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Discussion Starter #1
When I first got the car second hand, it looked like the tank was dented in from underneath, and every time I took off the filler cap it would suck in air. Now, whenever I take off the filler cap, it blows out air, and the tank is round and bulging, so much so that it has bent the strap holding it to the body. As I live on a farm, my road clearance is determined by my bulging fuel tank.

Where is the regulator (if there is one) that determines the pressure in the tank and how does it work, and is it something I need to clean or replace.

Thx, Grantley
 

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i know with some cars, the fuel cap is the pressure release etc. might need a new one...check yours and make sure its clean and all the seals are intact.
 

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Discussion Starter #3
checked the fuel cap; I can blow and suck through the key slot quite easily, so whatever is causing the problem cannot be a slow mechanism like engine using fuel etc, it must be some sort of pressure regulating pump that is working at full blast - or this is my thoughts anyway - is there such a pump or pipe connected to intake manifold that can do this?

Thanks
Grantley
 

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Which year and engine is your 307? Petrol or diesel? All pumps suck from the tank and normal atmospheric pressure should equalise the pressure inside the tank.

If you have pressure within the tank, and your car is a petrol version, have a look at the heat shield to the front right of the tank.

However, to inflate a tank, the pressure has to be pretty huge - I'd check the retaining strap.
 

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Discussion Starter #5 (Edited)
The retaining strap (strap under tank) has been bent by the pressure, the last few days, my tank seems to be under mild vacuum; so do not know what causes the pressure at times, mild vacuum like now, or at other times enormous vacuum causing the tank to look like it has been dented in by about 1/3 of its volume. The moment I take off the cap, there is an enormous movement of air (direction depending on whether under pressure or vacuum), and then tank returns to normal position. In this position, there is a space of about 20mm (3/4 inch) between strap and tank.

The cap breathes when I apply vacuum or pressure to it from my mouth, so whatever is causing this, must be producing faster than the cap can dissipate the effect.

Petrol 2005 307 2.0 SW
 

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I’ve been thinking about this. Let’s say that your car does 40mpg = 1 litre every 8.8 miles/ 14Km. At 60mph, it takes you 8.8 minutes to remove a litre of petrol from your tank. That’s a flow of 113 cc per minute or ~2 ccs a second, which is nothing really. If you can blow through it, your flow of air through the cap is probably OK, but see later.

If the tank swells, the only possibility is that cold fuel inside the tank is warming and creating a vapour pressure faster than 113 cc a minute; that’s why I asked you to check the heat shield. (If you live somewhere hot, it could be the local air temperature.) This seems unlikely.

The next problem might be your means of testing the cap. You may be able to blow through it whist it is in your hands, but can you blow through it when it is in place?

Another test is to remove the rubber sealing washer (if you can do this without damage) and see if the effect is still there. I don’t recommend this except for a short time (i) it will fail an MOT (ii)it is there because if you turn the car over, petrol does not splash out. If this is making too good a seal, then this could be the problem.
 

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Discussion Starter #7
Having had a look at the fuel tank, I found that the breather pipe goes into the main filler pipe; effectively making the system a sort of closed system. Two pipes go forward from the tank (cannot see exactly where on the tank, but towards the top), one seems to be the fuel line, the other line goes from the tank up to the wheel well of the front right hand wheel (I have a right wheel drive), there it goes into a box that according to a peugeot parts program I have calls it "canister depollution absorber", from this box it goes into the inlet manifold of the car. This then explains the high vacuum I see in the tank (assuming it allows too much air to suck through to the manifold). What I do not understand is what causes the pressure in the tank at other times.
 

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Discussion Starter #8
Just pulled out the "canister depollution absorber" which sits just above the right front wheel. Found 2 problems,
1: the pipes were connected the wrong way around on the device.
2: The breather from this canister was totally blocked with mud.
Washed out this breather, put the pipes on correctly, so now will need to see if the fuel tank problems are over.
 

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Just reading this old post.
Having a problem with vacuum in tank when i remove filler cap, sucking air in.
307 1.6l petrol. Doesn't seem right to be doing this.
Checked the de-pollution canister and valve seem ok.
Anyone had same problems?
Thanks all
 
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