Fuel Tank FAQ

Question

My question is this, is the fuel cap designed to be vented to the atmosphere? I noticed a collapsing of the filler tube when I go fill her up (it is a replacement rubber model-the rubber ends on the original steel tube were leaky causing a stink in my trunk) and there is obvious pressure on the cap when I remove it. There is also the stock vapor recovery tank mounted directly below the trunk lid/above the package shelf. Could that be blocked up causing the negative Pressure? I reconnected the tubes to the tank and tightened all connections. Any suggestions on modifying this system to relieve the pressure are appreciated.

Answer

From: Thomas Walter (walter@roadster.sps.mot.com)

What year Model? 510's had closed systems, the cap was not vented. [Not sure about Canadian models, nor early USA 510's. Seem to recall my sister's '68 having the evap tank... but this was not a requirement for roadsters until 1970!].

On the drivers side firewall is a little thing with three lines to it. One comes from the tank vent, the other two lines are connected to the air cleaner (when the tank becomes to pull a vacuum from the fuel being drawn out of it, it will get its "air" supply from the line connected to the air cleaner). The other line goes to the crankcase (the little tube above the PVC breather). If you fill up the tank on a hot day, and the full expands it will pressurize the tank... this allows the pressure to be "vented" into the crankcase.

So check the line from the tank to the little "2 way valve" is open and clear. Then check the vavle itself. (This is also the source off funny noise after you shut the engine off, and notice a "clicking" sound from the engine compartment).

To check the WHOLE system to be "air tight" I've used a air compressor - which is regultated to a MAXIMUM of 5 PSI - and connected this to the "vent" line. Once pressurized, you can easily check the system for either "air" of "fuel" leaks. Then The Fuel line was disconnected from the carb and held into a empty gas can. Pressurizing the system purged all the old fuel out of the car... easy way to drain a tank on a 510! (Car had been sitting for some years, and had a few gallons of 'soured' gas in the tank. Also with the pressure you'll locate leaks fairly quickly.