Technical Gas Cap Pressure

I have noticed lately that when I take off the gas cap on my 34 Dodge that it has a lot of pressure build up that releases. Is this okay or should I put a different type of cap on? Thanks for the info.

 

Pressure or vacuum? If you have the stock tank you have to have a vented cap so the tank can breathe both in and out but more in.
When the engine draws gas out of the tank there has to be air let in to take the space the gas vacated or it creates a vacuum in the tank that will get to the point where the vacuum reaches the point the fuel pump will no longer pull fuel out. On the other hand if the car is sitting out in the heat and the heat expands the gas in the tank it can build up a bit of pressure that has to have somewhere to go.

 

It should be vented to the atmosphere, either in the cap or a tank vent.

 

It is definitely pressure, The tank was built by the guy that built my car. I do remember at the end of last year I forgot to put my cap on after refueling and my buddy gave me a cap. It is probably not vented. Off to th auto shop again! Thanks

 

If the cap was not vented and there is no external vent to the gas tank, you'd only be able to drive a short distance as the vacuum created inside the tank would prevent gas from being successfully drawn up the line by the pump. The car would stall. After you remove the gas cap the pressure inside the tank would equalise with the atmosphere and allow the pump to draw gas again. Put the gas cap back on and it'll happen again after a period of time. Check that the gas tank vent hose is above the top of the tank and in a loop and that it vents external to the cabin at a lower level near the bottom of the gas tank, additionally check that it is NOT blocked.. Remove it and blow it through with compressed air to clean any possible blockages.

 

EFI will build pressure in the tank, without the proper venting.
Pre smog carb motors ran a vented cap.

 

Not necessarily… I had a vented cap fail to vent and the fuel pump collapsed the tank! The car stalled all right, but because it ran out of gas. With the tank sucked down to about half of it's capacity, the fuel gauge didn't read right.