Technical Partial vaccuum in 2.8JTD '03 tank

[blaven]

I've noticed, more often than not, when I come to refuel, there is an inrush of air into the tank when I remove the fuel filler cap. Is this "normal". Is it desirable? Why does it happen?

 

[errolmorris]

Are you sure there is a vacuum?
It is more likely pressure build up within the tank. The filler cap usually has a valve built in that will guard against over pressure.
The fuel filler cap should generally be released slowly to allow the pressure to ease out. It may well be that you are opening the filler cap quickly and the pressure is released in a 'gush'.
The problem with opening the cap quickly is that if there is sufficient fuel in the tank, you risk a blast of fuel vapour whether it be petrol or diesel.
If you have a vacuum, I think you may have other issues, but since I have not experienced a vacuum problem with any fuel tank I have ever worked on, I could be way off the mark.

 

[blaven]

Are you sure there is a vacuum?
It is more likely pressure build up within the tank. The filler cap usually has a valve built in that will guard against over pressure.
The fuel filler cap should generally be released slowly to allow the pressure to ease out. It may well be that you are opening the filler cap quickly and the pressure is released in a 'gush'.
The problem with opening the cap quickly is that if there is sufficient fuel in the tank, you risk a blast of fuel vapour whether it be petrol or diesel.
If you have a vacuum, I think you may have other issues, but since I have not experienced a vacuum problem with any fuel tank I have ever worked on, I could be way off the mark.

Thank you. No, I'm not sure its a vaccuum, and you're probably right. I'll have to check more closely next time I'm filling up. But what causes the pressure build up if that's what it is?

 

[errolmorris]

As the fuel 'sloshes' about in the tank, vapours are given off. Liken it to shaking a soft drink bottle and how the pressure builds...not the best analogy, but I'm sure you get the idea.
Anyway, the less fuel you have in the tank, the greater the volume of gas/vapour build up as there is more space available for the vapour/gas to accumulate. This is where the fuel cap vent comes into play and, if it's working properly, will prevent too much pressure build up.
I have never come across a blocked fuel filler cap, but that doesn't mean they don't exist. Try to remember to remove the filler cap gradually and let the pressure out slowly rather than a rapid removal. You should hear a soft hissing sound as you release the cap and the less fuel in the tank at the time, the greater the volume of gas that will escape.
Sorry for the long winded story, but rest assured it is quite safe and normal for this to happen.

 

[Shingy]

I've had three X244's over many years, and when releasing the cap for refuelling they have all gone "whoosh" with an inrush of air, particularly when the tank is very low, and so I've assumed that's normal.

 

[blaven]

I've had three X244's over many years, and when releasing the cap for refuelling they have all gone "whoosh" with an inrush of air, particularly when the tank is very low, and so I've assumed that's normal.

Thanks Shingy. Looks like that should now be an 'outrush' of air!

 

[Shingy]

You've got me thinking now blaven, as I've always taken it to me an inrush (vacuum) rather than a pressure within the tank being released. Whatever it is it always does it on mine and have never experienced any engine running problems!

 

[blaven]

I've noticed, more often than not, when I come to refuel, there is an inrush of air into the tank when I remove the fuel filler cap. Is this "normal". Is it desirable? Why does it happen?

Aaargh! Just read page 93 of the Ducato Handbook! (While looking up something else).
"IMPORTANT The airtight seal of the cap may lead to a slight increase of pressure in the tank. A hissing sound when the cap is removed is therefore quite normal."
I'll get my coat!