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MG MGB Technical - Vented Petrol Cap

Not sure my cap, UK car is venting properly,can anybody advise how to check?

Kevin.
Kevin Jackson

If the vent is blocked, drive until it starts misfiring from fuel starvation then remove the cap. If you get a big WHOOSH it is blocked, and especially if the car immediately starts right up and runs well again. However one of mine makes a slight 'gasp' when I refuel when on a run, so don't confuse that with it being blocked. What is the problem anyway?
Paul Hunt 2

Hi Paul,

The problem was not apparent until I accelerated at full power for the first time, and after about 5 seconds the engine cut out, and a few seconds later restarted whilst on the 'overrun'. Itried it again with the same result.

What mkes me think that the cap isn't venting properly, is that when pumping out some old fuel a while ago the tank started to "implode" from the vacuum created, so removed the cap and all was well, but as that was some time ago I had forgotten until doing the full power test, The fuel pump is a new pointless type and should be adequate for the V8.

Mine is the lockable type and I cannot see where the vent is, any idea?

Kevin
Kevin Jackson

There are 2 sorts of lockable cap. One is vented, one isn't. My car came with a non-vented cap that the PO had drilled a 1/16 hole straight through! Worked, but fuel dribbled out.
So I bought a vented locking cap-under a fiver.

The bit that twirls round to lock it has "vented" moulded on, other than that it looks the same as the old one.
Martin Layton

Thanks Martin,

I'll check mine, don't recall there being any markings on it.

Kevin.
Kevin Jackson

I drilled a small hole in the tank side of my cap. Now, while it vents to the atmosphere, it has none of those problems
Richard Morris

I wonder if anyone else has had the same experience as me.

Long ago I fitted a locking petrol cap on my MGB in place of the plain original.

One very warm summers day I wandered into the garage and noticed an overpowering smell of petrol. The sun had been baking the steel garage doors and the interior of the garage had become particularly hot.To my surprise I found a large and growing puddle of fuel under the engine.

So far as I could tell the heat build up in the garage had pressurised the vapour in the tank and forced fuel through the carburettor float chambers which then overflowed onto the floor. Unlocking the cap and releasing the pressure with a whoosh stopped the fuel loss. I opened the garage doors wide allowing the fuel dry up and dispersing the vapour. One small spark and it all could have been very nasty.

The moral of the story seems to be that unvented petrol caps are potentially very dangerous.

sam christie

There are two types of both lockable *and* non-lockable caps, you have to make sure you get the correct one in both cases. Vented caps usually have a spring-loaded valve which results in a small amount of vacuum developing in the tank before it opens and lets air in, and this is what results in the 'gasp'. This valve is supposed to prevent fuel leakage from the filler in a roll-over. Some filler caps may not have this valve, like my V8, which leaked in the sun and emits no 'gasp'.

American tanks from 1970 or 71 used a non-vented filler as the tank vented via the charcoal canister to trap fumes. If these cars are modified to remove the emissions kit and seal the tank vent they must have a vented cap fitted or some alternative. American tanks from the same date also had an inverted 'can' inside the tank with a bleed valve at the top, so even if the tank was filled right to the top of the filler, after a few moments the fuel level would rise inside the 'can', so lowering the level in the filler, and preventing overflow when the cold fuel from the station tanks heated up and expanded in the car tank. The bleed valve vented to the inside of the tank, not the outside of course, and the 'can' reduced the effective capacity by about one gallon.

My V8 has leaked out of the filler after being filled then parked on the drive, even though the front of the car was pointing downwards. If the filler cap prevents reverse leakage, like the cap valve is supposed to do, then expansion will force fuel to escape elsewhere, like from the float chamber in Sam's case. Fuel can drip from the float chambers due to other causes like 'normal' expansion of the fuel in the float chamber after switching off a hot engine on a hot day, if the fuel level is too high in the float chamber, like from an incorrectly adjusted or partially sunk float - I've had that too on the V8.
Paul Hunt 2

Paul.

Thanks for your detailed explanation, I will check my cap over the weekend and hopefully do some driving now that the steerings cured and the new suspensions on the car. See thread, Out of the fireplace in the conversions section.

Kevin.

Kevin Jackson

Had a look at my cap and it's definitely the vented type so not sure if the cap thats causing the problem.

I'll try the cap off my friends V8 roadster and see how it performs,it's a bit tricky when you live in the suburbs doing a full throttle test even in second gear!

Kevin
Kevin Jackson

"it's a bit tricky when you live in the suburbs doing a full throttle test even in second gear"

Especially when you have to wait for the tank to develop enough vacuum to cause fuel starvation :o)

Even if you have a vented cap the vent could be blocked, so borrowing one (buy one? Perish the thought) or possibly leaving yours loose is the way to eliminate it.
Paul Hunt 2

Yes, and I wouldn't want it to cut out when I was trying to overtake on the open road, where ever that is!

Kevin.
Kevin Jackson

This thread was discussed between 12/04/2007 and 20/04/2007

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