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When the key is first turned before cranking, the fuel pump comes on for a few seconds to prime the fuel system and bring it up to pressure and then shuts off. When you crank the engine, the fuel injectors start to function causing the fuel pressure to drop. So that's why the fuel pump kicks on while the engine is cranking and it will remain on until the ECM sees that everything is in working order and then commands the fuel pump to remain on until the ignition is turned off.
Is this a 4.3L Vortec Engine? If so, the (CRAB) injection assembly under the intake is faulty. It looks more like a spider than anything else. Be sure that you have adequate amount of fuel pressure because the (Crab Injection System) won't work at all. If it's an eight cylinder, may need to use a injector noid light. It's a special design light that plugs into the injector harness connector and lights up in pulses when engine is cranked. If there is no light, trace and check all wires leading from injector harness to ECU. Hope This Helps.
If you know for a fact the injectors are not opening, you would need to check for voltage and a ground signal from the computer. The computer grounds the injectors to open them based on the crank and cam sensors.
If the passkey system were not working the engine should not crank and the fuel injectors would be dead. The security light would also stay on. The fuel pump relay gets battery voltage and also voltage from the key switch. The computer grounds the relay to power the pump. The crank sensor does not come into play until the engine is cranking and it supplies a signal for the ignition and injector pulse. Are you not getting power from the key switch to the relay, or from the battery side to the relay ?
I'd use a gage and check proper fuel pressure, key on engine off, 60--66 psi with spider fuel system.
Engine cranking did you visually check for spark at the spark plugs? Engine cranking is there a fuel injector pulse?https://s8.postimg.org/50arofh11/fuel_pump.jpg
Click on the link for fuel pump relay, you can check the fuse. At the fuel pump, gray wire is voltage, black is ground.
There is a priming pump on top of fuel filter assembly. Had to have somebody else pump it while turning the motor over, It fired right up. Was looking for a while for this pump, because it doesnt look like a pump, it is metal on top of fuel filter.
I had same problem. Engine will start if you crank it for awhile. This is because the fuel pump has a . check valve that is supposed to keep fuel from running back into tank and hold fuel at pressure so it will start immediately. If valve leaks it takes awhile for fuel to be pumped back up to injectors. Replacing fuel pump fixed my problem.
if the engine wont turn over take to air intake hose off at the throttle control valve and while holding the valve open spray something that contains ether, i use brake cleaner or carb/choke cleaner (both avaliable at any auto parts store) if this does not fix the problem then you should check the amperage of the battery. it should be between 14 and 16 amps. also check the the fuel injectors are clean. a dirty fuel injector can severly drop the preformance of an FI engine. hope it works
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