Gas odor after running twin 350's

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#1
Probably a stupid newbie question...but i have twin 08 Yamaha 350's on my 360 Express and have noticed a mild gas smell from the outboards and a small sheen in the water after running the boat for a while. Both the smell and film dissipate after a while, and only occurs after running. There is no gas smell onboard. The engines have been running flawlessly. I have only had the boat for a few months so Im not sure if this is a new occurrence. Is this something that I should be concerned about?
 

seasick

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#3
Unless some sort of maintenance was done incorrectly, I doubt that you would develop a leak or whatever on both motors at the same time. Try running one motor at a time to see if indeed only one motor has a problem and go from there.
If maintenance was done to both motors just prior to this problem occurring, I would look at that work first.
 

journeyman

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#4
I would do as Fishtales suggests. I have the same motors and do not have any gas odor or sheen. Shouldn't be to hard to locate
 
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#5
If you are sure you are smelling raw gas, definitely figure it out asap.

If you don't find a gas problem, consider the possibility that its hydraulic fluid from one of the tilt rams.
That wouldn't explain a strong gas smell but sometimes seeing something out of the ordinary leads us to smelling things that were always there.
 

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Update....so I had a guy at the marina check out my engine and he thought he saw a dried up area of fluid under the fuel cooler (his term). He recommended a Yamaha service guy who can out, did a thoughrough check and was unable to find anything leaking. He merely stated that the engine runs rich, and doesn’t have any issues. He stated that the sheen in the water was simply exhaust.

I still notice a much stronger gas scent from that engine compared to the other though. I guess I will just keep a very close eye on things.

Thanks again for everyone’s advice. It is always really appreciated.
 

seasick

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#7
I mentioned, do both engines leave that sheen? You should be able to test at the dock. Since it seems to you that one Onegin smells of gas more than the other, you probably have an issue on the 'smelly' motor.
I don't buy the 'running rich' argument. Gas in the exhaust can get there in a couple of ways; one of course is a leak that drains into he outer exhaust housing. The second is excess gas in the exhaust itself, either too much fuel or incomplete combustion. Having a tech dump the ecu data might point to a problem. Over time, a leak will most likely get bigger and an over rich situation will lead to fouled plugs.
You could have a bad injector also. The diagnostic unit should be able to detect that (I think).
If you have fuel usage data on your displays, compare the usage and burn rate of each engine. Leaks will often show up as higher gallons used for the motor with the leak.
 
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#8
if the guy really did check the motor out properly, it sounds like either a tiny fuel leak somewhere reaching the water via gravity or else unignited fuel (not running rich), meaning a spark issue.

to be running rich enough to leave a visible sheen of exhaust in the water without throwing a code or symptom a yamaha tech would detect is hard to believe. o2 sensors get lazy before they fail but not that lazy.
 

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#9
Sounds like I may need a third opinion. I had the boat in Fort Lauderdale initially, super easy to service, but moved it inland to Lake Lanier in Georgia...much more difficult to find certified Yamaha mechanics who know a 350.
 

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I mentioned, do both engines leave that sheen? You should be able to test at the dock. Since it seems to you that one Onegin smells of gas more than the other, you probably have an issue on the 'smelly' motor.
I don't buy the 'running rich' argument. Gas in the exhaust can get there in a couple of ways; one of course is a leak that drains into he outer exhaust housing. The second is excess gas in the exhaust itself, either too much fuel or incomplete combustion. Having a tech dump the ecu data might point to a problem. Over time, a leak will most likely get bigger and an over rich situation will lead to fouled plugs.
You could have a bad injector also. The diagnostic unit should be able to detect that (I think).
If you have fuel usage data on your displays, compare the usage and burn rate of each engine. Leaks will often show up as higher gallons used for the motor with the leak.
The smell is definitely coming from the Starboard engine. I’m still not sure which side the sheen is coming from, but will check it out again this weekend. Thanks for the advice on checking the fuel consumption, I’ll take a look at this weekend.
 
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#11
So...as I continue to smell gas from the starboard engine, I went ahead and had a third Yamaha mechanic look and he too was unable to find anything after a thorough running of the engine.

So today, after a day of fishing, I noticed that there wasn’t a gas smell until I trimmed the motor all the way up. Doing this also caused a sheen to form in the water. In fact, some of the little amount of water exiting the splash well scupper by the starboard engine smelled strongly of gas.

Does the fact this occurs only with a trimmed up motor aid in a diagnosis?