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Why does this lead to a rough start? Because a defective purge valve doesn't allow for gas to flow from the EVAP canister to support combustion when the engine is starting up?
The valve can be stuck open or stuck closed. It is normally closed, so stuck closed doesn't usually cause any driveability problems aside from a check engine light. The most common situation is a stuck open valve, in which case it causes a vacuum leak through the fuel canister. Those fuel vapors coming from the canister create a rich condition at startup that can cause a bumpy start and rough idle. Also if the problem is worse after filling up, you can get fuel entering the canister which can cause a mix of raw fuel and charcoal to be drawn up through the stuck open purge valve, messing up the idle and even clogging up the purge valve seat.
