Vented or not gas cap

[Sonny]

The previous owner replaced the gas tank in my '70 Belvedere and it does not have a vent. The cap is not vented, and I noticed after a drive that some gas sprayed out at the fuel hose connection.

 

[OH71RRunner]

I think you need a vented cap if the tank is not vented.

 

[RJRENTON]

I believe that the tank IS vented thru a separate fitting on the tank and loop type drvice, not requiring a vented cap. This arrangement is usually for 49 states with California being different requiring a vapor storage/ purge device. All the '71 models had the California type system. This involved a stand pipe collection device at the tank and a vapor line running to the engine's valve cover breather, where collected gasoline vapors were stored in the engine's crankcase and later purged thru the PCV system. The valve cover device also had a nipple connected to the carbs float bowl to capture gasoline vspors. Later models used an activated charcoal canister for vapor storage and fuel tank venting. This charcoal canister was purged at a controlled rate thru the intake manifold vacuum. To reiterate, I believe the fuel tank should have a NON VENTED cap. If you are experiencing fuel spillage, it is likely that the vent pipe/vapor storage system is clogged or restricted. Because of the large surface area of the fuel tank, it is possible to generate a vacuum in the tank and possibly collapsing the tank or starving the fuel pump because it has to work harder.
This, of course is my opinion.
Bob Renton

 

[Ranger16]

The tank on my '69 Road Runner is vented and thus uses a non-vented cap so I'm sure your '70 Belvedere is as well.
The diagram below is cropped too much, but the vent line is in the upper right. The 3rd arrow from the right is where it comes through the floor of the trunk. It makes the loop (far right arrow) in the trunk and then vents to the atmosphere where it exits the trunk (middle arrow). I suspect yours is similar if not identical.

 

[Sonny]

The tank on my '69 Road Runner is vented and thus uses a non-vented cap so I'm sure your '70 Belvedere is as well.
The diagram below is cropped too much, but the vent line is in the upper right. The 3rd arrow from the right is where it comes through the floor of the trunk. It makes the loop (far right arrow) in the trunk and then vents to the atmosphere where it exits the trunk (middle arrow). I suspect yours is similar if not identical.

I'll check again but the tank they installed does not have any vent connections that ive seen and I've looked. Underneath or lines that loop in the trunk. Worst case it doesn't have one and I buy a vented cap?

 

[Dennis H]

Vent lines in mine go very high almost to the rear shelf deck height. It is necessary that they do.

 

[69L48Z27]

If you open the gas cap after a drive and it pushes air out like the tank is pressurized, then you need to either fix the vents or have a vented cap. Vented cap will still keep some pressure built up though.

 

[Ranger16]

I'll check again but the tank they installed does not have any vent connections that ive seen and I've looked. Underneath or lines that loop in the trunk. Worst case it doesn't have one and I buy a vented cap?

Oh, you had the tank replaced. That's may be a different story. If it is not vented, then you definitely need a vented cap.

 

[Vermont Rock]

I understand that the vented & non-vented caps had different attaching configurations that necessitated different fill pipes. I know I went through this with my 70 Road Runner before realizing that mine was vented like the California versions that ran a line up to the air cleaner. The caps even look different.
Just my thoughts. Tim