1991 Chevy C3500 454 CID;
Engine shut down instantly while driving (no sputter). Have spark at the plugs, engine will run if you manually feed fuel to the intake. Makes me think that it's the fuel system, don't know if its mechanical or electrical including any sensors. So far I have introduced 12 volts to the fuel pump harness at the tank and had good flow (have not checked pressure). Have a low voltage of 8.5 volts at the pump, oil pressure switch, fuel pump relay and fuel pump fuse when the ignition is on while the battery voltage reads 11.7 volts. Please help!
When the key is first turned ON, without the engine running, the control module will turn the fuel pump relay ON for two seconds. This builds up the fuel pressure to normal operating pressure. If the engine is not started within two seconds, the control module will shut the fuel pump OFF and wait until ignition reference pulses are present. As soon as the engine is cranked, the control module turns the relay ON, which powers the fuel pump. The control module continues to power the fuel pump during engine operation. If the fuel pump relay fails, it is backed up by the oil pressure switch, which continues to operate the fuel pump as long as oil pressure remains above 28.0 kPa (4 psi).
A faulty fuel pump relay can result in long cranking times, particularly if the engine is cold.
An inoperative fuel pump would cause a no start condition. A fuel pump which does not provide enough pressure can result in poor performance.
The low voltage could be a result of the truck not running, check the voltage with the truck running if you can to verify the low voltage problem. If you can't; then check the voltage with a battery charger on the battery. If voltage goes back to normal then check to see if you have the proper voltage at the fuel pump relay.
If you have a low voltage at the fuel pump relay and fuel pump, check all of the engine grounds. Check voltage before the relay and after to be sure it is the same. If voltage is low check the wires going through the bulk head connector in the fire wall of the truck. If the voltage is good going into the fuel pump relay and low going out then it could be a bad relay. If you can't get voltage to the fuel pump at all the worst it could be is a bad ECM.
The fuel pressure could be low because of a bad fuel pump pulsator in the fuel tank. The pulsator is attached to the fuel pump on the fuel tank sending unit.
Try these few suggestions and get back to me with your results. The more detailed information you give me the better I can help you.