Friday, January 27, 2012

How to test Electric fuel pump on GMC sonoma?

The procedure with details is as follows :----

The electric pump is attached to the fuel sending unit, located in the fuel tank.


See Figure 1
  1. Properly relieve the fuel system pressure.
  2. Leave the gauge attached to the pressure fitting on the fuel inlet pipe.
  3. If disconnected during the fuel pressure relief procedure, reconnect the negative battery terminal.
  4. If the engine will run, start the engine and allow it to run at normal idle speed. The fuel pressure should be 58-64 psi (400-440 kPa). Once the engine is at normal operating temperature, open the throttle quickly while noting fuel pressure; it should quickly approach 64 psi (440 kPa) if all components are operating properly (there is no need to proceed further). If the pressure was in specification before, but does not approach 64 psi (440 kPa) on acceleration, the pressure regulator in the CMFI unit is faulty and the assembly should be replaced.
  5. If the engine does not run, turn the ignition ON , but do not attempt to start the engine. Listen for the fuel pump to run. Within 2 seconds of turning the ignition ON pressure should be 58-64 psi (400-440 kPa) while the pump is running. Once the pump stops, pressure may vary by several pounds, then it should hold steady. If the pressure does not hold steady, wait 10 seconds and repeat this step, but pinch the fuel pressure line flexible hose and watch if the pressure holds. If it still does not hold, the CMFI unit should be replaced. If the pressure holds with the pressure line pinched, check for a partially disconnected fuel dampener (pulsator) or faulty in-tank fuel pump.

Click image to see an enlarged view
Fig. Fig. 1: CMFI fuel system pressure testing schematic
  1. If the fuel pump did not run or system pressure did not reach specification, locate the fuel pump test connector. The test connector is usually found on the driver's side of the engine compartment (on or near the fender), with a single wire (usually red) leading from the relay to the connector. Using a 10 amp fused jumper wire, apply battery voltage to the test connector in order to energize and run the fuel pump. The pump should run and produce fuel pressure of 58-64 psi (400-440 kPa). If the pump does not run, check the relay and fuel pump wiring.
  2. If the pump pressure was lower than specification, first check for a restricted fuel line, filter or a disconnected fuel pulse dampener (pulsator) and repair/replace, as necessary. If no restrictions can be found, restrict the flexible fuel return line (by gradually pinching it) until the pressure rises above 64 psi (440 kPa), but DO NOT allow pressure to exceed 75 psi (517 kPa). If the fuel pressure rises above specification with the return line restricted, then the pressure regulator is faulty and the CMFI assembly should be replaced. If pressure still does not reach specification, check for a faulty fuel pump, partially disconnected fuel pulse dampener (pulsator), partially restricted pump strainer or an incorrect pump.
  3. If during the previous steps, the fuel pressure was higher than specification, relieve the system pressure, then disconnect the engine compartment fuel return line. Attach a 5 / 16 ID flex hose to the fuel line from the throttle body and place the other end into an approved gasoline container. Cycle the ignition in order to energize the fuel pump and watch system pressure. If pressure is still higher, check for restrictions in the line between the pressure regulator and the point where it was disconnected. Repair or replace the line if restrictions are found or replace the CMFI assembly with the faulty internal pressure regulator if no other causes of high pressure are identified. If fuel pressure is normal only with the rest of the return line out of the circuit, check that remaining line for restrictions and repair or replace, as necessary.
  4. Once the test is completed, depressurize the fuel system and remove the gauge.

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