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Gas tank under pressure - is this normal? (title edited) [solved]


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I had to fill up yesterday and I noticed that as soon as I turned the key in the gas cap, the thing popped like a shaken up soda can. It did not do that the first time I filled up, but that was also in much cooler temps than we have had lately.

I recently cleaned the airfilter and had the fuel air vent tube pop off, but I put it back on without issue. Am I being paranoid and this is normal or do I need to find out where an obstruction is in the vent lines?

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Shouldnt have any suction like that, so you have a vent problem, depending on the bike either the cap itself is vented or the gas tank is vented. with a vent line running to the charcoal filter.

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From what you're describing I assume you are describing the tank being under pressure, not vacuum, based on your post. The title you chose for the thread is misleading and completely the opposite of what you are describing.

Pressure (a little) is normal, especially on a hot day. Vacuum however, is not normal. Even a slight ammount of vacuum in the tank will make it near impossible to open the cap without prying and likely damaging something. If you do happen to have a vacuum on your tank it means one of the tubes connected to the bottom of the tank is clogged somehow causing the tank not to be able to self equalize pressure with atmosphere.

WARNING, VACUUM INSIDE YOUR TANK IS VERY NOT GOOD!!!!!!

There are documented cases of vacuum inside the tank COLLAPSING the tank inward and basically ruining the tank.

Please clairify your problem so we can help you track down the source of your concern.

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Ah, excellent point. It was definitely under pressure and not vacuum. The thing literally flew open as soon as I turned the key.

I'll double check this in the morning when it is nice and cool and report back.

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My Kawasaki has a black fuel tank and does this on hot days. You can actually hear it venting just a little at times when it is above 90.

Funny you mention Kawasaki. One of my riding buddies has a ZX14 and he said that he has heard it whistle like a tea kettle at times! I think that would seriously bother me.

I tell you, this bike has so much more technology than my Nighthawk.

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I popped the gas cap and there was a little tiny "pfft" and that was it. I am going to chalk this one up as just really high temp causing pressure.

it shouldnt be under pressure in either direction, Im in south Texas, never have any breathing issues like that under any temp. if it flys back again in the heat, Id still look at the venting.

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I got up extra early today to check it because it was so much cooler. Still, I had the pressure from riding. So I popped the tank up and checked the tubes. One of them looked like it was being flattened.. So I rotated the hose on the tank nipple and I think that was the issue.

So it wasn't pinched, but it was definitely restricted.

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