New fuel tank question
 

My fuel tank in the 1969 Nova SS was leaking so I changed it and now it has a pressure sound when I remove the cap. This did not happen prior. The tanks look the same and I used the original sending unit. Any opinions on why this is happening?

It sounds like your new gas tank is not ventilated through the filler neck so a vented gas cap would be required. It is equipped with a special breathing valve so that it can be used on non-ventilated fuel tanks. The valve relieves the pressure that can build up in the gas tank due to expansion especially on hot days.

My gas cap is vented. And the tanks look the same.

It is normal to see some + pressure with a anti surge cap. It will appear more noticable with little fuel in the tank after driving on warm days.
These caps are not directly vented to atmosphere . If they were you would spill gas with a full tank during take off.



Mike

So today was the first day that the temp hit 90 degrees and the tank is very low on fuel. I will see what happens going forward. If anyone has any other suggestions, Please let me know.
THANKS

Yes vented one way.

The gas cap shown works good on an original tank. The reproduction tank likes the 1971 to 74 cap. GM 413755 or recent replacement GT 85 . Solved excessive pressure build up for my application. Earl

You should have repaired the original tank. Radiator repair shops will fix fuel tank leaks. Your red Nova is a all original unrestored car, that's the way you want to keep it... Just my two cents!!

That is a great idea. I still have the original tank and it was leaking at the filler neck, So most likely reparable. I will send it out and have it repaired. Thanks for your imput.
The new tank does look good under the car.

My old '70 Nova did the exact same thing with the OEM tank and cap, and I never did figure out why. The tank had no apparent issues with venting, and it didn't cause any performance issues, or tank damage, so I just lived with the "whoosh".

As I stated earlier, the original tank did not do that but it also had a leak around the filler neck next to the tank, Maybe the leak did vent it better. I just took the car for a ride to get gas and it performed well and it did "Whoosh" when I opened the cap.

Your leak allowed the pressure to vent.

Your building pressure caused the leak.

Yes I agree, What to do to stop the pressure build up ?

Have you had your gas cap tested to make sure it is working properly? I believe Jiffy Lube can do it.

FYI,I put a repo tank in my 69 bird.It would build pressure in my garage that faces west in the afternoon.If I opened the"vented"cap it would have a big woosh.I had my 366 NASCAR tunnel port engine in the car,tried to start it one day and it was locked up.Pulled all the plugs and cranked it over,shot fuel everywhere.It had built enough pressure to push the gas past the needle and seats in the factory holley.VERY lucky it did not fire and push a rod threw the unobtainable block.To be safe I drilled a very small hole in the center of the cap.The vent allows air to be pulled in but not out.My factory 350 auto non A/C tank had a vent in the neck.Wish I had just kept it in the car.Tom