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If your fuel line is hooked up to the motor shouldn't that allow for expansion/contraction if you close your vent? Also, if your tank is full (which it always should be, after all this is a sailboat, what do you need a motor for? ) wouldn't you expect fuel to leak out while you are sailing (heeling)?

That being said, I never close mine. I guess I really should though, especially if leaving the fuel line hooked up allows for expansion/contraction.

alanmoor wrote:If your fuel line is hooked up to the motor shouldn't that allow for expansion/contraction if you close your vent? Also, if your tank is full (which it always should be, after all this is a sailboat, what do you need a motor for? ) wouldn't you expect fuel to leak out while you are sailing (heeling)?

That being said, I never close mine. I guess I really should though, especially if leaving the fuel line hooked up allows for expansion/contraction.

Okay, you asked, so rant back ON: I've specifically been told by my dealer not to leave the gas hooked up to the engine, because the pressure from the ballooning tanks could damage the intake on the eTec.

Also, on heeling and full tanks, I would not expect a fuel tank with a properly sealed cap to leak just because the tank is not sitting level. That said, because of this, I try not to put more than four gallons in each of my 6 gallon tanks, just to reduce the leaking/smelliness while I sail. So, seems dumb that I'm limiting my range (under power) by a third just because the cap doesn't work the way it should.

Rant OFF.

You are correct on all your statements. I had a tank filled up to the top of max. fill line and with closed vent in the winter here in SW FLorida and noticed a strong gas odor down below. I went on deck and saw the gas tank expanded and fuel comeing out the closed vent.
I have a MacX and the chalking inside the tank floor edges created a leak and I had to reseal the joint.
I now only put in 5 gals. in a 6 gal. tank, vents can't be trusted.
The vent is needed for adding air back in the tank as you use the fuel and tank expansion. Many times I find people who can't start their motor due to no fuel supply as the vent is closed and won't allow fuel to engine .
Shadeing the fuel tanks from direct sun is highly recommended.
Dave

jbousquin wrote:Just to expand on Seahouse's rant:

I've got two 6 gallon Scepter tanks with "self-purging" vents. These little works of genius were apparently required by the EPA on the tanks I bought. Because they're self-purging, they don't have a vent you can open and close. They're designed to vent to the outside when a certain PSI is reached inside the tank by expanding vapors. I believe it's 5 lbs or something.

Anyway, the bottom line is, they don't work.

Basically, if I leave my gas tanks with the caps screwed on tight, they blow up like a balloon and don't allow the fumes to excape... UNTIL those expanded fumes put so much pressure on the self-venting caps that they blow the little gasket inside the tank and they start to fail. That means the expanding fumes can now get out all the time.... along with liquid fuel as well, especially when heeling.

My dealer just sent me a new cap to replace an old one where the valve was blown, and I'm going to have to get another one for my second tank, too, as it's started leaking as well. This is especially noticeable when the Mac heels, and you see a shiny sheen of gasoline coming down the front of the tank, and onto the cockpit floor where you're usually standing if you're sailing the boat.

My dealer told me to avoid blowing the caps in the future, I needed to unscrew the cap a quarter turn when leaving the boat to allow the tanks to vent... which, of course, defeats the whole purpose of these self-purging vents required by the EPA to "protect" our environment. I just don't see how regularly spilling fuel onto the cockpit floor, and eventually, into the water, accomplishes that.

Just incredibly dumb, if you ask me.

Okay, my rant switch now turned to OFF, too.

All this vent silliness could be completely resolved if the manufacturer's included a replaceable expansion bladder and fitting. Put the balloon over the expansion nipple and seal it with a screw-down ring. Evaporating gas will fill the balloon during the hot part of the day and condense back into the tank at night. Worst case scenario you return to find your balloon has popped and you vented some gas to atmosphere, which is what appears to be happening anyway since a lot of people leave their vents open. The balloon could be a T-hose off the engine intake, so it could be conveniently located atop the engine and would be deflated immediately upon starting the engine as the first gas.

Yeah, it would look stupid but it would solve the problem.

[On Edit] Unfortunately it looks like every stretchable plastic, rubber, or latex on the planet is attacked aggressively by gasoline. The closest you could get would be an accordion tube rotomolded out of HDPE (Milk jug material).

The more I think about it, the harder it is to believe that much fuel evaporated via the vent. I have small containers in the shed for lawn mowers that don't have sealed lids. I've left them for long periods of time and never noticed that kind of evaporation. I guess much depends on the air flow through the vent, but we have huge temp swings (90's high - 40's lows) in the summer.

I wonder if your fuel flow sensor is not correct.

RussMT wrote:The more I think about it, the harder it is to believe that much fuel evaporated via the vent. I have small containers in the shed for lawn mowers that don't have sealed lids. I've left them for long periods of time and never noticed that kind of evaporation. I guess much depends on the air flow through the vent, but we have huge temp swings (90's high - 40's lows) in the summer.

I wonder if your fuel flow sensor is not correct.

I wonder about calibration as well. It's the engine's EMM fuel flow sensor reporting to my Garmin head via NMEA2000. Of course, it's measuring tiny droplets of fuel over millions of droplets, so any calibration error at all could result in large swings.

It's the Garmin performing the fuel use calculation from flow rates reported by the EMM, and it's likely that it's not deducting fuel usage when the Garmin is not turned on. But I always run with the Garmin on--certainly I've not operated for 3 gallons of fuel without it on. I'll burn off a gallon without the Garmin on and see if that affects the calculation. But again, it's always on, so it wouldn't account for any more than a half-gallon of fuel use total if the number is inaccurate when off.

