Hey Ken. The answer lies in the purpose and operation of the system.
The whole purpose for the charcoal canister is to capture emissions as they evaporate from the fuel tank when the vehicle is NOT running. Vehicles do spend most of their time not running. Parked in a parking lot or your garage, let's say. Fuel tanks are not vented through the gas cap anymore. All venting - both directions if needed - is through the canister. There should never be a positive or negative pressure in the tank. The only thing between the tank vent and atmosphere is the roll-over valve and the canister.
In order to extend the life of the charcoal, and allow it to be used over and over again, the canister is "purged" with fresh, outside air to sweep the hydrocarbons out of the charcoal and into the engine. The purge valve is connected to the canister on one end, and manifold vacuum on the other end. Engine vacuum is used to suck the fresh air backwards through the charcoal during engine operation. Air is sucked back into the canister through the vent hose hanging down from underneath your Spyder. On the 1330 systems, there is a little filter in that line. On our system, it just sucks in whatever happens to be down there - dirt and all. In the case of the 998, the purge valve is only connected to the rear cylinder. Now, while it can be argued that the hydrocarbons being sucked out of the charcoal are flammable, the affects of the lower vacuum out-weigh that. It's a vacuum leak. Thus you end up with reduced efficiency and performance. On an engine with larger displacement, there is not much affect. On the 998, when only using the one cylinder, it has a bigger impact.
The ECM controls when the valve is opened and closed. It is closed at low RPM and idle conditions. It is opened / cycled at normal highway RPM. Some folks can even feel the slight surging that occurs on very smooth roads as the valve opens and closes. Missing and stalling occurs if the valve sticks open at lower RPMs and at idle. Again, it's a vacuum leak.
So, in your case, if the purge valve has failed to open, and / or after you do the canisterectomy, the engine will perform better and more efficiently by eliminating the loss of vacuum cause by purge valve operation. Thus, the fuel mileage will improve. In addition, right now you may also be subconsciously operating the engine differently because you have that light lit on the display. Going easy on the wrist will also improve fuel economy.
Your code indicates an electrical problem with the valve. It probably doesn't open anymore. Whether or not you do a canisterectomy, you'll need to replace the valve to clear the code.
