Gas tank pressure, how much is normal?

[1973Guppie]

local parts store....

 

[roadrunnernewmexico]

I'd like to share the results of my similar situation. After hours of research I can tell you that a "vented" gas cap, properly operating allows air ONLY to enter your tank. And then ONLY after the tank is in vacuum lower than the spec, usually negative 2 PSI. Positive pressure, either from heat, atmospheric pressure, sloshing around or any combination thereof is released ONLY through the evap charcoal system. Yes, the vapour separator allows ONLY fumes to move to the canister. It usually has a "roll over" valve nearby in the line. My vehicle (a 1985 Jaguar XJS with a GMC engine, which is a 12 page story for another day) has a vacuum operated purge valve attached between the canister and manifold vacuum by way of a thermal sensitive valve. When the engine is 1) Hot and 2) operating above 2500 rpm, the canister purges. This keeps it from stumbling upon cranking. If you have excessive negative pressure (a sucking sound) your cap is not venting properly INTO your tank. The vacuum is created by the space created by gas leaving the tank. Be aware that negative 2 PSI makes a much larger "suck" that you would expect. Emissions testing will not tell you if you have a PROPERLY venting cap. Only if you have a cap that is NOT properly containing fumes. In other words it is tested under PRESSURE not VACUUM. If you have excessive OUTWARD WHOOSHING, you have a problem in the evap system. Again, be aware that positive 2 PSI can make a much larger outward whoosh than one would expect. If your tank is "spitting" gas out, you defiantly have a failure to flow forward to the canister. A final point, this evap system CANNOT hurt your gas mileage or performance (unlike other emissions devices). Quite the contrary, it combusts the fumes into energy that would otherwise be wasted if vented to the atmosphere.

Be well
DaviD

 

[firestopper]

I'd like to share the results of my similar situation. After hours of research I can tell you that a "vented" gas cap, properly operating allows air ONLY to enter your tank. And then ONLY after the tank is in vacuum lower than the spec, usually negative 2 PSI. Positive pressure, either from heat, atmospheric pressure, sloshing around or any combination thereof is released ONLY through the evap charcoal system. Yes, the vapour separator allows ONLY fumes to move to the canister. It usually has a "roll over" valve nearby in the line. My vehicle (a 1985 Jaguar XJS with a GMC engine, which is a 12 page story for another day) has a vacuum operated purge valve attached between the canister and manifold vacuum by way of a thermal sensitive valve. When the engine is 1) Hot and 2) operating above 2500 rpm, the canister purges. This keeps it from stumbling upon cranking. If you have excessive negative pressure (a sucking sound) your cap is not venting properly INTO your tank. The vacuum is created by the space created by gas leaving the tank. Be aware that negative 2 PSI makes a much larger "suck" that you would expect. Emissions testing will not tell you if you have a PROPERLY venting cap. Only if you have a cap that is NOT properly containing fumes. In other words it is tested under PRESSURE not VACUUM. If you have excessive OUTWARD WHOOSHING, you have a problem in the evap system. Again, be aware that positive 2 PSI can make a much larger outward whoosh than one would expect. If your tank is "spitting" gas out, you defiantly have a failure to flow forward to the canister. A final point, this evap system CANNOT hurt your gas mileage or performance (unlike other emissions devices). Quite the contrary, it combusts the fumes into energy that would otherwise be wasted if vented to the atmosphere.

Be well
DaviD

This must be why I'm getting 13MPG lol.
Rig runs great but EVERY time I stop to refuel and pull the cap off it releases a fair amount of pressure.