'66 D200- Vented Fuel Cap or Vented Tank

[JimmyMustang]

I've seen a couple posts about this on the forum but this is my first old truck and so I'm trying to clarify.
When I bought my truck, it had a non-vented fuel cap. It definitely wasn't stock and every time I opened it.....Swoosh!
I assume all kinds of bad things can happen with that kind of pressure in the heat of Texas so I drilled a little hole in it, until I could get a new "Vented" Cap.
Well, the inside of the new cap is identical to the one that I drilled a hole in and its even more air tight. I even had gas splash back up at me.
What am I missing? Does the tank have a vent tube? Could it be blocked?
Thanks in advance!

[PwrWgnDrvr]

It is a common misconception that the vent lets air out. It does not. It lets air in,otherwise, as the gas is pumped out the vacuum created in the tank would collapse and destroy it. Every tank must be vented some way or another. Your vented cap has a check valve in it and only allows air to flow in. Learn to open the cap slowly to relieve pressure buildup if hot weather is a problem. Your hole drilled in the cap is going to leak fuel when cornering with a full tank.

[dodgeboykim]

I put a non vented locking cap on my 66 way back in 1976. Guess what. The gas tank collapsed while driving down the highway at the worst possible time. Going over a long major bridge.

[Kaegi]

what year is it? my 61 truck have a non vented cap and a vent line that comes off the filler neck and crosses over to passenger side then up into pillar then back down and out through floor. my vent line was plugged. I was getting the same problem as you till I unplugged it. you can just look at filler neck inside cab and you will see the vent line. My later trucks had no vent line and ran a vented cap. vented cap should release pressure and allow air in.

[dodgeboykim]

My truck currently has no vent line and the original tank filler didn't either.

[JimmyMustang]

what year is it? my 61 truck have a non vented cap and a vent line that comes off the filler neck and crosses over to passenger side then up into pillar then back down and out through floor. my vent line was plugged. I was getting the same problem as you till I unplugged it. you can just look at filler neck inside cab and you will see the vent line. My later trucks had no vent line and ran a vented cap. vented cap should release pressure and allow air in.

It's a '66 D200. The previous owner replaced the filler neck inside the cab. He did it because he said he could smell gas.
My original question and thought was when he did that, is it possible that he removed the vent line?
If not and a '66 D200 came stock a vented cap, maybe the new cap I bought online is not the correct one either.
Maybe a better topic question would have been: "How do I tell if a gas cap is vented?"

[PwrWgnDrvr]

If your tank has not collapsed, its vented.

[Jims68]

If your tank has not collapsed, its vented.

What causes the pressure? If the fuel is being sucked out by the fuel pump, then why does it "blow" when you remove the gas cap?

[PwrWgnDrvr]

If your tank has not collapsed, its vented.

What causes the pressure? If the fuel is being sucked out by the fuel pump, then why does it "blow" when you remove the gas cap?

The vent is one way....IN only, to keep from creating a vacuum and collapsing.
Now think about basic physics:
At night, air cools and contracts, including what is in the tank. The one way vented cap allows outside cool air to go IN to the tank, since the cooling tank air has created a vacuum and the one way vent maintains equilibrium.
When the daytime temp rises, the cool, dense air in the tank expands as the tank heats up. This expanding air creates a pressure that is not relieved by the one way vented cap and it releases when the cap is removed.
Modern vehicles allow for air both ways, thru various filters and systems to capture the fuel vapor that would otherwise be released as a pollutant along with the pressurized air.
U can watch this yourself by taking an empty one gallon jug or can, such as a paint thinner can, put the lid on tight in the middle of the day, then look at it first thing the next morning. It will be partially collapsed. Then try the opposite: Put the lid on before dawn, when it is the coldest, take it off mid afternoon and it will "whoosh" air out.