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-   -   Fuel tank suction (http://www.mastercraft.com/teamtalk/showthread.php?t=31238)

rudaire 07-15-2009 12:39 PM

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

bigmac 07-15-2009 01:30 PM

Quote:

Originally Posted by rudaire (Post 614630)
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.

rudaire 07-15-2009 10:57 PM

do you hear air rushing as you unscrew the fuel cap?
 
Quote:

Originally Posted by bigmac (Post 614663)
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

rudaire 07-16-2009 11:14 AM

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

mcmx2 07-16-2009 11:25 AM

You should have no air pressure either way. The tank is vented on your boat. The vent on some is on the outside of the boat. On others it is vented back up to the cap and the cap is vented. It does not matter that you have a fuel injected boat it is vented to the outside air somewhere.

bigmac 07-16-2009 11:28 AM

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.

rudaire 07-16-2009 12:44 PM

Quote:

Originally Posted by bigmac (Post 614988)
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

bigmac 07-16-2009 01:07 PM

Quote:

Originally Posted by rudaire (Post 615034)
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..

Although not free-vented, in and out, to the outside air like boat systems are. I'm sure that the EPA is concerned about atmospheric venting of fuel vapors in boats, just like they were in automobiles. Problem is that in this case they have to tussel with the US Coast Guard and it looks like the Coast Guard is winning. So far.

I had that problem on a previous boat. It was non-fuel injected and the vent line was plugged. It DID stop working until I looked at the vent line and saw that it had an excessive amount of hose, and had tightly kinked. Unkinking it did the trick on that particular boat

shunra 07-16-2009 01:21 PM

Quote:

Originally Posted by rudaire (Post 614858)
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.

rudaire 07-16-2009 03:01 PM

Quote:

Originally Posted by shunra (Post 615049)
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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