Loud thump

[Noah96]

When I was driving my truck back from college this past weekend on interstate my truck just died then I heard what sounded like a rubber mallet hitting the inside of my gas tank. Headlights didn't flicker at all or voltage didn't drop just died then the large thump came from the gas tank. Truck started right up after I came to a stop like nothing had happened. Any thoughts to what may have caused this?

[CG]

... ?

[HO455]

Just guessing here, but possibly your tank vent is plugged (fully or partially) and the fuel pump was pulling a vacuum in the tank and when the engine died (from lack of fuel) the vacuum was released and the tank popped back to normal. Or the tank popped out of shape from the vacuum before the engine died.

[hjewell2]

what HO455 said

[Noah96]

Its a 1969 GMC where can I find the vent on the tank?

[Fbks_Cruiser]

Did you by chance happen to replace your gas cap recently? I believe a 69 gmc should be using a vented cap. If so, maybe you got a cap for a later model with a vented tank. (Cap not vented). Or maybe the vent on the cap is plugged. If air is not allowed into the tank as gas is consumed the tank will suck in a bit. Your truck will run until the fuel pump can no longer overcome the vacuum in the tank. The thump you heard was probably the tank either collapsing a bit or returning back to shape. If it happens again, loosen the gas cap and see if you hear air sucking, your truck should then run.

[Noah96]

Yea I bought a stant cap off of amazon

[dmjlambert]

Your fuel pump pulled as much gas as it could out of the tank, without any air going into the tank to replace the gas. It reached the point of not being able to pull any more gas to deliver to the carburetor and then died. The fuel tank then popped back into shape. In 1969 there was no complex ventilation such as charcoal canisters under the hood connecting to a separate air line that goes to the tank. You probably just have the gas line coming from the tank and that is all. So that means you need a gas cap that will let a little bit of air in. The vented caps let air in, and don't let fumes out unless the pressure exceeds a certain amount such as from heat expansion when the parked truck is sitting in the sun.

[Sparky dave]

Tank vent plugged same

[slocityk20]

Sounds like you burped the fuel tank to me

[68 P.O.S.]

Originally Posted by dmjlambert

Your fuel pump pulled as much gas as it could out of the tank, without any air going into the tank to replace the gas. It reached the point of not being able to pull any more gas to deliver to the carburetor and then died. The fuel tank then popped back into shape. In 1969 there was no complex ventilation such as charcoal canisters under the hood connecting to a separate air line that goes to the tank. You probably just have the gas line coming from the tank and that is all. So that means you need a gas cap that will let a little bit of air in. The vented caps let air in, and don't let fumes out unless the pressure exceeds a certain amount such as from heat expansion when the parked truck is sitting in the sun.

This!