Gas Tank Pressure

[cowsaredelicious]

- 1960 Impala convertible 348 3x2

Hello again, I noticed the other day when I stopped to fuel up that when I went to take the cap off the tank that there was a significant amount of pressure built up in the tank. I would dare to say an excessive amount. When the cap was removed the pressure was significant enough to get the attention of another person at the station.

I'm looking to get a different gas cap that has built in vent but i'm wondering what is normal and should I be looking at venting the tank in another manner?

The tank is new on the car and there has been very low miles since the rebuild was complete.

Any advice would be appreciated.

 

[Don Jacks]

It sounds like you've got a non-vented cap.and your car must have a vented cap in order to work properly.You may need to go to a dedicated vendor,such as Show-Cars,since most cars went to the non vented type with the advent of the charcoal canister system in about 1969.

 

[63impalass409]

What he said

 

[Fathead Racing]

My understanding of the vented cap is that it will let air pass but one way. That way is into the tank to prevent a vacuum but not out to the atmosphere. I don't think there is enough pressure created to cause problems. Have I been wrong all these years? It's happened before, believe it or not.

 

[bobs409]

My Jeep is terrible for this! Especially when it get down low like 1/4 tank, it sounds like a tea pot ready to explode! I put a vented locking gas cap on but it sure doesn't seem to let any pressure OUT.

 

[La Hot Rods]

Not sure that you can buy a cap that will vent to the outside anymore.

 

[Don Jacks]

Drill a hole in the center of the rivet that holds the "ears" on the cap all the way through the cap.About 1/16 inch should do it.Bob,your Jeep needs work! There's a canister solonid that controls the tank pressure.Find out what's wrong with that part of the tank venting circuit.

 

[Carmine]

Had this same issue on my '64. An awful lot of pressure. I did what Don suggested. Drilled a few holes. No more pressure, Carmine.

 

[models916]

How does it get to be pressure when the fuel pump is removing fuel?

 

[Fathead Racing]

You won't have this problem in the winter months in northern climates. It's all about the air in the tank expanding with the heat.

 

[blkblk63ss]

My understanding of the vented cap is that it will let air pass but one way. That way is into the tank to prevent a vacuum but not out to the atmosphere. I don't think there is enough pressure created to cause problems. Have I been wrong all these years? It's happened before, believe it or not.

You are correct.....I will leave my vented cap as is ,fuel vapor''s stay in tank.and gas will not evaporate out of tank.

 

[Carmine]

Does maybe gas sloshing around in the tank also create pressure?? Not sure, Carmine

 

[Don Jacks]

Not really,Carmine.

 

[DonSSDD]

It may be sucking, not blowing? As the tank empties.

 

[bobs409]

It may be sucking, not blowing? As the tank empties.

 

[skipxt4]

 

[63impalass409]

It is sucking pressure. On cars like my 63 it uses a vented cap and in 65 they actually use a non vented cap as it has a vent in the tank which I had plugged mine for some reason when installing so when I drove it around the block to put gas in it I undid the cap on my 65 I couldn't believe the suction pressure in there , figure much further the tank would have sucked in. At work we have big shop vac things that use a 55 gallon drum as a canister and one time one of them got a little dent in it so when powered up it sucked the drum flat so that's the first thing I thought of when I undid my fuel cap.