either your vapor locking or your cap isnt really venting

 

Are you sure you fuel gauge is working OK?

 

another thing to concider is if you put an aftermarket fuel pump in you gotta have a fuel return line in the system.
with just a feed line with an aftermarket pump itll suck the fuel at a high rate causing vacum at the tank.

 

and your high HP dual exhaust will heat up the gas it the tank causing it to expand and build more pressure, the cap must vent or a return line needed might help too but we hear of alot of the gas caps out of the box that dont vent right.

 

I've seen the problem you describe before when a guy had a fuel pickup that was a little too high in the tank and sucked air when the fuel got low enough. Pressure in the tank should not cause a problem, since it will actually help push the fuel out. I've pressurized my fuel tank to flush my fuel lines and the fuel flows easily with just a moderate amount of air pressure.

 

it is building pressure to the point, when i go to remove my cap, if i'm not carefull the cap will fly into the parking lot.

 

If you're using the OE pickup with the new pump, it could be losing its prime as the pickup is from the top of the tank.

You should be able to "hear" the pump become unloaded if fuel isn't being pumped.

Do as the doctor says, if it hurts when you do that, don't do that! Fuel tank gets to 3/8ths, fill it.

 

Pressure in the tank is normal. I currently have three running 69's, and all three will burp pretty good when I remove the gas cap. It's a sign of healthy tank with no leaks.

A working gas cap (one that rattles when you shake it) will allow air 'in' to replace used fuel,..but the cap won't allow fumes or fuel out. Hence, temp and atmospheric changes will cause pressure in the tank,..and as was stated earlier, some pressure is created by the heat of tailpipes if they're running to your bumper.

Are you sure the floats are set to specs?

Also, next time she dies coast to safe place,..open the hood, take the air cleaner off, and watch your accelerator-pump squirters as you work the throttle linkage by hand. If it's not squirting of if it's sort of a weak "spray", you have some starvation that needs to be diagnosed.

What do you have your regulator set on?

 

We are seeing this pressure build up problem more and more. I believe that it is caused by modern gasoline mixtures heated by the exhaust pipes, since I never had this problem in the 70's with my Camaro. But I have had this same problem since about 1992. The gas cap is a two-way valve and does vent out if the pressure is very high, but it causes problems before that. I have taken a new GM cap apart by grinding off the rivet head, modified the valve, and put it back together. It works great, and doesn't leak fumes. It vents off pressure at a much lower point, and the car never has vapor lock any more.

 

I would like to add that I never had a vapor lock problem on a quadrajet equipped Camaro due to the return line. It seems to only effect Holley equipped cars like my ZL-1.

 

Interesting theory on the exapnsion of heated fumes of newer fuels,..perhaps someone has further input.

But I don't see how you could fab a gas cap that would allow air out, but keep fuel in. Upon acceleration, the fuel will be forced to the back of the tank and the fuller tube,..the cap would leak fuel. Or do I not understand the mod?

 

The mod works great. It is the same check valve as original, it just vents at a much lower pressure than stock. No fumes escape until pressure builds up. No gas escapes due to the check valve (just like stock). The check valve still lets air in as before as needed. The only problem is air pollution. But I dont use my car as a daily driver.
There was a thread a long time ago at CRG that talked about modern fuels and quicker evaportion rates. It was about quadrajet carbs and how the gas is gone from the bowl after a week or 2 of sitting-even sealed carbs with epoxy on the bowl plugs-and long cranking to get the fuel back to the bowl.