GMC 25168719 Fuel Pump

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The process for replacing a mechanical pump on carbureted models involves disconnecting the battery cable, detaching the fuel inlet hose, outlet line, and vapor return hose, removing the mounting bolts, and retrieving the pushrod. After cleaning the block mounting surface and applying grease to the pushrod, the new gasket and pump are installed and the hoses and lines are reattached. For electric pumps on fuel-injected models, the fuel system pressure is relieved and the fuel tank is removed. The fuel pump/sending unit assembly is unlocked by turning the inner ring counterclockwise, and then lifted from the tank. The rubber gasket and strainer are inspected and replaced if necessary. If needed, the fuel pump can be separated from the sending unit. The assembly is then inserted back into the fuel tank and the inner lock ring is turned clockwise to engage the locking cams. Finally, the fuel tank is installed.

The fuel pump is located in the fuel tank for both four-cylinder and V6 models and is non-repairable. When the key is turned on, the ECM activates the fuel pump relay for quick pressure buildup. If not started, the ECM waits for engine start, using the oil pressure switch as a backup if the relay fails. A malfunctioning relay may cause long cranking times. The ECM powers the fuel pump when ignition is on, relying on HEI distributor pulses. To test the pump, install a fuel pressure gauge and turn ignition on. If no pressure, check the fuse and wiring. For a good fuse but no pressure, test the pump by applying battery voltage. Inspect the system for pressure discrepancies, leaks, and faulty components, including the relay and connector.

To remove the fuel pump/sending unit assembly, start by disconnecting the cable from the negative battery terminal. Then, relieve the fuel pressure. Next, remove the fuel tank. Inside the fuel tank, the fuel pump/sending unit assembly is held in place by a lockring mechanism. To unlock it, turn the inner ring clockwise until the locking cams are free of the retaining tangs. If the rings are too tight, gently knock them loose with a soft-face hammer. Carefully extract the fuel pump/sending unit assembly from the fuel tank, making sure not to bump it into the outer lockring. Inspect the condition of the rubber gasket and replace it if necessary. Also, inspect and clean the strainer on the lower end of the fuel pump. If needed, separate the fuel pump from the sending unit by pulling the pump assembly into the rubber connector and sliding it away from the bottom support. Take care not to damage the rubber insulator and fuel strainer. For installation, insert the fuel pump/sending unit assembly back into the fuel tank and turn the inner lockring counterclockwise until the locking cams are fully engaged by the retaining tangs. Finally, reinstall the fuel tank.

To test the fuel pump relay, disconnect it and connect a test light to the ignition wire.

If fuel pressure is too low, you should conduct a fuel pump volume check, and if volume is insufficient, inspect for fuel line restrictions. If no restrictions are found, replace the fuel pump.

Check for restrictions in fuel lines and filters, and inspect the fuel pump for issues.

To begin, disconnect the negative battery cable to isolate the battery.

Inspect the condition of the seal around the mouth of the lockring mechanism.

Check for restrictions in fuel lines and filters, and inspect the fuel pump for issues.

On models equipped with the Theftlock audio system, be sure the lockout feature is turned off before performing any procedure that requires disconnecting the battery. This is to prevent any issues with the audio system.

After that, the fuel tank needs to be removed.

Ensure that there is enough fuel in the tank and relieve the fuel pressure.

After cleaning the block mounting surface and applying grease to the pushrod, the new gasket and pump should be installed and the hoses and lines should be reattached.

To test the fuel pump, start by disconnecting the fuel line into the carburetor and then crank the engine. Check if there is a strong spurt of gasoline from the line. If not, inspect for clogs or issues with the fuel pump. To test the fuel pump flow capacity, use a measuring container and a watch to see if the pump can pump one pint of gasoline in 30 seconds or less.

There may be a leaking injector or a faulty fuel pump.

For fuel-injected models, start by verifying that the fuel pump is running by listening for a whirring noise when the ignition switch is turned on. If the pump does not come on, check the fuel pump fuse and relay. If they are both functioning properly, check for an open circuit between the relay and the fuel pump. If the circuit is fine, disconnect the electrical connector at the fuel pump and connect battery voltage to the gray wire terminal of the pump to see if the pump runs.

The fuel pump should be reassembled to the sending unit bracket assembly and then inserted back into the fuel tank.

The final step is to install the fuel tank.