this is some info that i got from Rockauto i/c/w a possible P0446 cel....


Will a warning light on the dash light up immediately if the gas cap is loose? No, the "Check Engine" light or its equivalent may not come on until many miles and days have passed. Why is that?

The fuel tank cap is part of the Evaporative Emission System (EVAP). EVAP prevents gasoline vapors from escaping from the fuel tank. Onboard Diagnostics (OBD II) monitors the air/fuel ratio, ignition and some other systems continuously, but EVAP is one of the systems that is typically only tested once per trip.

EVAP may actually not even be tested once per trip. Most/all OBD II COMPUTERS DO NOT TEST THE EVAP SYSTEM UNLESS THE FUEL TANK IS BETWEEN 1/4 AND 3/4 FULL. The gas cap is loose when the car leaves the gas station, but the Check Engine light may not come on until the driver has burned through a 1/4 tank of gas and probably a significant amount of time. It can be hard tying together a gas station stop last Saturday with an illuminated Check Engine light on Thursday.

Even professional mechanics might get led astray if a customer drops off a vehicle with a full or nearly empty fuel tank. With the "wrong" amount of gas in the tank, a failing EVAP part would not trigger an EVAP diagnostic trouble code (DTC), but it might lead to a DTC from another system. A bad Vapor Canister Purge Valve/Solenoid (sends gasoline vapors to the engine) or Vapor Canister Vent Valve/Solenoid (lets fresh air into the EVAP system) could create a vacuum leak or electrical problem that triggers Oxygen Sensor, Fuel Trim or other downstream DTCs. The much more relevant EVAP DTCs would not start showing up unless the fuel tank got between 1/4 and 3/4 full.

 

Your assumptions in the last paragraph are a bit flawed.

The vent does not let air into the EVAP system; it lets air into the gas tank to avoid a vacuum lock. The gas is being sucked out and the fumes are being sucked out by the purge valve (which is PWM).

In the olden days the gas cap was vented, so no worries. Now, we worry about gas fumes escaping into the atmosphere, so the vent only opens when needed and any air that escapes from the tank is cleaned up in the charcoal canister. It is more complicated than that, but that's the reasoning.

The only harm leaving the gas cap off is to the environment, not the car.

BTW, P0446 is a performance code, P0455 is "large EVAP leak".