2003 Chevy Silverado P0455 EVAP emission system leak detect

The rough idle, or rough cold start could be relative to deposits in the injectors, or poor fuel quality. Too many misfires should trigger a check engine light, is that occurring. The fuel leaking needs to be addressed. That could indicate something is broken, or loose. The fuel pump whine is somewhat common, if you are lowering the tank to address leakage, you may opt to replace the pump. It may be the sender/O-ring that is leaking.

If you can locate the source of the fuel leak, it is quite possible that you can correct the p0455. The sender o-ring is just a suggestion at this point, and if it is leaking fuel, it is most likely leaking vacuum when the system runs the test, and flagging the evap leak code P0455. Was it leaking fuel while filling, or after it was filled, and was it topped off excessively? Without having a scan tool to activate functions for testing the evap system, you are relying on the vehicle to run the test on its own. Evap tests have some specific criteria, and it could take a while before they run themselves. That being said, after you have performed a repair you will not positively know right after, unless you find a hard fault. The way the system functions is, the engine control system closes the vent solenoid, and opens the purge solenoid to draw vacuum on the fuel system. When a specific vacuum reading is achieved, via a fuel tank pressure sensor, the purge solenoid is closed, and the rate of vacuum decay is measured and compared to a predetermined rate. It can determine a small or large leak. With this being said, several components can cause a fault. From the fuel cap, pressure sensor, vent/purge solenoids, tank, or lines connecting it all.