gas odor only when underway

[trykflyr]

Started getting a faint gas odor in the canvas enclosed cabin of my river runner last fall. Only happens when underway and then I need to be moving faster than a couple knots. No obvious leaks, hoses and vent lines look ok as far as I can see them and the vent hoses don’t appear to be plugged from the thru hull vents to the tanks. No noticeable odor when I opened it up for the first time this year after sitting since October; tanks are about 1/2 full each. There’s no signs of leakage around the carb, fuel pump, filter or tank selector valve. Both fill caps are tight, pulled the thru hull vent fittings and they’re free & clear. All the hose clamps are tight… I plan on pulling the floorboards this week and do my usual annual underfloor corrosion look. There’s no standing fuel or water in the bilge area… This one’s got me scratching my head. Had a client’s Cessna with the same type of issue, turned out to be a plugged vent inlet tube in the wing, which is why I‘m thinking a vent issue… Any thoughts are greatly appreciated!

 

[Chris1956]

I/O, inboard or outboard engine?

 

[racerone]

Stop using the boat.-----Put a " NO SMOKING " sign on it.

 

[dingbat]

How old are the fuel lines?

Had to fill hoses start to “seep”at the bottom of the bend where they transition from vertical to horizontal

 

[Tassie 1]

Isn't that a known issue with half cabins/cabin cruisers/ cuddy cabins or other enclosed type boats, specially at low speeds?
Where fumes are sucked back into the cockpit UNLESS a window or hatch is cracked open just a tad,

I noticed it in my son's cuddy cabin a couple of weeks ago when going through the 5 kt no wash zone...once he opened it up the smell got left behind.

I 'm sure it's also mentioned in a couple of older boating books l have

 

[dubs283]

Can be a sign of engine(s) running rich

Might be time for a tune up or carburetor adjustment/rebuild

 

[Grub54891]

Over time the hoses get saturated with fuel. They will leach odor through them, but not show any leaks. One of mine had no leaks and it smelled also. Replaced all hoses and issue gone.

 

[Scott Danforth]

Would help to know if this is a newer boat, an older boat, I/O or O/B, portable tank or in-hull tank

 

[cyclops222]

No winter fumes are the KEY condition !! The fumes are only with the engine running. !!!!!!
That leads us to........ Stuck on engine choke Carb. or F I. Carb is running to rich due to internal AIR MIXING JETS.
Mouse stuffed lifejacket stuffing into the CARB inlet throat. My boat 3 years ago.
If multi cylinder engine ? Bad sparkplug.
Exhaust manifold gasket leak.
Or a combination of poor maintenance in general.

 

[trykflyr]

Thanks for all the responses… I’ve owned this for 2 seasons, previous owner ( 3 years) is a good friend and fellow aircraft mechanic. I’ll fly in anything he wrenches on, so poor maintenance is way down the troubleshooting list. He bought it from the original owners. Boat is a 93 Hewescraft River Runner with a carb’d Redline 302 coupled to a Berkley jet pump, 341 total hours since new on it. New Edelbrock carb, air cleaner & gasket 2 years ago, less than 60 hours since install. Runs smooth, choke unloads normally, no misses or fouled plugs, 2 fitted aluminum 22 gal tanks under the floor midships; vents are on the left upper corner of the transom. I suspect the hoses are less than 10 years old just based on their condition and my experience with aircraft hoses. Couldn’tfindany weatherche king, cracks, wear or chaffing. I’m planning in chnaging all of them out regardless just as routine maintenance along with the coolant & heater hoses. The problem is recent…didn’t have it the first season at all and it only showed up towards the end of the year last year.

I did replace the original canvas enclosure between seasons…went from a 2 bow to a 3 bow that’s a bit taller. #Tassie 1 mentioned the issue with half cabins, maybe the height difference is causing a bigger burble behind the canvas… Can’t recall noticing any odor between uses, after the last use when I closed it up for winter or whether I had the center glass open or notehen I did notice it. It’s not a strong odor, just enough to notice and intermittently based on speed. I’m pulling the floorboards out this weekend to check things… Thanks again for the help!

 

[Stinnett21]

Seeping fuel sender gasket or screws. Assuming your sender is toward the rear of the tank, which most are, when you plane off fuel runs to the back and puts it under pressure.

 

[FunInDuhSun]

Isn't that a known issue with half cabins/cabin cruisers/ cuddy cabins or other enclosed type boats, specially at low speeds?
Where fumes are sucked back into the cockpit UNLESS a window or hatch is cracked open just a tad,

I noticed it in my son's cuddy cabin a couple of weeks ago when going through the 5 kt no wash zone...once he opened it up the smell got left behind.

I 'm sure it's also mentioned in a couple of older boating books l have

Yup, that’s called the “Station Wagon Effect”. Some of the manufacturers tried to minimize the problem by going to side exhausts.

 

[mike_i]

Can you tell or have you determined if the gas odor is coming from the exhaust or open air.

 

[trykflyr]

Update: Got the floor pulled out & removed all the void filler foam. That’s bone dry as is the bottom. Nothing leaking anywhere. I had the aft tank sender electrically fail about halfway through September, but it wasn’t seeping; it’s been replaced this week with new gasket & hardware. The forward one is tight as well. All the hoses are in good shape for being 3 decades old and have been a serious pain to pull off their respective connections. All are being replaced while I’ve got everything open. The selector valve is solid, however the transom vent screens appear to be crudded up a bit. They feel open when blown through, the vent hoses feel open too, but I’ve got replacements for both anyway. Replaced the spin on fuel filter; it was way tight and may have compressed the gasket too far, but I never noticed any odor when running the engine bay blower or opening the doghouse cover. All the tank connection tubes are free & clear. The exhausts are below the surface when running…

Thanks for all the help… I’ll post again when I get it all back together and I can run it after the ice goes out…

 

[trykflyr]

Can you tell or have you determined if the gas odor is coming from the exhaust or open air.

I’m confident it’s open air. The exhausts are submerged at the speeds I’ve noticed it.

 

[trykflyr]

Got it down to individual molecules…ended up drilling out the rivets holding the covers over the filler hoses to get them out. Everything looked solid and secure. Replaced all the old rubber & thru hull vents and on a whim I looked under the flip top gas caps. Both foam gaskets were dry rotted badly and the aft one is about half the size of the forward one. The old canvas was inboard of the caps, the new sides are outboard of them. I’ve a suspicion my odor is from fumes leaking around that aft gasket which is now inside the enclosure. New gaskets are unobtainium so I’ll find a sheet of closed cell neopreme and fabricate a couple. New rivets will be shot in with a 2 part polysulfide sealant we use to seal pressurized areas of aircraft fuselages. Lotta work for a couple 25 cent pieces of foam…..

 

[rxl650]

Lotta work for a couple 25 cent pieces of foam…..

Bahhhhahahaha, I know the feeling all to well. I buy used all the time and tear into what ever it is at the time. Beats being the guy towed in daily as he fixes problems as they arise.
It's nice knowing what you have at the end of the day. Especially on the water.
Well done.