Fuel - Gas - Petrol Cap Vacuum Vented

[john lewis]

u say slight vacuum, mine used to have a little or slight vacuum, last time I bought gas the gas attendant didn't know how to take it off, it was unscrewed just suction more then I ever felt before, if to much gas is going into canister filter would that create a bigger vacuum, thanking u in advance john

[HDRider]

The emission canister is plumbed into the low vacuum port which is in front of the carb throttle plate so it is not subject to full manifold vacuum. I don't think that there is more than a couple of inches of vacuum at that port.

If your tank made a huge sucking sounds it seems more likely that you have blockage in the system somewhere and the removal of the fuel via the fuel pump is creating a much higher vacuum than the design spec.

Edward Hamer
Petaluma CA

[Hasbeen]

When you stop the engine, the fuel tank vacuum should be relieved back through the charcoal canister, via the inlet manifold/crankcase etc.

If it is not, check the vapor separator, & the lines to the canister, but importantly the canister itself. In the 2 of my 7s that still had them, the canister was full of a thin black oily mess, a mixture of charcoal, water, petrol & dissolved charcoal. It would have been doing nothing useful, [if they ever did], & a lot of harm, including hold a vacuum in the fuel tank, if the cap had still sealed completely.

[john lewis]

after hearing of blockage, it registered, I turned down fuel pressure on regulator, I will turn it up and see what happens thanks

[Dave Dyer]

Hi,
Does anyone know how the vented fuel cap should work?
My tank was pressurising and when I removed the cap there was a great woosh and the tank even made a sound as it returned to normal size. I knew there was a problem as my fuel line joints started leaking.

I stripped the cap and can see there are 2 small rubber plugs held in position by springs, both opposite to each other. It looks like when the pressure in the tank increases this moves the rubber plug from its seat against the spring pressure and vents. The other plug works in the opposite direction, when theres a vacuum in the tank it allows air to enter.

I fitted it back to the car but it hasn't cured the problem.
Does it work in the way I've descibed above?
I'm tempted to remove the rubber plugs so the holes are always open.

Thanks for any help,
Dave

[Spike]

I'm interested as well, when I undo the fuel cap there's a small whoosh of air rushing in as well. Doesn't seem to affect performance though.

I do remember a boss a long time ago had a Vauxhall Frontera, and twice the fuel tank vent blocked and actual caused the tank to be crushed by air pressure.

Graham

[Hasbeen]

You may have been supplied the wrong cap.

Here in Oz we have the US sealed cap. Our tanks breath through a vapour separator behind the hatch in the boot, connected to a charcoal canister in the engine bay. The carbs breath through this canister, & the engine breather is plumbed to this & the inlet manifold.

If you were accidentally supplied with our fuel cap, you would be pulling a mild vacuum in the tank.

Hasbeen

[FI Spyder]

The small vacuum does sound familiar. You can hear a high pitched sucking sound when I drive into my garage (I live in a very quiet location) that lasts for quite a long time. I find it hard to believe you have the sealed gas cap unless you bought one off eBay from outside of UK. Usually it's the other way around, N/A people getting UK caps (locking gas caps all seem to be UK venting types) with a resulting sudden unexplained small of gas in garage or even while driving. I suspect it is much more common for the vents getting plugged but not seeing one you're on your own as to unplugging it.