LCV Fuel Tank Vacuum

Unsure if this is (cold) weather dependant or the sign of another problem. Six month old LCV Jimmy and when I unscrewed the fuel tank cap there was  a rather loud hissing noise as air was sucked into the tank. Now the weather was frosty, the tank was almost empty after a Motorway run and I just thought the hiss of air sounded rather unnatural?  Can't say I ever noted this sound before and wondered if there's a blocked or trapped vent? Maybe the garage would say "Ah Sir, they all do that"! Any thoughts from more knowledgable forum members?

I sometimes have that, not regularly but enough to not worry about it.

I also get that, mostly only happens when the tank is quite low and you have been on a decent run. Perhaps you have let you tank get lower than usual so don't normally get it. I, however am alway sqeezing the last bit out of it 

Normal for it to hiss when opening the cap. I suspect the noise changes depending on how low the tank is as there will be more space for air. 

On modern cars the fuel tank is sealed for emmission regulation reasons.

This is to prevent fuel vapour being released to the atmosphere and being considered a pollutant. As "spare" fuel is returned to the tank from the high pressure fuel pump there is a constant fluctuation in the pressure in the tank, or indeed vacuum if there was a lot of vapour in the tank and the temperature suddenly dropped and the vapour condensed to fuel. 
The normal pressure relief system from the fuel tank routes via the EVAP canister under the bonnet. This canister consists of active carbon to clean the vapour and return it either direct to the atmosphere or to the exhaust manifold.

Depending on the timing of you opening the fuel filler you will be interrupting this process and will hear the pressure release.

To add to what Martin said-

There is a one way valve at the cap to let air into the tank as fuel is used up, but it can't let anything out. (no fumes, or fuel if there is a roll-over)

There is a one way valve in the tank vent to the evap canister that only lets fumes out of the tank into the evap as the fuel heats up.

(The vent is in the deadspace at the top of the tank that you can't normally fill with fuel. If you overfill the tank by rocking the car and continually topping up, and don't immediately use the fuel, when the fuel expands in the heat neat fuel goes out that pipe into the evap, and when it purges at the next startup the extra fuel gets into the engine and it missfires for a while)

Anyway, after a run most likely there is a slight negative pressure in the tank, not enough to open the valve and the hiss is air going in.

The MOT tester unscrews the cap and listens for the hiss to check the cap is effective and the hose isn't split (section 6.1.3)

I forgot to say, all modern petrol cars hiss when you undo the fuel cap after a run. Diesels most probably do too but I'm not familiar with the venting systems on diesels. (My 200TDi had a strange anti-rollover valve in the fuel cap with ball bearings and levers IIRC)

 

Just like to thank those forum members who responded and helped educate me about modern fuel systems. I was becoming increasingly concerned that the fuel pump might be under too much load creating a vacuum and that my fuel consumption was going to head from 'not very good' to 'truly awful'. I'm off to the fuel station in the morning - a happier man following your responses - advantages of a forum with participating folk that 'know stuff'.

Never try brimming the tank to get the best mileage. On my Gen.4 it caused the engine to stutter and stop. It quickly restarted and as it only happened after brimming the tank, I stopped doing it.

Roger

Hasn’t happened to my two Gen 4s and I always Brim them.

Maybe a different interpretation of brimming. I would let the nozzle cut out and then remove it enough to trickle some more to fill to the top. Now I just fill until the flow stops and that's it Either way it solved the problem for me

Roger

wrote:

Maybe a different interpretation of brimming. I would let the nozzle cut out and then remove it enough to trickle some more to fill to the top. Now I just fill until the flow stops and that's it Either way it solved the problem for me

Roger
 

The tank bulges up above the filler spout to create a deadspace that you can't fill with fuel unless you are determined and keep going after the pump cuts off- or rock the car up & down to slosh the air out.

If you do fill the deadspace, when the fuel expands neat fuel goes into the evap cannister along the vent, and is drawn into the engine when it purges, making it run too rich, and missfire.