This is our story...my son and I are restoring our 69 SS396 Elky and we replaced the gas tank last month. He had been driving it and had problems with the original tank cap that was on it. On hot days it would leak out gas with a full tank. So when we got the new tank we got what they said was a proper non-vented cap. We got under the car today and found out that the tank is collapsing due to a vacuum inside it. We opened the cap and the tank expanded a little, but now we will have to drop it back out of the car and try to get it back in shape. The fuel sending unit(a new one) also quit working last week. Obviously the sending unit was smashed in the vacuum as well. Any suggestions?

Thanks,
Marty and Nick Lemons

How is the vent hose at the top of the tank? The reason I ask is that my elky has a vapor canister. I decided to get rid of it so I, without thinking put a plug over the vent tube which caused the tank to pressurize-not a good thing. I put the canister back on, no more problems.

There is no vent at the fill neck. During hot weather with a vented cap the gas runs out all over the ground. The original tank did not have a vent anywhere as well. The replacement tank looked exactly like the original.

Maybe the later model tanks had one to prevent that sort of thing. Sorry I couldn't help. Of course the fuel injected models are surposed to be pressurized.

On hot summer days with a full tank of fuel my 66 does the same thing, pressurizes and spills fuel all over the ground. I've checked again and again, I have the correct cap and the vent line is open. I just don't keep the car topped off during the summer unless I'm going on a road trip.

I had this problem with my 70 Chevelle SS 396, I would get to the car show and if I did'nt loosen the cap to release the pressure it would also seep out and down the neck,and of course every body around me would get nervous.I'm not sure about a 69, but my 70 has a return line 5/16 from the sending unit assembly along with the 3/8 fuel line,both running to the fuel pump, and the fuel pump has an place to connect this return line from the tank.Since nothing else worked to correct this pressure build up, I drilled about a 1/16 hole in the center of the gas cap[With Gas Cap Removed From The Tank],This worked on mine, it was small enough for gas not to come out and also it kept the pressure from building up.If you choose to try this, use your old cap,punch a center mark on the large rivot head and carefully drill all the way through it.When nothing else works you got to try other methods.By now I hope you found the problem or better advice, but good luck and don't give up.

I just came in from putting the tank back in. We took it out and found out when the tank partially collapsed, it bent the fuel pickup tube from the new sending unit and put a crimp in it(found that out when it quit in the driveway). We got it straightened back to the original shape and re-installed it. I called my friend whom I sold it to 10 years ago, and he suggested drilling a hole in the top of the filler neck, but now after reading your post I will go the gas cap route, as it will be easy to fill back in. Thanks for the reply's. Are Forums great or what?

Whatever you do, DO NOT drill a hole in the filler neck. Sparks from the drill + gasoline fumes = BOOM!

NO, I drilled the hole in the cap on the workbench inside the garage.....car outside, but it did sound like" Hey Bubba, watch this". haha

With the hole drilled in the gas cap, does fuel spill out during a hard right turn when the tank is full?

Yes, there is already discoloration on the top of the gas cap where it is seeping out. My main concern is the structural integrity of the tank. The original didn't collapse. This new one is obviously too thin of material..I know they won't refund my money on it, and does anyone know of where to get one that would be exactly like the original?

Well it happened again! I'm "hobie1dog"s son and I'm sitting at the computer while my dad gets on the phone with NPD.

All of the parts catalogs that I've seen don't have any type of canister for venting the fuel lines/tank on the earlier modes ('pre 1970). The 'new' gas tank is a lot thinner than the original...so that might be the problem. If all of the replacement gas tanks are as thin as the one we got from NPD, then I'm afraid that we're royally #*$&@%'d. Either that or we come up with our own way to vent the tank without gas spilling out. :(