Gas on water

[Kingfisher06]

Have a 2005 90hp 4stroke Merc on my Lund. Recently, last 5-6 times I've launched the boat, as soon as the engine hits the water there is a little gas sheen around the engine. It's really not much, maybe 3-4 sq ft that doesnt grow in size, just instantly appears after dunking it in. I think this is really the first time I've seen this after a decade of use. Motor runs great, nothing has changed on that front. Is it just excess gas from the exhaust from the previous time out? Any thoughts? Appreciate it.  

[Bozeman Bob]

If it has a carburetor ,when you tilt the motor up , some gas may come out of the bowl and drip down the leg ,where it meets the water when you dunk it . If it's fuel injected I would pop the shroud and make sure all fuel line hoses and clamps are good . Raise and lower engine when inspecting . A two stroke will have the sheen when dunking , not a 4 stroke . 

[MCF]

Hook it up to the hose and start it and idle it and make sure there is not a pin hole leak coming from one of the hoses somewhere.  

[Kingfisher06]

Will do, thanks for the tips! 

[sherman brown]

At times if the motor is idled or made short runs, you'll get residue in your exhaust ports that will leave a small gas film on the water. this could or could not be your case but something I thought you should know about. Try taking her for a long run then stop shup it down and tilt your motor up for a few min minutes and trim it down and start it back up and see if you're still getting a film. Just something you can check.

[Kingfisher06]

Good to know. I'll do that experiment. Also, I am gassing it pretty hard to get onto trailer each time I take it out-wonder if that big push can leave excess gas somewhere that finds its way out on the next launch. Thank you, much appreciated. 

[DtD]

An old school 2-stroker does this for the past 70 years ; knowing how far to the dock  I poll the tank line and let the engine die at the dock. Now I have an empty engine and no possibility of fouling the lake or the splash well or my sense of guilt. Six to eight pumps to the bulb and off we go. Just sayin'

Dave

[fisherdude]

Could the sheen on the water be caused by an oil leak like low unit oil Maybe got slight drip from a vent hole maybe a seal going bad.

Edited June 24, 2025 by fisherdude
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[Kingfisher06]

Friday I am going to have some time to go through all these suggestions, much appreciated. I'm not the best mechanic, hoping the problem is staring me right in the eyes lol. 

[Bozeman Bob]

So is it fuel injected or has carburetor's ?

[Kingfisher06]

So, after pulling my hair out for a few weeks and going through all ya'lls suggestions, I finally realized my power trim has a very slow leak. The little bit of sheen on the water after launching each time was power trim fluid. I must have a bad seal. Maybe I over filled the reservoir this spring and/or the seal has just gone bad. Anyhow, thanks for the suggestions, and I thought important to update ya'll in case you were wondering. 

Edited August 4, 2025 by Kingfisher06

[sherman brown]

21 hours ago, Kingfisher06 said:

So, after pulling my hair out for a few weeks and going through all ya'lls suggestions, I finally realized my power trim has a very slow leak. The little bit of sheen on the water after launching each time was power trim fluid. I must have a bad seal. Maybe I over filled the reservoir this spring and/or the seal has just gone bad. Anyhow, thanks for the suggestions, and I thought important to update ya'll in case you were wondering. 

 

 

Glad you found the problem! Now for the fix, good luck, and let us know if and when and how you fixed it. Sometimes, a sheen on the water is as simple as unburnt oil in the exhaust of 2-stroke motors.