Fuel System Diagnosis , Testing fuel pump

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Fuel System Diagnosis , Testing fuel pump

Postby HenryJ » Sun Sep 02, 2007 6:10 am

How to check the fuel pump pressure and the regualtor.

Edited and condensed partial testing GM data:

When the ignition switch is turned ON, the powertrain control module (PCM) energizes the fuel pump relay which turns the fuel pump ON. The pump remains ON as long as the engine is cranking or running and the PCM receives reference pulses. If there are no reference pulses, the PCM shuts the fuel pump OFF 2 seconds after the ignition was switched to the ON position or if the engine stops.

An electric fuel pump pumps the fuel through an in-line filter to the fuel meter body assembly. The pump attaches to the fuel level sender assembly inside of the fuel tank. The fuel pump provides fuel pressure above what is needed by the fuel injectors. The pressure regulator keeps the fuel available to the injectors at a regulated pressure. Unused fuel returns to the fuel tank by the fuel return pipe.

Inspect the fuel system for damage or external leaks before proceeding with this diagnostic.

1. Turn the ignition on. Listen at the fuel tank for a buzzing sound from the pump. This step verifies that the fuel pump is operating.

1. Turn OFF the ignition.
2. Turn OFF all accessories.

Wrap a shop towel around the fuel pressure connection in order to reduce the risk of fire and personal injury. The towel will absorb any fuel leakage that occurs during the connection of the fuel pressure gauge. Place the towel in an approved container when the connection of the fuel pressure gauge is complete.

{photo courtesy S-10Planet.com}

3. Install the fuel pressure gauge.
4. Place the bleed hose of the fuel pressure gauge into an approved gasoline container.
5. Turn ON the ignition, with the engine OFF.
6. Bleed the air out of the fuel pressure gauge.

* You may have to command the fuel pump ON several times in order to obtain the highest possible fuel pressure.
* DO NOT start the engine.
7. Observe the fuel pressure gauge with the fuel pump commanded ON.

Is the fuel pressure within 60-66 psi?

The fuel pressure may vary slightly when the fuel pump stops operating. After the fuel pump stops operating, the fuel pressure should stabilize and remain constant.

Monitor the fuel pressure gauge for 1 minute.

Does the fuel pressure decrease more than 5 psi? This step tests for an internal fuel leak. If the fuel pressure decreases during this test, an internal loss of pressure is indicated.

1. Relieve the fuel pressure to 10 psi.
2. Monitor the fuel pressure gauge for 5 minutes.

Does the fuel pressure decrease more than 2 psi? (fuel pump or injectors)

1. Turn ON the ignition, with the engine OFF.
2. Monitor the fuel pressure gauge.
3. Start the engine.

Does the fuel pressure decrease 3-10 psi when the engine is started? This step tests the fuel pressure regulator. The fuel pressure regulator is controlled by engine vacuum. With engine vacuum applied, the pressure should decrease by the specified value.

These tests can help to determine the component to be replaced. Fuel pump and module, pressure regulator or injector assembly.
Last edited by HenryJ on Thu Dec 27, 2007 9:16 am, edited 2 times in total.
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Postby T Man » Sun Sep 02, 2007 8:00 pm

amp draw of the fuel pump is a big one. Ive seen bad pumps come through my shop that would make and hold pressure, but had incredibly high draw. To test, an inductive pickup dvom, or dmm and a jumper wire with a blade type fuse are needed. Remove stock fuse, and insert jumper wire. (The one I have at the shop has 2 male spade fittings trimmed to fit a fuse holder) Using inductive pickup ammeter, measure the amp draw.
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Joined: Wed Jun 27, 2007 7:53 pm

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