I have a 2004 yukon xl flex fuel. I'm having fuel pressure problems. I am only getting 50 psi at key turn only and 40 at engine running. I change the fuel pump, fuel filter and fuel regulator. My yukon will not start at the first complete key turn from a cold start, but it will start the second time and as long as I don't let it off long it will start first crank. I tested the pressure yesterday and at turn key it goes to 50 psi,and quickly drop off to 0 psi. Can someone help me please