Petrol cap hiss - caused by vacuum or expansion?

[youngsyr]

Following on from a discussion in the MX-5 forum, can anyone finally put to rest whether the "hiss" that you get when you unscrew your petrol cap is caused by:

a) the vacuum from the removel of fuel that you've used since last fastening it; or

b) the expansion of petrol/petrol vapour following putting cool petrol into your tank and it being heated to ambient; or

c) either, depending on the circumstances (full tank vs empty tank)?

[Roger Dodger]

Unsure, I usually get a mighty hiss from a jerry can which is full (therefore expansion).

p.s my mk1 MX5 sounds like a vortex when opening the cap!

[StottyZr]

I've always presumed vacuum.

[hondafanatic]

Just before you open unscrew the petrol cap, light a match, hold it right up against the seal and watch which way the flame goes as you unscrew.

If the flame gets sucked into the filler, then it's a vacuum. If it flickers away from the filler, then it's expansion.

Let us know how you get on.

[Munter]

hondafanatic said:

Just before you open unscrew the petrol cap, light a match, hold it right up against the seal and watch which way the flame goes as you unscrew.

If the flame gets sucked into the filler, then it's a vacuum. If it flickers away from the filler, then it's expansion.

Let us know how you get on.

Although if you could use some "disco smoke" that method may work...

[Mr MXT]

OP - Why do you assume that the petrol going into the tank is cooler than ambient?

[AUDIHenry]

I don't recall last time I had this. I believe California cars (and maybe others now) have a charcoal system to recycle the fumes. I always thought the hiss was expansion.

[LuS1fer]

I thought it was to do with the system being pressurised for fuel injection.

[youngsyr]

Mr MXT said:

OP - Why do you assume that the petrol going into the tank is cooler than ambient?

Because having filled jerry cans in the past I have seen the effects of expansion when I've opened them and assumed it must be due to temperature changes as there were no other variables.

[Mr MXT]

If the product has come straight from a refinery rather than via a terminal, it can be "hot".

Just to add a bit more confusion.

[Sam1990]

I always seem to get more of a hiss with a near empty tank after having not filled up for longer than usual. Perhaps it's the fumes that have accumilated escaping?

[Mroad]

It's fumes escaping unless you have poorly/blocked EVAP system in which case it's the other way round.

[Gaygle]

a) the vacuum from the removel of fuel that you've used since last fastening it

This ^^

Fill up your tank right to the top, use all the fuel and then open the fuel cap and you will get a massive hiss.

AFAIK this is because the small amount of air in the tank when it was full (ie. at normal atmospheric pressure) has now been "stretched" to fill the majority of the tank (ie. the air in the tank is now at low pressure) now the fuel has gone and so when you open the fuel cap, air rushes in to equalise this pressure (ie. to equalise the higher pressure air of the atmosphere, with the lower pressure air of the fuel tank.

[J4CKO]

Everyone makes that noise when they are get a well earned drink !

[thinfourth2]

I think its a sign your tank vent is blocked

[bqf]

'Tank vent'??

The only vent system is in the fuel filler cap - if the tank is overfilled and is very hot, fuel can escape through the filler cap. Never happens, of course, given that as soon as you drive off you start 'making space' in the tank!!

The hiss is the air rushing in because it's at a higher pressure than the air in the tank (which was put very eloquently by an earlier poster)...

[thinfourth2]

bqf said:

'Tank vent'??

The only vent system is in the fuel filler cap - if the tank is overfilled and is very hot, fuel can escape through the filler cap. Never happens, of course, given that as soon as you drive off you start 'making space' in the tank!!

The hiss is the air rushing in because it's at a higher pressure than the air in the tank (which was put very eloquently by an earlier poster)...

So what happens in cold weather when there is not even heat to make the fuel evaoprate and you keep sucking out the fuel.

What stops the tank from crushing like a beer can

[bqf]

thinfourth2 said:

bqf said:

'Tank vent'??

The only vent system is in the fuel filler cap

So what happens in cold weather when there is not even heat to make the fuel evaoprate and you keep sucking out the fuel.

What stops the tank from crushing like a beer can

The vent in the fuel filler cap?

[Munter]

bqf said:

The vent in the fuel filler cap?

But if there is a vent in the cap to equalise the pressure. Why is there a pressure difference?

[bqf]

Is this the bit where every car owner in Europe comes on here and say 'my fuel tank has a separate tank breather'??

It'll be quite funny if that happens