Morning All


Does anyone ever suffer from pressure in the fuel tank on hot days? went to put fuel in yesterday and it blew the cap out of my hand!


Cant see any breathers either which cant be right surely?


Had a quick search before posting this and cant find anything so hoping its nothing serious...


Thanks in advance..


Rob

I am led to believe that there is a breather built into the filler cap---if there wasn't, you would suffer fuel starvation because the tank would 'de-pressurise' as you used the fuel up (from the tank). Check that the breather isn't blocked/jammed. I have never run into this problem, but it needs to be sorted before we get to the 'hot' (!?!) part of the year.

Yeah I had a look at the filler cap but couldn't see anything but having said that I didn't give it close inspection.......Just looked on VDL and my cap doesn't look like any of them, wonder if its the wrong cap with no vent in it?

My fuel cap is obviously vented, with the valve protruding in the center, but I see Axel G offers them with & without vent, for Fiat 500. Strange as there is no other vac/press outlet...

For the money they cost I think I'll just buy a new one.....


Really surprised me, when I started to loosen the cap it hissed but was really surprised how much pressure it held to literally blow it out off my hand.....

May be a daft question but isn’t the vent to allow air in and not out? I am ready to be knocked down on this.....

The Whitakers said:

Does anyone ever suffer from pressure in the fuel tank on hot days? went to put fuel in yesterday and it blew the cap out of my hand!

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Some people here would pay good money for that built-in, labour-saving, easy-starting, fuel-pressurising modification; beats blowing into the tank any day.

I think tintin is right and that the breather in the cap will be a one-way, inlet valve. If so, the normal action for any vapour-pressure on a hot day could only be to push more fuel along the supply pipe towards the fuel-pump. I think there should be sufficient surplus capacity in the system to accommodate that expansion as long as there's no blockage in the pipe or stiff valves in the pump.
I don't think any of that actually applies in normal circumstances and that you've just been caught in a"perfect storm" of a very empty tank on a very hot day; obviously there is less space for vapour when the tank is full and it would normalise when the temperature dropped.

My tank is also under pressure. No obvious breather / valve in the cap. I always undo the cap slowly and job done.

I've had other cars where pressure builds up in the tank. Never been a problem. Maybe an advantage with regards to better fuel flow?

R

Ditto here.
On a hot Summer day here (30degC +) I get a definite pffft when I undo the filler cap.

I always thought that the breather in the cap let air in, not out.

Chris

Very interesting responses thank you......


I will look at it in more depth and report back...

Just another thing on the extreme pressure improving fuel delivery....not sure I am happy with all that pressure on the pump diaphragm as a constant scenario.......


If the diaphragm failed is there a potential for the fuel to be pushed through to the engine oil? not had one in bits as yet to understand the fundamentals of its design.


Rob

Regarding the diaphragm failure scenario, yes, with the original setup petrol will enter the crankcase and contaminate the oil. However the Axel G fuel pump rebuild kit has a neat development; TWO diaphragms separated by a spacer with a drip hole in it. Should one of the two membranes fail, you'll be warned by either petrol, or oil, dripping out of said hole.

As a motorcyclist, I just KNOW tanks need to be vented both ways, just imagine a bike & its tank parked for a couple of hours in the noonday sun...
If in doubt about a 500's fuel cap, I'd just drill a 1.0 mm hole through it, problem solved

giardini said:

Regarding the diaphragm failure scenario, yes, with the original setup petrol will enter the crankcase and contaminate the oil. However the Axel G fuel pump rebuild kit has a neat development; TWO diaphragms separated by a spacer with a drip hole in it. Should one of the two membranes fail, you'll be warned by either petrol, or oil, dripping out of said hole.

As a motorcyclist, I just KNOW tanks need to be vented both ways, just imagine a bike & its tank parked for a couple of hours in the noonday sun...
If in doubt about a 500's fuel cap, I'd just drill a 1.0 mm hole through it, problem solved

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Exactly, My VW did this and syphoned the tank of fuel into the oil and that wasn't even under pressure.......


Biker myself and fully understand too hence my concerns.....


Will have to look into those rebuild kit's, you wouldn't have a link at hand would you please? TIA if so...




Rob

There you go Rob, fuel pump kit: https://webshop.fiat500126.com/en/fuelsystem-und-carburetor/fuel-system/fuel-pump-repair-kit

DMI

Or fit an electric pump?

Whoops time to duck again....

Having just tanked-up with 18 litres in a very empty tank and not getting a "pop", it got me thinking. Rather than fuel vapour pressure kicking the lid off, might it be that a blocked or missing inlet valve on the filler cap is causing a vacuum, which sucks in the fuel tank walls to an extent. At the moment the lid is loosened air would rush into the tank with a clack as the walls straighten out.
With the tank being in front you wouldn't want an actual breather hole to positively expel vapour because the constant smell would drive you nuts.

fiat500 said:

With the tank being in front you wouldn't want an actual breather hole to positively expel vapour because the constant smell would drive you nuts.

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Just flip open the heat flap then, get the full 2-stroke high Ahhhh A bong on wheels !

giardini said:

Just flip open the heat flap then, get the full 2-stroke high Ahhhh A bong on wheels !

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I have a heat flap???????

The Whitakers said:

I have a heat flap???????

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You should have two but unless the delivery tubes are damaged there's no direct connection with the front locker area. However, the thoughtful design of the bulkhead, with its many holes and gaps ensures a plentiful supply of petrol tainted/enhanced? air if you have a leaky tank or fuel cap.

fiat500 said:

You should have two but unless the delivery tubes are damaged there's no direct connection with the front locker area. However, the thoughtful design of the bulkhead, with its many holes and gaps ensures a plentiful supply of petrol tainted/enhanced? air if you have a leaky tank or fuel cap.

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Ah I now know which flaps (always make me giggle) you mean and yes they have no delivery tubes........


Leaky fuel tank is something I don't have ...