gas cap

[TNT]

What kind of gas cap do I need, when I turn off the car the gas cap hiss's I think its under vacuum, I guess

 

[addo]

What year of car? Many have a vented tank - yours may be blocked.

 

[woodbutcher]

Yep,make sure that you have the correct cap and also that the vent is open.I saw the results of an incorrect gas cap many years ago.The guy had his gas tank collapse from vacuum not being relieved by the UNVENTED cap.
Leo

 

[TNT]

ok sorry I didn't give all the info its a 68 mustang with a new tank new gas line and there was only one line to the carb. so if there is a vent where would it be I think the cap needs a vent

 

[addo]

There is usually a little ¼" spout near the filler neck.

 

[MustangSix]

There won't be a vent on a 68. The cap is vented.

If you have a non-vented cap it will seal tightly and create a vacuum in the tank. In some cases, the tank could actually collapse, but most of the time, the car will simply stop running as the fuel pump is unable to overcome the vacuum seal.

 

[jamyers]

You can often drill a small (1/16") hole in the center of the gas cap to make a vent.