Gas Cap question

Johnny Canuck · Aug 16, 2015

When returning after riding on a hot day, prior to parking in the garage, you could twist off the fuel cap to remove the excess pressure, then tighten as normal and park. This may reduce the subsequent "hissing" and fume concern you have. Might not work, but suggest you give it a try on a hot day.

bonth123 · Aug 16, 2015

After putting the clamp on the line I mentioned earlier, I have not had any fume issues, and the pressure in the tank is way down after I fiddled with the cap adn moved the internal parts with a thin wire. It may have been stuck from lack of use, but it works ok now. As for carb cleaner, I thought it would be safe on the rubber...but if not, then I can always do a solvent dip.

huntingdog · Mar 14, 2021

Can anyone tell me if this is normal operation of the gas cap?
When I open the gas cap of my ST1100 after a ride, or even when it has sat for a week, there is this hissing soung ( air goes into the gas tank). So the cap doesn't let outside air in when the engine is running and this creates vaccum. Is this normal? I think this can make the life of the gas pump hard. Sprayed the vent hole with silicone spray and blowed some air into it to make sure the valve isn't stuck. I've been thinking of drilling a small hole in the gas cap to resolve the problem, but I'm concerned of gas fumes. Any suggestions?

Smudgemo · Mar 14, 2021

Mine doesn't seem to properly vent either. The red hose is clear, but typically pressure builds up in the tank past the point where I'd think it would vent. I'm hesitant to take apart the cap as outlined in one of the tutorial pages until I'm at least sure it's not functioning properly. I was thinking about soaking it in something. I've got a bucket of carb cleaner but will that wreck the internals? Got me.

Upt' North · Mar 14, 2021

huntingdog said:
Can anyone tell me if this is normal operation of the gas cap?
When I open the gas cap of my ST1100 after a ride, or even when it has sat for a week, there is this hissing soung ( air goes into the gas tank). So the cap doesn't let outside air in when the engine is running and this creates vaccum. Is this normal? I think this can make the life of the gas pump hard. Sprayed the vent hole with silicone spray and blowed some air into it to make sure the valve isn't stuck. I've been thinking of drilling a small hole in the gas cap to resolve the problem, but I'm concerned of gas fumes. Any suggestions?

They all seem to do the same.
Just ride.
Upt'North.

Al st1100 · Mar 14, 2021

Smudgemo said:
Mine doesn't seem to properly vent either. The red hose is clear, but typically pressure builds up in the tank past the point where I'd think it would vent. I'm hesitant to take apart the cap as outlined in one of the tutorial pages until I'm at least sure it's not functioning properly. I was thinking about soaking it in something. I've got a bucket of carb cleaner but will that wreck the internals? Got me.

Leave it alone, pressure buildup is normal. You have a hot engine right in front of the gas tank.

Al st1100 · Mar 14, 2021

huntingdog said:
Can anyone tell me if this is normal operation of the gas cap?
When I open the gas cap of my ST1100 after a ride, or even when it has sat for a week, there is this hissing soung ( air goes into the gas tank). So the cap doesn't let outside air in when the engine is running and this creates vaccum. Is this normal? I think this can make the life of the gas pump hard. Sprayed the vent hole with silicone spray and blowed some air into it to make sure the valve isn't stuck. I've been thinking of drilling a small hole in the gas cap to resolve the problem, but I'm concerned of gas fumes. Any suggestions?

If it was a vacuum it would stop running, it's pressure and completely normal. Leave it alone.

Oldbikefixr · Mar 14, 2021

I rode mine home from the dealership...about 90 miles. Checked the bike over really well after a day or two riding, looking for issues as well as enjoying the bike. First time I removed the gas cap, I got that sucking/vacuum sound, but had experienced no running problems with it. I replaced the vent hose and gas cap because the cap vent appeared to be plugged. Same vacuum sound afterward, but didn't seem to be as pronounced. Chose to ignore it.

CYYJ · Mar 14, 2021

When troubleshooting fuel tank venting issues, keep in mind that the cap itself is not the only possible cause of the problem.

The vent hose leading from the cap down towards the bottom of the bike can become plugged at the bottom from road debris (usually mud), or kinked somewhere along its winding path. So, take a moment to disconnect the vent hose from the cap and blow a little air down it - or pour a little soapy water down it - to make sure that the vent hose is not obstructed.

