I have a 69 Camaro when i loosen the NEW! gas cap their is alot of pressure in the tank.It seems like it is worse when the tank is nearing empty.What is causing this and what kind of problem will it create?

Tailpipes running next to the tank, plus road heat, plus engine heat due to the car direction, and high ambient temperature, all heat up the air in the tank.

The fuller the tank, the less pressure buildup. Air expands more than a liquid.

You might try a 71 & later gas cap. They have a relief valve to relieve pressure, plus a vacuum valve to provide an equalization when cooler temperatures arrive.

Are you sure it's pressure and not a vacuum? Is the tank properly vented? Check to see if the tank is vented, or does the car require a vented cap.

Put your hand on the bottom of the tank after a long drive and see how warm it is. I had a leaking exhaust pipe that heated the fuel to a boil!!!

some pressure is normal.

Wether it is pressure or vacuum a properly vented tank shouldn't have an issue. I would bet your NEW cap is not vented. Is that the only thing you changed? Did you have this problem before?

69 caps are vented, but they only allow air into the tank, not out. This keeps your tank from venting when it's parked in your garage and filling your house (or gargage) with gas fumes.

Mark, You are right. There should also be a vent on the tank going up towards the front of the car. I sold my 69 so I can't confirm this, I do know on 1970's it is there. I doubt it is pressure though, if that was the case I would expect there to be flooding issues, from gas getting forced by the pump and into the carb.

The vent line went up to the filter on those cars so equipped, the system wasn't open to the atmosphere anywhere other than the gas cap and off the top of the carb bowl. First gens didn't have a charcoal canister system like the later model cars did. Pressure will build up as the temperature in the tank goes up.

Single fuel line 69's had no vent, either at the tank or elsewhere - they take a vented cap.

:beers:

No first gen tank is vented, only the fuel cap is vented. The fuel cap has a valve that will allow air in, but won't allow fumes out. A good fuel cap will rattle much like a PCV valve.

The retrun line used on Q-Jet cars returned fuel to the tank and had nothing to do with venting. This was an effort to move fuel through the main fuel line to prevent vapor lock.

Your car is probably fine. Just make sure all exhaust components are at least 1.5" away from the tank.

Beginning in 1971, all Chevy cars (I think in 71) received fuel tanks with vent lines that connected from the tank, to a fuel separator (usually mounted on the tank), to a charcoal can, then to a vacuum source at the carb. This system created vacuum in the tank which prevented the tank from burping when you opened the fuel cap. These systems did not use a vented fuel cap but a closed one,..all in the name of reducing emissions.

I had same problem. Tried the 2 line sending unit (available in 69) using one as a vent , 3 different caps, gaskets etc. What occurs is the original tanks were vented thru the cap, but in acceleration I found gas would escape (literally pour out) of the cap; so I applied thick gaskets to the cap, got a great seal, but a vacuum was created due to the pumping of fuel, this eventually effected performance. Talked with hot rodders famliar with 69's they solved the problem by drilling a small hole into the center of the a tight fitting cap (non vented). Works like a charm. You can use the 2 line sending unit but the cap hole is easier. If you prefer the 2 line method, I have two available.
Dave

SS-Dave,..I'd rethink drilling holes in your gas cap and/or using the return line as a vent. Raw fuel will escape and when it's not, fumes are. All this is taking place within dripping distance of a hot exhaust system.

What's wrong with the factory set up?...a vented cap that will only allow air in,..but won't allow fuel or fumes to escape.

I never used the 2 line set-up, but if we did we would have safely routed the line, the gas cap hole is very very small, approx. 1/16th, but it has to be drilled thru the entire cap length. Using the vented cap I got raw fuel spilling out like a faucet on acceleration. If I could find a vented cap that worked I would use it, I tried 4 different caps.