2000 Chevy Express codes p0137 and p0452

My Express is giving codes P0137 and P0452. The P0452 is indicating low voltage on the evap pressure sensor located in the fuel pump assembly. I have not checked the fuel pressure but wonder if the evap sensor could be bad and causing the problem. I replaced the fuel pump and sensor last fall when the fuel pump died. I know that the fuel pump wiring harness is a problem on these vehicles but I don't know if that is the problem. The van is running so rich that the outside of the exhaust pipe tip is black and it is getting terrible mileage. The P0137 is the downstream O2 sensor which is not supposed to affect the fuel mixture but I believe that indicates a lean condition after the cat which appears to be the opposite of my problem. The van runs poorly when cold and is down on power all the time. It stumbles when cold before the O2 sensors are supposed to affect anything. I also know that the fuel pressure regulator is a problem on these vehicles. One additional thing is that when siphoning the gas out when I relaced the fuel pump, I bent the silver flap in the fuel filler so that it does not close all the way now. Could that cause a problem?