Air hissing noise from fuel filler cap on 456...

I noticed this too, and found many posts about it as a common problem with some mentioning outie-air vs innie-air :|

After getting my 456 couple months ago, I, also, made the sad mistake of tightening my gas cap an extra torque and COULDN'T for the life of me unscrew the cap when it came time to filler-up...looked like a strong dork while wife looks at me and starts to open her little black book
Thing is, I believe mine builds pressure and really becomes an issue if it gas gets too low, like it is trying to consume the last fume of petrol it can.....

 

This is usually pretty normal.

The fuel system pressure is designed to go though the fuel evaporative emission system to be burned by the engine so the gases don't normally vent into the atmosphere.

The warmer the ambient temperature the more noticeable is that pressure upon cap removal.

 

The problem is caused by the fact the fuel is cycled over and over again by the fuel pump through the injection system and back to the tank, as the fuel pump has a relatively high capacity and not all the fuel gets used by the engine, particularly at idle or at low power output.
As the fuel level gets lower in the tank the problem gets worse, so when you need to fill up the fuel in the tank is at its warmest temperature.
The M type uses a heat exchanger between the fuel and the A/C system to cool the fuel returning to the tank (that is the hump in between the two intake plenums), the 550 has the heat exchanger in the tank itself, I believe.

 

That makes a lot of sense.

I remember in 1975 when I imported my Dino, part of the certification process was building a fuel evaporative emission system. All the parts were available from Fiat and Ferrari at the time. It was just the labor to do it all that was tough.

However, I remember after that system was done, everytime the gas cap came off it would release pressure.

I was told if it did not the FEES was not working or there was a leak in the system.

I had to change out the gas cap to a non-venting cap as part of the certification process, and DOT and EPA checked and verified everyone of the required items.

 

There also could be a blockage in the vapor recirculation system charcoal filter.

Steve

 

That is a good point and certainly one worth having checked

 

... bumping this thread up from its last post 5 years ago...

Actually I have just had the fuel filler hose on my 456 replaced, it was leaking and I did not dare filling up the car beyond 2/3 anymore - the fuel smell especially in LH corners was not as awful as it was worrying, and definitely in need of addressing!
No more fuel smell now anymore, which is great, but I do get the hissing sound now when getting gas - sounds like it's normal?
I do notice that the car seems to start better now when warm, can that be related in any way?

 

Just did a quick search through the web, as I believed this hissing to be not normal. What I found looks serious, actually. Looks like the canister may indeed be blocked, and that is an issue, as that uninteresting thing does actually quite an important job. Blockage can lead to different kinds of problems, not in the least a cracked or imploded fuel tank. So I guess I'll get it checked out...

 

All the 456's I know hiss when you undo the tank cap. Normal.

 

I took out my canister and mine still hisses, especially when it is hot outside. Actually, I have noted that the air is coming out, not in...

 

What comes out when one opens the filler cap is not air, but rather gasoline vapor.
This happens only when filling up a car that has been running for a while and the gas level in the tank is low.
If one is filling up with a car that has just been started up and the fuel level is not too low, there is little or no pressure in the tank.
The problem was corrected in the M version by using a heat exchanger for the fuel that returns into the tank.
The heat exchanger sits under the cover in the middle of the fuel rails and uses the A/C as coolant.
This becomes extreme during the summer months, particularly in the South.
During the winter months, in cold climate, the opposite may happen: the fuel in the tank in cold weather contracts and when the cap is opened air may be sucked in for a brief moment.