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Old 09-04-2009   #1
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Fuel cap

My Fuel cap has a pin hole at the centre which is quite strange, is there supposed to be something fitted on the cap, a screw perhaps?

I am sure the fuel would start to evaporate otehwise, let me know if there is something fitted on the fuel cap as someone told me there hsould be an air intake to the tank
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Old 09-04-2009   #2
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Re: Fuel cap

With out that hole you would not get fuel to the front of the car it allows a tiny amount of air into the petrol tank without you would form a vacum and nasty things could happen to your petrol tank
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Old 10-04-2009   #3
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Re: Fuel cap

oh I see so you mean this hole is necessary and a standard thing on the fuel cap?

Wont the fuel evaporate faster?



Quote Originally Posted by allen fletcher View Post
With out that hole you would not get fuel to the front of the car it allows a tiny amount of air into the petrol tank without you would form a vacum and nasty things could happen to your petrol tank
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Old 12-04-2009   #4
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Re: Fuel cap

Quote Originally Posted by Shanil View Post
oh I see so you mean this hole is necessary and a standard thing on the fuel cap?

Wont the fuel evaporate faster?
No - not even in a warm climate.

You must have a means or ventilation of the engine will stop.

It will be fine!

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Old 09-12-2019   #5
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Re: Fuel cap

Since the world around me has cooled down recently I have noticed a smell of petrol when I first get into the car. I think I tracked it under the bonnet to the fuel cap. I dismantled the mechanism inside it which turns out to be a rubber disc which the vapour-pressure inside the tank will push up to seal the tiny vent hole. When fuel is drawn by the engine this pressure will be reduced to the point where the rubber disc is drawn away from the hole and allows entry of air. The equalisation of air-pressure is obviously essential in order to allow fuel to flow.
I have smoothed the underside of the cap and cleaned the rubber and give it a slight wipe with Vaseline.
But the smell remains.
Always a theorist, I have decided that this might be happening because there is currently very little evaporation from the petrol and therefore a reduced internal pressure inside the tank. So I'm putting off buying a new cap for now. But I am interested to know if anyone else has a similar experience?
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Old 09-12-2019   #6
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Re: Fuel cap

Mine when it is sat wrapped up warm, always smells of petrol inside the car. There are no obvious signs of leaks anywhere. As you say it is much stronger inside the car than outside. The only joint inside is where the rubber pipe from the tank joins the copper pipe and that looks fine.

I could be suffering the same problem as you where the pressure is dropping off therefore the seal you describe is opening up and allowing it to vent from the cap. Especially when you take into account that I have this little problem where fuel seems to drain back from the carb when it has been sat for a while, which sort of implies I might have a little air leak on one of the joints somewhere and pressure is dropping off? As you are aware itís only happened since I put the 650 engine in with the additional ĎTí piece, so lots of additional joints could be the cause, I need to give them the once over and check they are nice and tight.
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Old 09-12-2019   #7
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Re: Fuel cap

The inside of my car also has the smell of petrol. I've replaced the entire fuel line, but still have the smell of petrol!
In the summer I removed the fuel tank and checked it for leaks by blocking the breather on the cap and blowing through the petrol outlet. Fortunately, my lungs are still in good shape for someone of my advanced years! No leaks were found so I reinstalled the tank and as an experiment put a piece of insulating tape over the fuel expansion hole in the cap. The smell of fuel stopped and the insulating tape started to lift and expand above the expansion hole. The expansion hole is necessary to allow for temperature changes that affect the volume of the petrol and the vapour that is given off. In hot weather more space will be needed, in cold weather the petrol will have less vapour given off. If there wasn't a hole in the cap the petrol tank could buckle or leak and the engine would suffer from fuel starvation. I think I may need a new fuel cap as the metal disc in the fuel cap expansion drilling may not be seating properly and be allowing vapour out all the time (not sure if that's normal). The speedo in my 500F is missing the expensive rubber moulding that creates a seal between the bulkhead and the luggage compartment, so that may be making matter worse!
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Old 09-12-2019   #8
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Re: Fuel cap

funny leaks.....
for a long time I was trying to work out what was living in the roof of an extension... it sounded like pigeons, or was is wasps...
but it was too regular.... banging the ceiling did nothing...
people came they listened.... "cut a hole we ned to know".....
Then a few wees ago a guy came to pick up some bits.. and I said listen to this.....