Michael

John OoSTerhuis · Mar 14, 2021

CYYJ said:
The vent hose leading from the cap down towards the bottom of the bike can become plugged at the bottom from road debris (usually mud), or kinked somewhere along its winding path. So, take a moment to disconnect the vent hose from the cap and blow a little air down it - or pour a little soapy water down it - to make sure that the vent hose is not obstructed.

BTW, ST1100 trivia: that little hose has a T connector* inserted inline towards the bottom. It’s not connected to anything. The WoTL over the years is that it serves as a backup vent. This little “issue” comes up every few years when a STrider finds it while venturing into the bowels of their beaST for the firST time. ~~IINM,~~ it can be found by looking closely at the online fiche ~~(forgot which one)~~.

edit: item 23*: LINK

huntingdog · Mar 15, 2021

Checked the hose, it's clear.
Good to know about that T connector

.

John OoSTerhuis · Mar 15, 2021

JuST to be clear, the cap vents both ways (poSTs #12 and #14). While underway, the fuel level drops creating a vacuum that at a certain point the cap relieves. Take the fuel filler cap off immediately after stopping and you’ll hear the rush of air entering the tank. If you juST leave the ST sit without opening the cap, engine heat will eventually pressurize the tank and the cap will release it, often generating a moaning/groaning/humming sound, and a forum query.

John

ST Gui · Mar 17, 2021

John OoSTerhuis said:
often generating a moaning/groaning/humming sound, and a forum query.

jrp · Mar 17, 2021

Smudgemo said:
Mine doesn't seem to properly vent either. The red hose is clear, but typically pressure builds up in the tank past the point where I'd think it would vent. I'm hesitant to take apart the cap as outlined in one of the tutorial pages until I'm at least sure it's not functioning properly. I was thinking about soaking it in something. I've got a bucket of carb cleaner but will that wreck the internals? Got me.

I noticed the same thing a couple of years ago. I disassembled the cap and found that it was clean. It appears it takes quite a bit of pressure to overcome the valve.

John OoSTerhuis · Mar 17, 2021

Hmmm.... there should be a spec for pressure release, and for vacuum. I’m sure there’s a standard for the cap manufacturer to meet. Now to devise a test using my Mityvac... :think1:

John

Larry Fine · Mar 17, 2021

John OoSTerhuis said:
Now to devise a test using my Mityvac...

Have a spare upper tank handy?

STRider · Mar 17, 2021

John OoSTerhuis said:
If you juST leave the ST sit without opening the cap, engine heat will eventually pressurize the tank and the cap will release it, often generating a moaning/groaning/humming sound, and a forum query.

Yes, my 1100 has been moaning and groaning for years. I get concerned if it doesn't!

NotSoLilCrippseys · Jul 23, 2022

I'm just dropping a note of thanks into this years-long thread - and sparing those who know another forum query: I have a new-to-me ST1100 ('92, 40 amp alternator swap) with the "hissing" sound from the cap. It's good to know that it's basically normal. I'm not getting any (or much!) vacuum in the tank, as there's no fuel starvation issue. Today, while working in the yard, I heard it even though the bike hadn't been started in a day. I guess that sitting in the sun on a 90+ degree day led led to expansion of the fuel - and a need to blow off a bit of pressure.

Slydynbye · Jul 23, 2022

NotSoLilCrippseys said:
I'm just dropping a note of thanks into this years-long thread - and sparing those who know another forum query: I have a new-to-me ST1100 ('92, 40 amp alternator swap) with the "hissing" sound from the cap. It's good to know that it's basically normal. I'm not getting any (or much!) vacuum in the tank, as there's no fuel starvation issue. Today, while working in the yard, I heard it even though the bike hadn't been started in a day. I guess that sitting in the sun on a 90+ degree day led led to expansion of the fuel - and a need to blow off a bit of pressure.

Don't let that Beautiful 92 sit in the Sun too much, it fades the Paint.

Bush · Jul 23, 2022

Slydynbye said:
Don't let that Beautiful 92 sit in the Sun too much, it fades the Paint.

Was it the '92, or the '93 that had the candy glory red paint? Anyway, red is the worst colour for sun fade. It will turn pink!

Gas Cap question

Johnny Canuck

bonth123

huntingdog

Smudgemo

Intermodal Man of Mystery

Upt' North

Al st1100

Al st1100

Oldbikefixr

CYYJ

Michael

John OoSTerhuis

Life Is Good!

huntingdog

John OoSTerhuis

Life Is Good!

ST Gui

240Robert

jrp

John OoSTerhuis

Life Is Good!

Larry Fine

STRider

NotSoLilCrippseys

Slydynbye

Will ride for Pie

Bush