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  #1  
Old 07-15-2009, 12:39 PM
rudaire rudaire is offline
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Fuel tank suction

Just added fuel for first time to 02 xstar (obviously just got the boat..). Gas cap was REALLY hard to remove, and as I was unscrewing it, when the seal finally started to break I could hear a LOT of air rushing by the seal. I don't mean a small psst like popping a soda can. Like 5 full seconds of whooosh. I assume it was air rushing into the tank, and I also assume it was suction created by gallons of fuel being pumped out of the tank and no air being allowed to come into the tank to take that space that the fuel used to occupy.. Is this normal? Should a check-valve allow air into the tank to prevent this? Or is the system designed to run with negative pressure in the tank and a sensor monitors the negative pressure and triggers a check engine light if the negative pressure is lost, indicating a compromise in the system allowing air in, or possibly fuel out???

thanks
dave
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  #2  
Old 07-15-2009, 01:30 PM
bigmac bigmac is offline
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Quote:
Originally Posted by rudaire View Post
Just added fuel for first time to 02 xstar (obviously just got the boat..). Gas cap was REALLY hard to remove, and as I was unscrewing it, when the seal finally started to break I could hear a LOT of air rushing by the seal. I don't mean a small psst like popping a soda can. Like 5 full seconds of whooosh. I assume it was air rushing into the tank, and I also assume it was suction created by gallons of fuel being pumped out of the tank and no air being allowed to come into the tank to take that space that the fuel used to occupy.. Is this normal? Should a check-valve allow air into the tank to prevent this? Or is the system designed to run with negative pressure in the tank and a sensor monitors the negative pressure and triggers a check engine light if the negative pressure is lost, indicating a compromise in the system allowing air in, or possibly fuel out???

thanks
dave

The tank is vented...shouldn't be any negative or positive pressure buildup.
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  #3  
Old 07-15-2009, 10:57 PM
rudaire rudaire is offline
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do you hear air rushing as you unscrew the fuel cap?

Quote:
Originally Posted by bigmac View Post
The tank is vented...shouldn't be any negative or positive pressure buildup.
So nobody else hears a rush of air when they pop their gas cap to refuel?

If I'm the only one, then I assume my vent is 100% plugged... I'm surprised the fuel pump can pump fuel out of the tank under than much negative pressure.. Of course I haven't confirmed yet whether air was rushing out or in while the gas cap was being removed...

dave
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Old 07-16-2009, 11:14 AM
rudaire rudaire is offline
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Please respond...

bump..
Please... anyone with a fuel injected mastercraft is qualified to answer this question..

Do you get a rush of air as you remove your fuel cap to refill? I don't want to look for a blockage in my vent line only to learn the system is designed this way for a reason. Car tanks also have vent lines, but run a suction, which is how they know to trip a check engine light if you're gas cap is leaking...

d
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Old 07-16-2009, 11:28 AM
bigmac bigmac is offline
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Automobile gas tanks aren't vented, boat gas tanks are as required by the USCG.

For my part, and to answer your question, NO, my fuel injected boat does not get any kind of rush of air when I remove the gas cap. If it did, I would be alarmed and start looking at the vent for a blockage.
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  #6  
Old 07-16-2009, 12:44 PM
rudaire rudaire is offline
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Quote:
Originally Posted by bigmac View Post
Automobile gas tanks aren't vented, boat gas tanks are as required by the USCG.

For my part, and to answer your question, NO, my fuel injected boat does not get any kind of rush of air when I remove the gas cap. If it did, I would be alarmed and start looking at the vent for a blockage.
Thanks guys for the responses.. Although it isn't what I wanted to hear.. I'm amazed my fuel pump could continue to pump against that amount of negative pressure, and am equally surprised at how completely sealed my the blockage in my vent must be.. I'll look into the situation more tomorrow. May be a mud-dobber's nest, but I'm amazed at how well sealed it must be..

Auto gas tanks are vented, in that they do allow air to come in to replace the fuel that is pumped out. But the vent is associated with the charcoal canister system that holds the vapors from the fuel and later introduces them into the intake of the engine to be burned..

I'm sure I can find the vent lines coming off the top of the tank in my 02 xstar.. but if anyone has any tips or suggestions for me I'd be eager to hear them.

Again thanks for the responses.
dave
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  #7  
Old 07-16-2009, 01:21 PM
shunra shunra is offline
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Quote:
Originally Posted by rudaire View Post
So nobody else hears a rush of air when they pop their gas cap to refuel?

If I'm the only one, then I assume my vent is 100% plugged... I'm surprised the fuel pump can pump fuel out of the tank under than much negative pressure.. Of course I haven't confirmed yet whether air was rushing out or in while the gas cap was being removed...

dave
You are not the only one. I have the exact same suction sound you described. I have always wondered if it was normal. I don't know where the vent line is though.

Last edited by shunra; 01-16-2012 at 12:44 PM.
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  #8  
Old 07-16-2009, 03:01 PM
rudaire rudaire is offline
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Quote:
Originally Posted by shunra View Post
You are not the only one. I also have a 2002 X-Star and I have the exact same suction sound you described. I have always wondered if it was normal. I don't know where the vent line is though.
peculiar that the only two reported instances of this thus far are on the exact same year and model boat.. I'm going to chase the vent line, which I think I've seen when I pull the center floor panel, this weekend. I ran a tank from full to 3/16 and had no problem with engine performance in spite of the vacuum.. I assume you also haven't had engine troubles caused by this either??

d
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  #9  
Old 07-16-2009, 03:13 PM
rudaire rudaire is offline
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Quote:
Originally Posted by shunra View Post
You are not the only one. I also have a 2002 X-Star and I have the exact same suction sound you described. I have always wondered if it was normal. I don't know where the vent line is though.
peculiar that the only two reported instances of this thus far are on the exact same year and model boat.. I'm going to chase the vent line, which I think I've seen when I pull the center floor panel, this weekend. I ran a tank from full to 3/16 and had no problem with engine performance in spite of the vacuum.. I assume you also haven't had engine troubles caused by this either??

d
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  #10  
Old 07-16-2009, 03:26 PM
rudaire rudaire is offline
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I just spoke to a third 2002 xstar owner who said that 'every now and then he'd hear pressure or vacuum escape from the tank as he unscrewed the gas cap, but it didn't last more than a second'...

May there be a one way check-valve that allows air to enter the tank to replace fuel that leaves the tank, but doesn't allow air (or fuel if the boat sinks) out of the tank?? Maybe the temperature rose during the day and the fuel and air expanded causing the tank to pressurize if a check-valve won't let anything out?..

Can I be sure the vent is literally an open line to the atmosphere on these boats? I also have a scarab, and on that boat the vent most certainly is an open line to atmosphere.
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