My 92 sentra with a 1.6L wont start. I replaced the plugs, fuel pump, distributor rotor, ignition relays, fuel pump relay, and ignition switch. It will start once or twice after a while of sitting but when you shut it off and try to restart it, it wont start again, it will just crank. It is getting good spark and fuel. WHAT IS THE PROBLEM???? Im stuck....