Gas overflow from filler

[trogdor1138]

My '71 240Z is leaking gas from the filler, specifically the actual port. I can fill the tank just fine with no leak to the ground. However, after sitting for a few minutes at home (and I believe even probably while driving) fuel begins to seep between the tank cap collar and the filler neck, sending fuel down the side of the car. Today it was bad enough that a puddle formed.

I believe that the cool gas from the pump is heating up and expanding as I drive home. The question is how can this be dealt with? I'm guessing a return line blockage?

- The tank was boiled out and cleaned at a radiator shop a few months ago
- Rotted hoses from fuel tank to evap tank were replaced
- Carbs were cleaned and rebuilt with new valves
- Fuel is not leaking from carb bowl overflows
- Gas cap is new part from MSA
- Fuel pump is warm/hot after car has been running, even with spacer installed

The leak has gotten bad enough to bubble and lift pant on the rear quarter panel. Not a huge deal since the car needed new paint even if I decide to restore this one, but it indicates that this is a problem that needs to be fixed.

Thanks all!

[GreenZZZ]

The stress of the gas tank work might have caused a crack in the filler neck. Mine lasted 30 years before cracking. Other thing to check is if the gas cap has an rubber gasket in it or the spring loaded pressure relief valve in the cap is stuck open.

[Captain Obvious]

I don't have a 71 FSM loaded, but if the 71 has the same emissions setup as 72, then I don't think there's a pressure relief on the gas cap. When the gas tank pressure goes above 10mmHg, it's supposed to force the flow guide valve open and pass the gas vapors into the crankcase. (EC-14 and EC-15 of the 1972 manual)

Maybe your flow guide valve is stuck shut?
Maybe the vent line between the tank and the flow guide valve is clogged?
Maybe it's simply a split in the rubber filler neck?

Does the tank "woosh" out some vapor when you pop the cap after sitting in the sun on a hot day?

[GreenZZZ]

I don't think there's a pressure relief on the gas cap.

Negative pressure!!!


Symptom is full tank of gas will leak out the crease on both side of cap when making right turns.

[Captain Obvious]

Negative pressure!!!
Symptom is full tank of gas will leak out the crease on both side of cap when making right turns.

Haha! Gotcha! The negative pressure relief will certainly do that if the seal is dead. Good call!

I recently had a friend that didn't believe there was any sort of vent in the cap at all. Didn't believe me until I showed him. I think Datsun did a neat job of incorporating that feature into the cap.

[GreenZZZ]

So trogdor1138, check your gas cap negative pressure valve gasket

[Captain Obvious]

And, as stupid as a question as it is (seeing as it's a new cap and all) I just have to ask.

Are you sure that the rubber seal that's supposed to seal the cap against the filler hole is in good shape and is contacting in the correct location?

I know, I know...