I have been fighting the same problem for 4 weeks now.I am down to thinking its ECM related? '91 Z28 5.7 TPI. I have put in a new fuel tank, fuel pump, fuel filter, TPS sensor, ECM temp sensor and pigtail,new fuel regulator, fuel rail seals, new upper and lower injector O rings, new ignition module, new idle control valve, coil wire, cap, plugs, rotor, wires. When it is cold outside ( below 55 degrees) it will not start.I can hear the pump click on, but no pressure at the Shrader valve. Once the day warms up, it will start and start every time all day long and evening until the next morning and it wont start then. Yesterday morning, I put a small heater in the passenger floorboard, aimed at the kick panel, for about 15 minutes. Got nice and warm and it fired right up. Did the same thing this morning. Could ambient temp really affect the ECM? Should I just replace the entire ECM? Still needing help guys. anything would be greatly appreciated.