase oil getting diluted by fuel. To the best of my knowledge, the fuel always gets into the crankcase via the carburetor (or injectors), through the intake tract, into the combustion chamber, and then seeping past the piston rings, washing the cylinder walls on its way to ruining the oil. This can happen while the engine is running, but any gasoline engine with that much excess fuel in the combustion chamber would run really rough, would be prone to sparkplug fouling, and the exhaust smoke would be quite noticably black. Obviously with a boat, the black smoke symtom would be invisible due to the underwater exhaust. Fuel can also leak into the crankcase when the engine isn't running if propelled by gravity (motorcycles), a pressurized fuel tank (sealed to control emissions, or in rare cases to actually move the fuel from tank to carb), or an electric fuel pump (more common in late model cars).

Enough rambling tangents, back to the Yamaha. As I recall, that model motor doesn't have a 'traditional' choke (butterfly plate upstream from venturi), but has an enrichening circuit (as in fuel passages) controlled electrically by a separate set of windings in the stator (under the flywheel). The general idea seems to be that when the motor is first fired up cold, the enrichening circuit (in its default mode) adds enough fuel for the mixture to ignite consistantly in a cold combustion chamber. As soon as the engine starts spinning, the extra set of windings in the magneto supplies current to a heating element in the enrichening circuit control valve which heats up a bimetalic element (or something like that) which then closes off the enrichening circuit as it gets hotter.

So... if there's anything wrong anywhere in the starting mixture enrichening system (and I'm thinking of the electrical portion in particular), the result would be like leaving the choke on, which would supply the engine with more fuel than it can burn. Some of the excess fuel would tend collect at the low point of the combustion chamber (against the cylinder wall), and seep past the piston rings propelled by the pressure of combustion. So, to make a long story finally end, I suggest anyone who suspects their motor may have this problem take a close look at the electrical portion of the cold starting fuel system.

Also, don't overlook the possibility of a leaky float valve needle and seat in the carb. The needle and seat can wear out, can be damaged by corrosion or mishandling, or prevented from fully closing by crud in the fuel.

I agree with ClamBeach about the cold piston and rings due to a stuck open thermostat being a red herring. Yes, fuel vapor might condense back to a liquid against cold head and cylinders, fuel-washed cylinder walls can lead to accelerated ring wear, eventually causing excessive ring end gap, making it a little easier for fuel to get past the piston (as well as reducing compression). But that's not what's going on here.

-- Leon Sisson {to whom there's no such thing as a simple answer to a simple question

}

Edited by - on

Arlyn Stewart
Master Marine Consultant

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USA
2980 Posts

Response Posted - 07/22/2006 : 13:24:27

I'm in agreement with Val, Dave and Clam here... I'm dubious of the tech answer.

I'm thinking that one of two situations are happening during non running time.<ol type="1"><li>the tank is above the motor and gas is draining/syphoned to the motor through an unseated carb float valve or </li><li>the tank is lower than the motor but the vent is being closed and night/day air temp variations are pumping gas into the motor again through a leaky float valve</li></ol id="1">

I do buy the choke sticking theory and think that if it stuck, cylinders would see raw gas but the motor would never come up to good rpms and the problem should easily be detected with a poorly performing motor.

The other way is if one cylinder were dead... and again performance would be so bad that it wouldn't go unnoticed.

Motors not coming fully up to operating temps however will suffer premature wear because the cylinder doesn't expand enough, perhaps the tech had a moment of confusion.

Edited by - on

Gary B.
Admiral

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USA
969 Posts

Response Posted - 07/25/2006 : 10:32:58

Leon,

As usual, your not-simple explanations are incredible and useful. Thanks again.....I always learn so much from your tech help.

Arlyn....no, my engine is not running on one cylinder, and no, it doesn't run rough. The vent is NOT closed on my tank.

I will confess to not running the fuel out each time I use it and disconnecting the fuel line, but I guess I should. I thought the issue was stale fuel and varnish build up. Since I use my boat several times a week, I thought I needed do this......

My father was a great mechanic.....and taught me enough to also have some basics, but things are MUCH too 'electronic" for me these days; obviously, mechanical ability is not inherited.

Thanks.....

Gary B.
s/v Encore!

Hoping I can get through the season without having to take in the engine. I would have to wait WEEKS at this time of year. I will just keep checking the engine before each use and changing oil frequently if it starts acting up again. Last few times....no issue....

Edited by - on

Sloop Smitten
Master Marine Consultant

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USA
1181 Posts

Response Posted - 07/25/2006 : 13:05:29

Gary,
I am not at all familiar with Yamaha engines but I remember this same situation being a problem with old air cooled VW engines. The fuel pump was powered by a shaft that intersected with the cam and if the fuel pump diaphragm sprung a leak, fuel could be pumped into the crankcase. If the Yamaha fuel pump is of a similar type it might be pumping small amounts of fuel into your crankcase. Just an idea in case the thermostat solution doesn't hold water. (Pun intended)

Edited by - on

atgep
Master Marine Consultant

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1009 Posts

Response Posted - 07/25/2006 : 13:38:43

The fuel pump would be a prime suspect. Most small engines use a vacum operated fuel pump. Any leakage would allow raw fuel to enter the engine intake or crankcase. My experience with small fuel pumps it to replace them instead of rebuild.

Tom.

Edited by - on

Leon Sisson
Master Marine Consultant

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USA
1893 Posts

Response Posted - 07/25/2006 : 15:12:28

Very good points about the possibilities of the fuel pump leaking into the crankcase, and of the unvented fuel tank heating up and pushing gas past the carb float valve! I humbly apologize for my previous omission of those possibilities.

I seem to recall a lot of car fuel pumps have a drain hole specifically to keep fuel from getting into the crankcase in the event a pump diaphragm leaks. I'm pretty sure the old Honda B-series fuel pump gets its power from being bumped by a valve rocker arm, or maybe a lobe on the camshaft, very similar to a mechanical fuel pump on a car. I think the Yamaha 10HP 4-stroke fuel pump works similar to the Honda, although I don't have one in front of me right now to check.

-- Leon Sisson

Edited by - on

tinob
Master Marine Consultant

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USA
1883 Posts

Response Posted - 07/25/2006 : 15:45:16

The old Fiat Topolino( 1947 ) had such a fuel pump and when its diaphragm went south it sprayed oil all over the engine compartment. The 1963 Fiat Multipla had the same problem I seem to recall. Neither contaminated the oil, just everything else

.

I'd pay a pretty penny for the Topolino today. The one I had was a 500 cc convertible station wagon. Gas milage to die for.

Val on Calista # 3936, Patchogue, N.Y.

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