Gas Cap Mystery

[RRoller123]

I have been tracking down a small gas smell for literally weeks. Found a slight leak under pressure at the CSI, fixed that but the smell didn't go away. No smell under the hood, none inside the car at rest, but slight gas smell in the trunk and in the garage in the morning. No apparent leaks anywhere, after a full inspection of the system. So I have finally found that it seems to be coming from the gas cap. Now this is interesting, because we could smell it while driving. I didn't think the air circulation would allow that, but it does.

I have an FI car (sealed system) and replaced the cap with a new FI cap because the plastic was cracked on the old one. The old one would thread on for about 3-1/2 turns, and then another 1/2 turn as it compressed the rubber seal, the new one only threads on about 1-1/2 turns. And I don't feel it compressing the seal. After putting the old gas cap back on, the smell has gone away immediately.

So I guess I have the wrong new cap, but it seems odd to me. Here are pictures of the whole system. Do these Caps look right to you folks?

Here is the Filler Thread system:



Old Gas Cap:





New Gas Cap:

[Zippy]

The new one looks like a vented cap. Does it have a small hole in the top (other side)? It also looks like there are five threads on the old cap and only three on the new one.
My 78 has a newer cap that looks like your old cap.

[RRoller123]

Thanks for responding, Zippy. It does look like a vented cap, but there is no vent on the outside. It just doesn't thread and seal correctly, so I am wondering if there are different threaded sections?

There are actually 5 threads on each cap. It remains a mystery to me, but I guess maybe I will just go buy a new sealed cap, unless someone has one in really good condition that they want to sell. Odd problem.

[76was124]

The old cap has a two step gasket that straddles the flange on the i.d of the filler tube. Does the new cap gasket sit at the same depth from the open end of the cap as the old cap's gasket's high spot, or as its low spot?

[seabeelt]

Pete, this may seem obvious, but looking at the two caps and your filler neck, I'm betting that the inner lip of the filler neck is hitting the metal center section of the new cap and not letting it screw on all the way ( 1-1/2 turns vice the 3-1/2 turns of the old on) and thereby not making contact with the seal. Your old cap does not have this center section...... Just thinking out loud as they say...

[RRoller123]

Hmmmm... Good ideas, I will do some measuring. Thanks.

[jfrawley]

I see we're discussing a fuel smell. I had that same issue for quite some time. Every morning the garage smelled like a gas station. Found out it was the rubber fuel filler hose. The one my car had on it was not ethanol resistant and therefore the vapors came right through the hose. Replaced that hose with a hose found at NAPA (part number 1047) that is ethanol resistant, and the problem went away that very day, and has not returned.

Worth a look.

[4uall]

This might help as well

http://fiatspider.com/f08/viewtopic.php ... hilit=+gas