My 1997 Fiesta 1.25 occasionally smells quite strongly of petrol from the filler cap area. There are no leaks from the filler pipe or any evidence of petrol on the floor at all. I don't think that the seal on the cap is faulty because when you release the cap you get a "pssst" from it.
I am correct in thinking that the Fiesta (along with most modern "eco friendly" vehicles is supposed to collect it's unburnt petrol vapour in a canister. Then when you run the engine a valve controlled by the PCM opens to release the vapour into the inlet manifold where it is burnt and passes out through the cat as normal exhaust etc?
Suppose this valve was stuck closed and the only place for the vapour to escape was through the seal in the filler cap when the pressure was high enough (IE with a full tank that has been rattled around in hot weather).
Just a thought - anybody any experience of a similar fault?
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The cap sounds OK, so it is likely that the latest warm weather has caused vapour to be expelled via the carbon cannister. This comes out under the car and can be smelt wherever the wind blows it.
The carbon only absorbs so much vapour and anything more is expelled unless the engine is run to purge it. It is possible that the cannister has dropped off or there is some other rupture in the breather system.
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Cant add much to say why you smell petrol, but you may possibly have a problem with the fuel tanks venting system. That hiss you hear could be air ENTERING the tank. Drawing out fuel into the engine is causing a vacuum in the tank.
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Interesting I thought the the Carbon Filter was only used to collect the vapours form the inlet manifold and nothing to do with the Tank. Your Psst noise is of concern as this is not a pressurised tank and either you have a block vent pipe or the cap is supposed to vent. I may as previously mentioned be a vacuum which would indicate something is wrong. Is it a ford cap, can you find the vent pipe, These vent pipes often have an inversion valve in them so in case the car rolls over is closes the vent pipe to reduce spillage. I have replaced a couple of the these on Nissans with that problem. Check it out even ask Ford. Regards Peter
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The carbon filter collects vapour from the tank(which is sealed to atmosphere)and vents it to the engine when it is running.
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Subsequent to my first comment, there are some incorrect comments. The hiss noise when releasing cap is normal. It is fuel vapour escaping from the tank, as it is designed to maintain a slight positive pressure.
The vapour cannister does vent to atmosphere and thereby vents the tank, otherwise the tank would distort or rupture if vehicle was left in the hot sun for a long time. The cap is not vented because this would fail legal regulations for new vehicles.
There is a separate vent in the filler which allows vapour out of the tank when refuelling. This is sealed off when the cap is fitted.
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>as it is designed to maintain a slight positive pressure.
What puts the slight "positive pressure" in the tank?
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A valve restricts inflow and outflow slightly. The pressure normally becomes more positive when you run the car, due to warm fuel being returned to the tank and the vehicle generally warming up. When cooling down overnight it may go negative but because petrol is so volatile it is positive much more than negative.
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ahhhhhh right
and is this enough to pressurise the tank even when the volume of fuel inside is going down? ie being consumed?
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The answer to your question is 'yes' but I suspect you don't understand the physics of vapours very well? Very little if any air needs to be drawn into the tank as the level goes down because of the volatility of petrol.
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Perhaps I should just mention before someone else does that if the tank is completely sealed, then a partial vacuum can be created when a lot of the fuel is used and stop the engine from running.
Air does need to be drawn in then but the major flows in and out will occur due to temperature variations e.g. standing in the hot sun for hours and then going into a cold garage at night.
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