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Old 24-02-2021, 13:44   #1
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Tank vent - what if I top the tanks up to the filling cap?

Im running new hoses in a new tank setup. I understand there must not be any dips in the vent line....but what happens if I top up my tanks so even my fill hose gets filled? Cause then there will be fuel standing in the vent hose as well. So what happens when running my engine?

And yes...the vent fitting sits higher than the fill cap
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Old 24-02-2021, 13:45   #2
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Re: Tank vent - what if I top the tanks up to the filling cap?

It'll pull air into the vent hose to make up for the fuel being used, pulling the fuel from the vent back into the tanks. So no issues. But realistically, you won't want the tanks that full, as you'll end up spilling fuel just from sloshing as the boat moves, or if the weather warms up and the fuel expands a bit.
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Old 24-02-2021, 15:03   #3
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Re: Tank vent - what if I top the tanks up to the filling cap?

It's why I've learned not to completely top up the tank before winter storage - expansion / contraction - doesn't do any real harm but leaves a small trail of diesel down the transom by spring.
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Old 24-02-2021, 18:37   #4
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Re: Tank vent - what if I top the tanks up to the filling cap?

Quote:
Originally Posted by rslifkin View Post
It'll pull air into the vent hose to make up for the fuel being used, pulling the fuel from the vent back into the tanks. So no issues. But realistically, you won't want the tanks that full, as you'll end up spilling fuel just from sloshing as the boat moves, or if the weather warms up and the fuel expands a bit.
Haha, yeah of course! Sometimes my brain is having a vacation
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Old 24-02-2021, 18:49   #5
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Re: Tank vent - what if I top the tanks up to the filling cap?

Quote:
Originally Posted by andreas.mehlin View Post
Im running new hoses in a new tank setup. I understand there must not be any dips in the vent line....but what happens if I top up my tanks so even my fill hose gets filled? Cause then there will be fuel standing in the vent hose as well. So what happens when running my engine?

And yes...the vent fitting sits higher than the fill cap
Depending on engine, 60 to 80% of fuel is returned to the tank. That fuel will be much warmer than when it went through the injection pump and therefore much expanded. It is extremely likely that you will leave a trail of fuel in the water from your vent. This cost a friend of mine a $10k fine in Wilson NY.
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Old 25-02-2021, 09:12   #6
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Re: Tank vent - what if I top the tanks up to the filling cap?

JUST FOR CONSIDERATION -

The US DOT, Canadian DOT and IMO require a maximum filling of a fuel tank filled with DIESEL to be NO MORE THAN 95% capacity of the TANK.

The official wording is .. NOT LIQUID-FULL AT 55C.

This is to allow for expansion of the fuel due to atmospheric pressure or temperature changes.
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Old 25-02-2021, 09:29   #7
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Re: Tank vent - what if I top the tanks up to the filling cap?

All vent lines should have a loop that goes ABOVE the through hull. This way, if fuel gets in from inside, it drains back in (hopefully) and if water gets in from outside, it drains back out (hopefully). The loop above should prevent any fluid from resting IN the vent line.
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Old 25-02-2021, 09:57   #8
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Re: Tank vent - what if I top the tanks up to the filling cap?

Preparing for a long cruise filled tank to the max on a cold day. When it warmed up a day later boat was surrounded by a fuel slick from expanding fuel leaking out the vent. Had to run engine for an hour to lower fuel level in tank. Fuel dispersed/evaporated before the CG could haul me off to prison.
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