Author Topic: Gas Tank hissing, blowing fuel, boiling fuel when hot!  (Read 487 times)

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Offline David

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Gas Tank hissing, blowing fuel, boiling fuel when hot!
« on: August 10, 2019, 04:04:53 pm »
Hello Concours Community,
This is my first post. My occurrence of potential tank venting issues was dangerous! While taking an MSF riding course yesterday, I rode my 2003 C10 around the course in the hot sun. After a few hours, the tank started hissing. I opened the tank cap and fuel sprayed out! After it stopped spraying, I looked in the tank and the gas was boiling (bubbles, froth). What would cause this? I heard it might be a tank vent problem (same thing happened with my chainsaw a few weeks ago...unclogging the tank vent fixed it). Is there a tank vent line I should clean out? I appreciate your suggestions. I'm particularly interested to hear from Steve in Sunny Florida (he did a great job on my carbs!). Thank you.
« Last Edit: August 10, 2019, 04:08:08 pm by David, Reason: Realzed that I\'d responed to a tank vent post; I didn\'t make a new thread »

Offline batboy

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Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #1 on: August 10, 2019, 07:45:33 pm »
Not sure what to make of the boiling... but, the tank is vented. If this is the standard US version, take the seat off and look at the back of the tank, you'll see a vent hose coming off the tank, That's the tank vent. There's another vent/drain that goes to gas cap filler area (in case fuel is spilled while pumping gas).
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Offline Steve in Sunny Fla

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #2 on: August 10, 2019, 08:08:00 pm »
The tank vents through a series of poppet valves in the gas cap. If they fail to work, I have seen tanks develop vacuum OR pressure. You're going to need the address the valves by cleaning them, spraying with some silicone, etc, or actually removing on or both valves. I removed the bottom most valve from my gas cap. The caveate is that if the bike falls over, gas can seep out. HTH, steve
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Offline David

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #3 on: August 10, 2019, 08:17:51 pm »
Thanks batboy and Steve in Sunny Fla.  I will check the vent hose at the back of the tank and the other vent hose which goes to the cap. I will also check the cap, try to clean it, etc. I appreciate your help!

Offline JPD

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #4 on: August 11, 2019, 12:18:01 pm »
Not sure why the gas was boiling but it sound like it was making more pressure than the vent system could handle. The vent was working as you could hear the hissing. The passages in the cap for the vent are very small and are there to equalize the pressure in the tank. A large over pressure may take some time to equalize. There are 2 check valves in the cap so the air will only flow where it is designed to.

Offline RWulf

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #5 on: August 11, 2019, 02:49:43 pm »
David, if I remember correctly the only tanks that had 2 vent hoses were California models.
The rest of the 49 state models had only one vent hose. That hose was a drain hose from
around the gas cap, in case you spilled gas while filling up. The California also had a insulating
pad on the underside of the gas tank.
As gas will boil at 102 degrees Fahrenheit according to Siri ( I thought it was higher). Where
you are at you indeed may be boiling the gas while riding slowly. Check for that insulation
on the underside of the tank it may be missing.
By the way most of the tanks hiss some with temperature changes, and you do need the tank
to vent inward to avoid a vacuum from stopping gas flow out to the carburetors.

Offline David

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #6 on: August 12, 2019, 01:09:38 am »
Thanks for all the advice. I removed the gas cap, took it apart and cleaned it (per SSF's advice). Perhaps my bike is a CA bike, as it does have a tube on the back of the tank and one going to an electronic device on the underside of tank (has vent hose and a few wires), and an exhaust gas re-breathing apparatus on top of the engine, which I've removed and capped off (per advice I found on COG). However, there is no insulation on underside of the tank. Anyhow, I will ride and see what happens with the gas/tank on the next hot day. Thanks again to all!

Offline bajasam

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #7 on: August 12, 2019, 03:39:29 pm »
Dont worry about boiling gas if you were riding real slow in a safety class all day, not a big deal, you should hear a XR650 with a plastic tank on a hot day!makes you want to break out the pasta an cook some dinner.

Offline connieklr

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #8 on: August 12, 2019, 03:54:45 pm »
Thanks for all the advice. I removed the gas cap, took it apart and cleaned it (per SSF's advice). Perhaps my bike is a CA bike, as it does have a tube on the back of the tank and one going to an electronic device on the underside of tank (has vent hose and a few wires), and an exhaust gas re-breathing apparatus on top of the engine, which I've removed and capped off (per advice I found on COG). However, there is no insulation on underside of the tank. Anyhow, I will ride and see what happens with the gas/tank on the next hot day. Thanks again to all!

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Offline jelbon

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #9 on: August 15, 2019, 10:14:00 pm »
There is a bit of cowling/baffle that is above the exhaust headers between the valve cover and triple tree. If it is missing exhaust heat would go directly up under the gas tank, even more so once the radiator fan kicks on.
Bit of a long shot, but worth a peek.

Offline Pbfoot

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #10 on: August 16, 2019, 01:53:30 pm »
California tanks have 3 vents at the back of the tank.
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Offline Woody

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #11 on: August 17, 2019, 07:30:46 am »
uh oh, i may need to be looking for a California tank, just bought an indiana bike in Cali.

Offline David

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #12 on: August 17, 2019, 05:06:21 pm »
I really appreciate everyone kicking in suggestions on this issue. It's great to be part of the Connie community! Regarding the hose to the electronic unit on the underside of the tank, yep, I figured it had to be related to the gas gauge, but not sure why a vent hose would go to the gas gauge electronics. I blew into that hose and air came out from the electronics unit area. So I've got 2 vent hoses: 1 at the back of the tank and another going to the gas gauge unit. No insulation on underside of tank. I'll take a look for the suggested cowling above the exhaust headers next time I have the tank off. I've cleaned out the tank cap and we'll see how it performs on the next hot day, riding slow. Thanks again. Stay safe out there.

Offline Bud

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #13 on: August 17, 2019, 05:58:22 pm »
Quote
not sure why a vent hose would go to the gas gauge electronics.
It's there in case the fuel gauge sending unit leaks.  With that in place, the leaked fuel isn't dripping down on a hot engine. :-\  HTH
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Offline David

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #14 on: August 18, 2019, 02:49:18 am »
Thanks Bud. I didn't realize the fuel gage electronics unit processed any fuel, that there's a sending unit in there. Yep, gas leaking on top of the engine would indeed be bad! I appreciate the tip!

Offline batboy

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Re: Gas Tank hissing, blowing fuel, boiling fuel when hot!
« Reply #15 on: August 18, 2019, 12:17:24 pm »
My understanding is the vent hose (it's actually a drain hose) under the  tank is for the gas cap area in case you spill a little gas during refilling operation, then the excess gas is drained away by the hose. It's good you blew through it to check for blockage. Sometime insects will plug up drain and vent hoses.

Yours is not a Californification bike, you'd have three vent hoses (the third one would be on the left side of the tank right at the back where the seat covers the tank).
« Last Edit: August 18, 2019, 12:22:15 pm by batboy »
1988 Kawasaki Concours ZG1000 - Ninja Edition
Bike has the usual accessories and modifications