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MG MGB Technical - Fuel tank filler cap

I have a 72 roadster which I have rebuilt, with a new fuel tank, fuel lines etc. I am using a lockable fuel filler cap that came with the car, not sure if it is original equipment or not. However my question is this, I noticed that the cap did not have a rubber seal ring, and the cape was loose and rattled when locked on. I made a neoprene seal ring and it locks and forms an effective seal, so much so, that now the tank is pressurising and an expanding depending on temperature etc. Should I revert to not having a seal in the cap and letting the loose filler cap act as a breather or should there be some other set up for a tank breather. Any advice would welcome.
Regards
Graham
Graham

Graham,

The cap should be a vented type. With the tank sealed the fuel pump will be trying to pull a vacuum and if it is effective enough the tank will collapse. After running the car remove the cap and you will probably hear the inrush of air. Changing the cap is the answer.

Geoff
Geoff King

Not to dispute Geoff, but I know my 71 Roadster is supposed to have non-venting fuel cap, which it does. However, I had a similar problem as you describe...fuel tank pressurizing. In my case, the tank would pressurize in the sunshine, and fuel would then vent to the charcoal cannister, in the engine compartment. The result was a potential explosive mess, and for some reason the engine would quit at this point too. I solved the problem thusly: removed the tank, and inspected the vent system. The vent pipe enters the tank, makes a 90 degree bend, and then is 'open' for vapors to enter the pipe. Sadly, my vent pipe was sagging, about 3 inches into the tank. So when the tank full gas, or more than about 5/8's of a tank, and the tank heated up in the sunshine, raw fuel was forced into the vent pipe and to the cannister. I fixed it by bending the pipe so that it's now at the top of the tank. Another part of my solution was re-orienting the fuel/vapor separator in the trunk. A previous owner had moved the vapor separator and it wasn't oriented correctly. So I'd be sure it's installed as originally intended. My problem is now apparently fixed. Good luck!

dave
David

Dave,

Tanks in the UK don't have fuel/vapour separators. The tank has two connections, one to pour the fuel in and the other for the pump to suck it out. If the filler cap is not vented air cannot enter to replace the fuel the pump has removed and there will be a vacuum.

Geoff
Geoff King

Even with a vented cap the tank will develop some pressure/vacuum because the vent includes a spring-loaded valve to prevent fuel seepage in a roll-over. Thus if you remove the cap after a decent run you will probably get a 'gasp' and this is normal (it's also very useful for stopping siphoning when you have to change the pump at the roadside ...). If you inadvertantly fit a non-vented cap you will get fuel starvation when the vacuum in the tank overcomes the suction abilities of the pump.
Paul Hunt

This thread was discussed between 28/02/2002 and 04/03/2002

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