I have a nice shiny new cad plated gas cap with a new chain and a new piece of rubber protect-o flap thingy on my Z. IT looks just beautiful. Very OEM. However It does have one little drawback. During track days, on most left turns, if my tank is even somewhat full, I have a stream of gas that pours down the side of my right rear quarter fender. This mixed with dust, dirt, and tire debris are quite a unsightly and it stinks. So if I decide to go crazy, is there an modern aftermarket gas cap that would keep the gas in the filler neck better than the OEM one. Perhaps my replacement cap (from motorsport) is just not quite up to OEM standards.

Any comments or recommendations on a functional gas cap, that, you know actually CAPS the GAS!

I don't have documentation back to 71, but I've messed with later year fuel caps and if the OEM cap is functioning properly, I don't think it should be leaking like that.

Back to at least 72 there was a check valve in the cap that was supposed to let air in (to replace gasoline taken out to run the engine), but not let air back OUT.

Are you sure your check valve is working properly? If you've not ever messed around with one of the caps, it's the spring loaded metal disk on the inside center of the cap. If you pry that disk away from the cap, it opens an air passageway to the little radial bumps bent into the outside of the cap. The ridges that run perpendicular-ish to the grab handle?

On edit - Found some pics. here's the cap where you can see the air intake passage bulges:

Here's the air intake passage from the side:

Here's the check valve. I pried it up with a screwdriver:

And here's the air intake hole on the under side of the cap where the air intakes get to the edge:

Edited September 21, 201410 yr by Captain Obvious

Page EF-22 of the 73 manual page has a decent cutaway dwg of the cap. It also describes how to test the check valve:

1. Wipe clean valve housing and have it in your mouth.

2. Inhale air. A slight resistance accompanied by valve indicates that valve is in good mechanical condition. Note also that, by further inhaling air, the resistance should be disappeared by valve clicks.*

3. When valve is clogged, or when there is nor resistance, replace cap as an assembled unit.

Nice, huh?

* I used #2 from the 74 manual because it's funnier. The other two are from 73.

Edited September 21, 201410 yr by Captain Obvious

If the collar that the cap attaches to is not sealed well against the "rubber ... thingy" you can get a slosh leak even if the cap is sealed tightly. Fix: remove the collar and run a bead of silicone seal all around it, re-install. Been there, done that!

Was going to ask that question too. I've taken my 72 Z to a few Solo events with a bit more than a quarter of a tank, and it leaked out. Gas cap was about 5 years old. Bought a brand new Nissan gas cap, new flap, went to another event, still leaked. I'll try a bead of silicone, as you suggested, and let you know. Last event next Sunday. ....... thanks

Also keep in mind that it may not be the cap. I can be the seal between the tank filler neck and the cap. These filler necks get pretty brittle with age and they can crack as well at that interface.. this too can cause fuel spills right under the cap…

FWIW,

Carl B.

If the collar that the cap attaches to is not sealed well against the "rubber ... thingy" you can get a slosh leak even if the cap is sealed tightly. Fix: remove the collar and run a bead of silicone seal all around it, re-install. Been there, done that!

I can second that. Had it happen on my track car. Make sure your sealant is rated for gasoline use.

Marty

Also keep in mind that it may not be the cap. I can be the seal between the tank filler neck and the cap. These filler necks get pretty brittle with age and they can crack as well at that interface.. this too can cause fuel spills right under the cap…

FWIW,

Carl B.

Pretty much describes my physical condition!!! The (used) fuel spills can be especially embarrassing.

Thanks gents, I am humbled by your knowledge and witt, as usual. I will give it a go and perform all the tests mentioned this friday. I will include some pics. If I recall, I think the replacement cap from MSA did not look exactly like my OEM, I may be remembering that wrong. I have been cleaning so many roadster parts lately my poor Z is feeling neglected. She needs a drive.

Any luck trouble shooting?

I just two brand new caps from nissan. They're still around. Before u buy from msa. Check the dealer. It's usually cheaper too.