My personal schema is to place my knee on the tank while turning the vent screw until the hissing stops, then back it out 1/8 to ¼ turn or so, if I want to leave the tank "open". Note that just setting the screw ¼ turn out from full closed will not, in all cases, insure that the vent is open. (Mine would not be). Depends on the specific design of the valve.

I do the same thing, but turn the screw until the hissing just stops if I want the tank “closed”. This way if a catastrophic pressure builds, the pressure should first release (slowly) at the vent, because it is “just” closed. For transporting, I just close it all the way.

Of course, I could not do the above knee thing at all on the leaky tank, because fuel will spew from the connector fitting not matter what.

The whole problem exists since the introduction of plastic tanks – metal tanks (both by strength of the material, and by their shape) easily withstood the vapour pressures of the fuel with a large margin of safety. And metal tanks had O-rings you could easily replace if you lost a drop or two when switching tanks.

Another reason (other than fuel degradation from oxidation, and emissions) to keep the valve closed if possible is to keep the ratio of the fuel components within original spec. Some components in gasoline are more volatile than others (winter fuel has a higher ratio of volatiles for easier cold starting). Leaving the tank open allows the volatiles to evaporate, leaving a higher ratio of less-volatile components behind, which in time works its way up until it, at some point, becomes an issue with starting and operation.

As I see it, we won’t likely see a solution to all this for a while. One solution is to use a carbon canister, similar to what’s in a car, that captures the fuel vapours, and then draws them back into the engine using a purge valve after it’s started, and burns them. The canisters can be physically large, like a coffee can, and associated several tubes are required. And they should be changed periodically to keep them at maximum performance. Yeah, that will be popular.

-Brian.

Ps. While the exact details escape me now, it is the evaporative emissions captured in the carbon canister that allowed a modern car to do a 200 mile trip (I do remember that I could drive to Toronto and back, = 200 miles at the time, which was late 1990’s) and put out fewer, or the same, pollutants than an older car (like pre 1970’s or so) that was just parked and was not started for the day. As I recall, a lawnmower just sitting was just as bad. So evaporative emissions are still significant, if not as high as you reported, Mastreb.

Here in the Northeast, the temperature drops at night and the humidity rises. The temperature - dewpoint spread quite often gets close to within five or six degrees of each other. If they are within three degrees or less, it gets hazy and foggy. This means the air is nearly saturated with moisture short of rain. It seems to me that closing the fuel vent would eliminate moisture gravitating to the inside of the tank. Every morning our grass and cars are wet with dew.

It seems to me that closing the fuel vent would eliminate moisture gravitating to the inside of the tank.

Hey WASP18!

Absolutely will! Close the valve and your moisture buildup in the tank will be zero. Just like in a modern auto’s (closed) fuel system.

Moisture builds up when the valve is open through the daily and seasonal temperature swings. The repeated “breathing” in and out from expansion and contraction introduces new air and moisture each time, and when the fuel and tank walls are cooler (they cool at a slower rate than the air) condensation happens inside the tank that doesn’t leave when the tanks “breathes” out. Note that it’s at night when the tank is “inhaling”, so that’s when all that moist air is going in. So with each new cycle the amount of moisture ratchets upward.

Oxygenated fuels (the euphemism for those with ethanol added) will take care of some of this moisture by dissolving it and burning it in the engine with the fuel. So it’s not normally a problem.

But get too much moisture built up and you encounter “phase separation”, where the water and alcohol reach a saturation point and come out of solution in the bottom of the tank, which you can see- looks kinda like dirty chicken broth. Then, no amount of alcohol will remove the water-alcohol layers, and the only way to clean the tank is to physically remove the goopy layers (in spite of the magic snake oil you might hear about).

Your grass will start to get greener now (= come out of “hibernation”- I’m assuming it’s brown like ours are here) because of the longer (cooler) nights and dew formation. That effect starts to become pronounced around mid August, whether rainfall increases or not.

The dew forms on any surface (like inside the tank) that is cooler than the air temperature. If there is air movement (= wind) it will evaporate as quickly, the surface temperatures will more rapidly equalize, and you won’t see it.

A related lesser-known fact, you might already know it: you will notice on cloudless nights it forms more quickly (and sometimes exclusively) on horizontal surfaces such as the hood, roof, and trunk that face the sky. You are witnessing “radiation cooling” into space, first-hand. (It’s not just heating that happens by radiation). Amazingly enough, as high as they may be, clouds will function as an insulating blanket to reduce this effect, so it won’t happen when clouds are in the sky, or under trees, roof overhangs, etc.

- Brian.

I forgot to open a vent on one of my 6.5 fuel tanks & it blew out the gasket on the top fuel line base connector & these are not cheap tanks they are merc, tanks I have 4 of them I usually only keep two of them full at any one time only bought the 4 so as I have the capability to go any distance if needed

J

Well one can always buy a gasoline propane fume detector for about 100.-150# if one feels embelished or u can just do what I do on these damp nights & days burn a nice Apple & Cinnimon Candle smells like someone was baking an apple pie in the boat

J

I thought about that - actually went ahead and bought two for my starboard side. I never did buy the second pair, I found that the port area is very convenient for storage and I can always take some extra 5 gallon cans if I think I will need them - doesn't happen much.

Highlander wrote:I forgot to open a vent on one of my 6.5 fuel tanks & it blew out the gasket on the top fuel line base connector & these are not cheap tanks they are merc, tanks I have 4 of them I usually only keep two of them full at any one time only bought the 4 so as I have the capability to go any distance if needed

J