MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGF Technical - Tank Vacuum

I'm sure there is a simple answer but can someone please explain why there is always a vacuum when I undo the perol cap on my F? As there is a breather in the neck how does a vacuum occour? Perhaps the breather is blocked.

Thanks for any comments

Tom Grayling
Tom Grayling

Tom , A possible breather pipe kinked or blocked sounds the most likely cause.
Mike
Mike

Tom,
Does your cockpit smell of petrol too ?

Stew.
Stew B

It's a normal function of the pressurised fuel system used in fuel injected cars. Think it's even mentioned in the driver's handbook?

Tim
Tim Jenner


Tim, I think you've hit the nail on the head and are correct on both counts.
Paul Nothard

Mine does exactly the same thing and I must agree with Tim, it is a function of a pressurised fuel system.

Also I would say that if the tank was not venting properly, then the fuel injection system would not be able to get the required quantity of fuel, which would obviously be noticable where engine performance is concerned. Either that or the fuel tank would implode!!
Ross

The fuel system in the MGF is designed to prevent fuel vapour escaping into the atmosphere to comply with regulations. The fuel tank filler breather pipe returns to the top of the tank. The evaporative canister is also connected to the top of the tank via a vent line. The canister has a purge valve which allows fumes/fuel vapour, which have collected in the charcoal filter when the engine is not running, to be drawn into the engine at speeds above 1800 rpm and only then when the engine is warm.

So, fuel vapour in tank expands and is collected by evaporative canister. The canister has a pipe to atmosphere but after the vapour has been filtered, so no emmisions there.

Canister is purged only when the engine is warm and running. 75 degC /1800 rpm.

Air can only get into the system via the canister. So the purging of the canister causes a slight vacuum in the system when the engine is running. This vacuum remains once the engine is switched off.

I think!

Bruce
Bruce Caldwell

Thanks guys for your quick responses.

Yes I do get fuel fumes inside but just live with it by keeping a window open when not in use.

I thought that perhaps the vacuum would adversley affect the performance, but if that were the case I guess I would have been asking the question long ago!
As long as you all get the same on your car then I reckon I will go with Bruce's excellent explanation.

Thanks

Tom
Tom Grayling

The fumes thing is apparently normally to do with tank sealing rather than any breathers - there was an (unofficial?) recall on it. I'm sure there's stuff in the archives about it.

Tim.
Tim Jenner

Tom,
Don't smoke whilst you drive or your F will really go like a bomb!
Tony H

Nitreous---for a moment!
Tom
Ps Dont smoke fortuneatly!
Tom Grayling

This thread was discussed on 29/05/2003

MG MGF Technical index

This thread is from the archives. Join the live MG MGF Technical BBS now