He replied yes I know that.. it's air bubbles....
Basically I had capped two pipes on the hot/cold while I was doing some work on a property.... using those push-fit things....
He explained (as there were no water leaks)
the water is actually being pulled back through the pipework... creating a small vacuum in the system...
Air was being sucked past the rubber O ring in the push fit end cap to equalize the vacuum, obviously in the form of bubbles, which amplified by the pipe and ceiling made the noise...

Moral of this story... A leak or such may not be in the most obvious place or be the most obvious thing....
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Old 09-12-2019   #9
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Re: Fuel cap

Quote Originally Posted by Vitesse View Post

........Especially when you take into account that I have this little problem where fuel seems to drain back from the carb when it has been sat for a while, which sort of implies I might have a little air leak on one of the joints somewhere and pressure is dropping off?
I think your leak-back problem is partly normal, although it never takes me more than three short spins to get fuel through to the carb. I don't know the working arrangements of the solenoid valve you had in your later carb, but might that hold at least part of the secret to that particular problem.
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Old 09-12-2019   #10
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Re: Fuel cap

Quote Originally Posted by Hobo1960 View Post
The expansion hole is necessary to allow for temperature changes that affect the volume of the petrol and the vapour that is given off. In hot weather more space will be needed, in cold weather the petrol will have less vapour given off.
I don't think the hole is intended to allow fuel vapour out. My reading of it is that it should only work to allow air into the tank.

It's useful to know that you've been onto this before me though....great to hear about your elderly lungs too. Tony @Vitesse works out his lungs on Luigi, but his are usually tested in reverse....sucking petrol through the pipework
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Old 09-12-2019   #11
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Re: Fuel cap

Quote Originally Posted by Bigvtwin996 View Post

Moral of this story... A leak or such may not be in the most obvious place or be the most obvious thing....
Good advice...my mind is open. Recently I did a lot of additional pipework so my first thoughts were the sender-unit gasket, a bit of pipe-soldering I did and the additional pipe-joins I made.

But I've checked all those and they seem good and my setup has no pipe-joins inside the passenger compartment in any case
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Old 09-12-2019   #12
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Re: Fuel cap

Quote Originally Posted by fiat500 View Post
I don't think the hole is intended to allow fuel vapour out. My reading of it is that it should only work to allow air into the tank.

It's useful to know that you've been onto this before me though....great to hear about your elderly lungs too. Tony @Vitesse works out his lungs on Luigi, but his are usually tested in reverse....sucking petrol through the pipework
I got sick of the taste of petrol and invested in that little hand pump thing in the end!

Youíve got me thinking as well, so if the concept is to allow the air in, to equalise the pressure? Obviously the fuel is gravity fed essentially to the fuel pump. I suppose if there is a pressure rise in the tank due to heat, then the extra pressure is going to release itself by pushing more fuel down to the pump and effectively to the carb. Under what circumstances would you expect the vent to open on the cap to allow air in?
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Old 10-12-2019   #13
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Re: Fuel cap

Quote Originally Posted by Vitesse View Post
Under what circumstances would you expect the vent to open on the cap to allow air in?
....good point...

In warm conditions for certain, there's going to be a positive vapour pressure above the petrol in the tank because the rubber disc in the cap will be forced to seal the pinhole.. But I can't see that ever becoming dangerous because at a certain point the pressure will prevent any further evaporation; just think about a bottle of liquid gas...it's the same situation. If the valve in the cap is working properly, no vapour shouldget out.

As the fuel level in the tank reduces, (and as you say, with the vapour-pressure actively helping the flow to an extent), the volume of vapour in the tank will increase, the pressure will equalise with atmospheric until eventually, the volume above the fuel could get to a negative pressure......but, at this point the rubber disc in the cap gets sucked downwards, (or pushed downwards by atmospheric pressure if my schoolboy physics serves me right), and air rushes in until equilibrium is reached. This inward flow of air will continue as long as fuel is being consumed. As long as air is drawn in, fuel vapour can't leak out.

Then you stop the car, park up and we're back to square one.

(at least, that's what I think)
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