Fuel Tank Pressure When Removing Gas Cap?

I have 71 Plymouth and this car has an unusual amount of pressure when removing the cap. I will crack the cap for a couple of seconds prior to removing and after that I fully open it has a slightly shaken Soda/Pop bottle pressure release. Is this normal? Do I have the wrong fuel cap on the vehicle? Let me know your thoughts....

You sure it's pressure when you open the cap and not suction? A 73 tank was installed into my 74 and because the venting system is different it looks like the guy who did it capped off the vent lines. That caused it to build suction inside the tank so I simply changed to a vented, locking gas cap and no issues since.

Yes, You are right it is suction... I will try a vented cap. Do you know if this problem will cause any adverse affects?

I've had several vehicles way back that had that whoosh when you took off the cap (I assumed it was vacuum) & no starvation (but they were daily drivers not raced).

You can also try to drill 1 or 2 small (1/8") holes in you cap. You shouldn't need much.

My buddy lost a couple pistons in his nitrous motor from fuel starvation, had a stock tank with a sump and the vent line was kinked. Pump couldn't push enough fuel, motor went lean, all done Mr. 351.