Fuel tank pressure?

Hi Everyone,

I've been smelling fuel fumes for a long time and it's finally been fixed. A small crack in one of the hoses. But an interesting side effect of the fix is that when I open the fuel cap, there's a bit of pressure in the system. As soon as the cap gets near the last threads, it literally pops off. Is this normal?

I've owned the car for a number of years and I don't ever recall that being the case. If it's not normal, is it something to be concerned about?

Thanks

 

I do not recall ever noticing that.
A crimped hose or blockage?

 

I have had that happen. If you fuel the car in cold weather then remove the cap in hot, the fuel vapor inside the tank will have expanded because of the temp change.

When you had the cracked hose it was allowing the pressure in the system to escape so it did not build up.

Modern fuel systems have a one-way valve to allow air in (so the fuel tank won't deform / collapse into itself as fuel is used) but not out because that would be smog-causing unburned fuel vapor. If your charcoal canister is still present it is supposed to soak up fuel vapor and release it into the intake system when the car is run.

 

It's a warm weather phenomenon. Should go away when temps are back in the 60's or lower. Pantdino is correct. Gasoline boiling point begins at 100F, a temp. easily reached in a closed container on a 90-95F day like we have been experiencing here in NY for the past week or two.

 

tanks are vented from a pipe taken off the fuel filler pipe so there should be no back pressure.

 

That's what I thought. Thanks for all the replies guys.

 

Only on a euro version - a US version has a more complex system (sealed at the filler neck & cap).

A small positive pressure is "normal" on a US version as the check valve inside the 3-way valve has a non-zero opening pressure (also, a euro properly "converted" for US importation would too). However, these systems often get removed/hacked/modified without the Modifier understand how the system is designed to work. Do you still have all the components+plumbing shown in this SPC figure?:

Ferrari 246 Dino Parts : Table 49 - FUEL SYSTEM (VARIANTS FOR USA VERSIONS)

The critical gizmos are item 8 (the 3-way valve) and item 12 (the charcoal canister); however, if it's a converted euro, sometimes the valving in item 8 is built into the aftermarket charcoal canister.

 

Hi Steve,

My car is a non-converted US car. And as far as I know, I have a completely stock setup. I'll confirm this with my mechanic the next time I'm in for a service.

Thanks for the reply,

Manny

 

Don't quite follow this description -- are you saying:

1. It is stock US version that has never been modified, or

2. It is a euro version, now in the US, that has never been modified, or

3. Something else?

 

#1

 

Then all is probably OK (if all the components + plumbing are still present per the SPC figure).