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Truck only runs for 3 seconds

My 1994 GMC Z71 will only run for about 3 secs. I have checked the voltage to the pump and it is okay while the motor is cranking. I can hear the pump prime up when the key is initially turned on. I have loosened the fuel line to the throttle body and gas gushes out. But looking at the injectors while it is running for the few secs. it’s getting plenty of gas and then it shuts down. Have been told that this is a pump problem. The previous owner had the pump replaced about 3 yrs. ago from O’rielly’s. The fuel filter is 2 wks old. Any help will be greatly appreciated.

If you have a timing light, connect it to one of the plug wires. If there’s spark while cranking the engine over but there is no spark when the engine starts, the problem might be with the ignition module.

Ignition modules function in two modes. These are the START and RUN modes. While cranking the engine over and the ignition switch is in the start position, the ignition module is in the START mode and allows full battery voltage to the ignition coil(s). This ensures that when a cold engine is started and when the fuel mixture is the richest, there’s a hot enough spark to ignite this rich mixture. Once the engine starts and the ignition switch is allowed to go into the run position, the ignition module goes into the RUN mode. The ignition module then steps the voltage down to the coil(s). This is done because if the voltage to the coil(s) reaches the 13.5-14.5 volts from the battery being charged from the alternator, it will result in damage to the coil(s)/secondary ignition system.

So it could be that the START mode if the ignition module is functioning, but the RUN mode of the module is failing.

Tester

Well we thought about that, but I should ad that my Dad can squirt gas into the barrels and it will keep running after the injectors stop, and will run as long as gas is squirted in. And the motor sounds good.

Check the fuel pressure regulator. Disconnect the vacuum hose. If fuel runs out, replace the regulator.

Have you done a fuel pressure test at the fuel rail?