Join Date: Mar 2004
Location: Valley Center, CA
Re: Built up pressure in gas tank
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.
69 SS350 ZL-2 4 speed, original paint, unrestored -SOLD
1972 AMX 401ci, 4 speed, cowl induction, console, gauges, posi, A/C, unrestored original