Pressurizing Fuel Tank (Fix Pics inside)

[Claviger]

I’ll post some pics so people know what to look out for if they decide to change out their cap in the future.

Makes a lot of sense that it pressurized when you turn off the motor, all the built up line pressure has to go somewhere when it stops feeding the motor, that somewhere being into the tank trapped.

 

[barbagris]

Fancy pants gas cap isolates the area between the actual tank and the opening with no flow between that path and the vent lines.

Which one? so we know to avoid it.

 

[Claviger]

It’s not a rocket part, I’ll have to look up what it came off of.

This was my thread on it, which I’ll update on the “fix”. For anyone considering doing the same.

EXIT: Forgot to link the thread:
Cut 3lb, Gain convenience, add bling One Item

 

[BigNorm]

I’ll post some pics so people know what to look out for if they decide to change out their cap in the future.

Makes a lot of sense that it pressurized when you turn off the motor, all the built up line pressure has to go somewhere when it stops feeding the motor, that somewhere being into the tank trapped.

NO SMOKING!

 

[WyldCFH]

Problem identified

Fancy pants gas cap isolates the area between the actual tank and the opening with no flow between that path and the vent lines.

A small hole will be added, I knew it had to be something stupidly simple.

@WyldCFH MVP for pointing my brain in the right direction

The only reason I though of it, to be honest, was I noticed my aftermarket cap has a seal. CNC Billet Motorbike Keyless For Triumph Daytona 600/650 T595 Rocket III TT600 | eBay

 

[R-III-R Turbo]

Makes a lot of sense that it pressurized when you turn off the motor, all the built up line pressure has to go somewhere when it stops feeding the motor, that somewhere being into the tank trapped.

Think you're confusing liquid pressure with gas pressure there dude

The pressure in your tank is gas pressure from vapour (gas) that can't escape out the breather due to your aftermarket gas cap gasket. Liquid pressure is a different thing

 

[DEcosse]

Not being a smart-ass but not 'pressure' coming out but air going in because of reduced (LESS than atmospheric) pressure (vacuum)

 

[Claviger]

No it’s pressure, I’m positive it’s not a vacuum. It tries to push cap off not hold it down.

 

[DEcosse]

That doesn't compute - gas is being pulled out of the tank reducing its volume relative to the volume of the tank - normally air has to flow into the tank to take the place of that displaced gas.
THAT is the purpose of the vent line. (but also if the tank gets hot - as in summer or from engine heat, without any gas being consumed, gas vapour can be released and this is also dealt by with the vent line and why CA systems have the Evaporator Canister)
If the vent line is plugged, then no air can flow into the tank to replace the displaced gas and it creates partial vacuum.
If you really do have the tank pressurizing positively, then somehow you are leaking compressed air into your fuel system, possibly from your forced air system?
You can do a test if you drill the hole in the cap fixture to open it to the vent - then put your finger over the hose and see whether it is pulling or pushing.
If you are creating positive pressure in the tank, then it might suggest you have bigger problem than it just not venting

 

[Paul Bryant]

My thoughts were that if your vent lines were blocked, or as you say the new cap was cutting them off then the heat from the Engine was "warming" the air inside the tank thus causing this pressurization.