I have a 2004 Chevy Avalanche that is periodically showing the "Tighten Fuel Cap" message and throwing a P0455 Gross EVAP Leak engine code. I have replaced the fuel cap twice, the fuel pump, the vapor canister, the purge valve/solenoid, and the vent valve/solenoid. I recently replaced the valley pan gasket (unrelated repair) and while I had the intake manifold off, I cleaned all of the injectors and replaced the intake manifold gasket as well as the valley pan gasket. In cleaning the injectors, I emptied all of the fuel from inside the rails. After reassembly, the truck will now periodically (and usually after sitting for a while) will turn over but will not start (it acts as if it has lost its prime) on the second attempt to start the vehicle (turn key off and back on the reprime fuel pump) the truck starts right up. When I had the fuel tank down, I didn’t see any cracks in any of the lines. I am at my wit's end... Any advice?

 

does the problem occur after after refueling? if yes....the purge solenoid is not sealing. check the new valve unplug the connector...run the engine and check for vacuum at the line from the valve. should be zero.
if no...put a fuel gauge on the rail and monitor for leakdown.

the code sets when the ftp sensor does not see pressure change when the system is purging. So either no vacuum is reaching the tank or the ftp sensor is f'ed or the system has a leak so large it can't pull vacuum.
if you have a scan tool....monitor the ftp sensor data....its should drop when purging and return to o"h20 when off and the cap removed.

 

It does not happen just after refueling. It usually occurs when @ or below 3/4 of a tank and usually within about 10 minutes of startup.

update one new symptoms. Now it seems as every start is a second start if sitting longer than 10 minutes. First attempt will crank over for as long as you hold the key and will never start. Second attempt starts right up.

I did replace the cam shaft position sensor when I had everything apart to do the valley pan as I read it could be leaking oil there due to a bad o-ring. I replaced it with and Oreily part

 

cycle the key to run for 2 seconds then off...repeat 4x and then try and start. this primes the fuel rail....if starts up first time....you have fuel pressure leakdown.