[NitrousGuy]

Your talking about the gas cap.
I am talking about the evap vent valve post charcoal canister.
GM has a spec that says a vehicle has to sit for 40 days without the battery dropping below a certain voltage. If the vent valve was energized, meaning it closed, for the 40 days, the voltage would not meet our spec.

While the vehicle is parked, the fuel system is not sealed. sure, the gas cap is sealed, but the vent valve is open.

I don't know which car you're thinking of (and you may know something I don't) -- the only GM cars I've owned were a '74 Nova (350 V8 ) and a '79 Cadillac Coup (425 V8 ). In both cases, the charcoal cannister was connected to the carb bowls (very similar to the routing on the California spec Kats) and not to the gas tank... unless there was another cannister I never found...

Hmmm...


Cheers,
=-= The CyberPoet

Mr. Poet,

I only have extensive experiance with 2000 model year gm's and on. 2000 is when I hired in, so I cant comment on the nova and caddy. Hell, those cars are older than I am, lol.

On the 2000+ cars, here is the evap flow in reverse order....
Intake plenum to the purge valve. From the purge valve to the fuel tank. From the the fuel tank to the evap canister aka charcoal canister which is located near the fuel tank. From the evap cannister to the vent valve and from the vent valve to the atmosphere.

While the vehicle is sitting ( key off) the PCM does not energize the vent valve which would seal the fuel system. We may go to a normally closed vent valve soon, but its not the case as of '06.

Current charcoal canisters do such a damn good job of absorbing escaping fumes from the fuel tank, that its un-nessassary to keep the system sealed during a key off event.

A downside to a normally cloesd vent valve would be that if the vent valve fails, then there is no way for the evap system to be purged once the purge valve is commanded to its duty cycle. It woudl basically but the evap system in a vaccum. The charcoal canister woudl not be allowed to empty and over time would be 'water logged' with fuel. That is a lawsiut just waiting to happen.

Leo

[The CyberPoet]

Thanks for the lesson... just goes to prove I learn something new every day!

Cheers,
=-= The CyberPoet