The Perils of An Unvented Gas Tank

Four years ago I removed the vapor canister on my 81 280zxt in order to route my intercooler piping. Before removing it, I researched on Zcar and Hybridz the impact of leaving the tank unvented. The research results were 50/50, some saying that leaving the tank unvented would not cause any issues while others suggested adding a vent or tucking a small vapor canister from a different model car in the wheel well (I should have listened Tonyd!!).

I decided I would not initially vent the tank and would monitor the tank when filling, etc for odd behavior. My Z is driven at most 2k miles a year. During that time I only experienced one odd experience when the tank blew a 4 foot stream of gas onto the adjacent gas pump when I cracked open the filler cap. I might add my boost is set at 12 PSI and is fueled by a fuel pump from a Porsche 911 twin turbo.

Last fall, three years after having removed the vapor canister, my Z started experiencing fuel starvation issues. Whenever the gas level dropped below 3/4's the fuel pressure would fall off and it would run lean.

After a very frustrating period of investigation and head scratching, I finally realized what the issue was. When viewed from underneath, it appeared that my gas tank was crushed! Which was even more evident when I dropped it and saw the condition of the top of the tank.

It appears that after an unusually long day of spirited driving to and from Lime Rock Park on our club's annual Bob Sharp Lime Rock day last summer, the vacuum built up in the tank to a point that it permanently deformed the tank, forming an impressive crease in the metal. The heavy fuel pickup tube was bent to the point that the strainer was snorkeling above the fuel level at 3/4 tank. Needless to say, the replacement tank is going in soon and will be vented.