Fuel System Diagnosis. Geo Prizm 1997

For 1990-2009 cars

Schematic Reference: Refer to Engine Controls Schematics

### Circuit Description

When the ignition switch is turned to the "START" position, the circuit opening relay will be energized to allow the fuel pump to operate and pressurize the system. The fuel pump will continue to operate if the Engine Control Module (ECM) receives an ignition signal when the engine is started. The fuel pump operates as long as the circuit opening relay is energized by the ECM. When the circuit opening relay is de-energized, the fuel pump will stop operating. The fuel pump delivers fuel to the fuel rail where the pressure is maintained by the fuel pressure regulator at 284 kPa (41 psi) higher than the pressure inside the intake manifold. Excess fuel is returned to the fuel tank through a return pipe and hoses.

### Diagnostic Aids

Check for leakage around all fuel hose/pipe connections. A plugged fuel injector could be the cause of a hard/no-start condition or loss of power/poor fuel economy. Refer to Symptoms for mechanical checks and inspections if fuel spray and pressure is normal.

### Test Description

The numbers below refer to the step numbers in the Diagnostic Table.

1. The OBD System Check prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.

2. To relieve the fuel pressure, refer to the Fuel Pressure Relief Procedure. With terminals "Fp" and +B jumpered and the ignition switch "ON" with the fuel pump running, the fuel pressure indicated by the fuel pressure gauge should be 265-304 kPa (38-44 psi). This pressure is controlled by the amount of pressure the spring inside the fuel pressure regulator can provide.

3. A fuel system that can not maintain a constant fuel pressure if it has a leak in one or more of the following areas:

• | The Fuel Pump Check Valve.

• | The Fuel Pump Flex Pipe.

• | The Valve or Valve Seat within the Fuel Pressure Regulator.

• | The Fuel Injector(s).

1. Fuel pressure that drops-off during acceleration, cruise, or hard cornering may cause a lean condition. A lean condition can cause a loss of power, surging, or misfire. A lean condition can be diagnosed using a scan tool. If an extremely lean condition occurs, the Oxygen Sensor(s) will stop toggling. The oxygen sensor output voltage(s) will drop below 500 mV. Also, the Fuel Injector Pulse Width will increase.

2. When the engine is at idle, the manifold pressure is low (high vacuum). This low pressure (high vacuum) is applied to the fuel pressure regulator diaphragm. The low pressure (high vacuum) will offset the pressure being applied to the fuel pressure regulator diaphragm by the spring inside fuel pressure regulator. When this happens, the result is lower fuel pressure. The fuel pressure at idle will vary slightly as the barometric pressure changes, but the fuel pressure at idle should always be less than the fuel pressure noted in Step 2 with the engine "OFF".

3. A rich condition may result from the fuel pressure being above 325 kPa (47 psi). A rich condition may cause a DTC P0130 or a DTC P0172 to set. Driveability conditions associated with rich conditions can include hard starting (followed by black smoke) and a strong sulfur smell in the exhaust.

4. This test determines if the high fuel pressure is due to a restricted fuel return pipe or if the high fuel pressure is due to a faulty fuel pressure regulator.

Step | Action | Value(s) | Yes | No
--- | --- | --- | --- | ---
1 | Was the Engine On-Board Diagnostic (OBD) System Check performed? | -- | Go to Step 2 | Go to Powertrain On Board Diagnostic (OBD) System Check

2

1. Turn OFF the ignition.
2. Turn the Air Conditioning System OFF.
3. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
4. Install a Fuel Pressure Gauge. Refer to Fuel Pressure Gage Installation and Removal .
5. Connect a fused jumper from the DLC (underhood) terminals "Fp" to "+B".
6. Turn ON the ignition.
7. Observe the fuel pressure indicated by the fuel pressure gauge with the fuel pump running.

Is the fuel pressure within the specified limits?

265-304 kPa (38-44 psi)

Go to Step 3

Go to Step 20

3 | Does the fuel pressure indicated by the fuel pressure gauge remain constant? | -- | Go to Step 4 | Go to Step 14
4 | Is the fuel pressure suspected of dropping-off during acceleration, cruise, or hard cornering? | -- | Go to Step 5 | Go to Step 10

5

Visually and physically inspect the following items for a restriction:

• | The In-Pipe Fuel Filter

• | The Fuel Feed Pipe

Was a restriction found?

--

Go to Step 6

Go to Step 7

6 | Repair the restriction as necessary. Is the action complete? | -- | Go to Step 29 | --

7

Visually and physically inspect the following items:

• | The fuel pump strainer for a restriction.

• | The fuel pump flex pipe for leaks

• | Verify the fuel pump is the correct fuel pump for this vehicle

• | The fuel pump pick-up position.

Was a problem found in any of these areas?

--

Go to Step 8

Go to Step 9

8 | Repair the condition as necessary. Is the action complete? | -- | Go to Step 29 | --
9 | Replace the fuel pump. Is the action complete? | -- | Go to Step 29 | --

10

1. Remove the fused jumper.
2. Start the engine.
3. Allow the engine to idle at normal operating temperature.

Does the fuel pressure indicted by the fuel pressure gauge drop by the amount specified?

21-69 kPa (3-10 psi)

Go to Symptoms

Go to Step 11

11

1. Disconnect the vacuum hose from the fuel pressure regulator.
2. With the engine idling, apply 14-18 inches of vacuum to the fuel pressure regulator.

Does the fuel pressure indicated by the fuel pressure gauge drop by the amount specified?

21-69 kPa (3-10 psi)

Go to Step 12

Go to Step 13

12 | Locate and repair the loss of vacuum to the fuel pressure regulator. Is the action complete? | -- | Go to Step 29 | --
13 | Replace the fuel pressure regulator. Is the action complete? | -- | Go to Step 29 | --

14

1. Leave the fused jumper connected to the DLC (underhood) terminals
2. Turn On the ignition.
3. Wait for the fuel pressure to build.
4. Disconnect the fused jumper.
5. Slowly pinch off the fuel feed hose.

Does the fuel pressure indicated by the fuel pressure gauge remain constant?

--

Go to Step 15

Go to Step 17

15 | Visually and physically inspect the fuel feed pipe for leakage, between the fuel rail and fuel pump. Is the fuel feed pipe leaking? | -- | Go to Step 16 | Go to Step 9
16 | Repair the fuel feed pipe. Is the action complete? | -- | Go to Step 29 | --

17

1. Connect a fused jumper from the DLC (underhood) terminals "Fp" to "+B".
2. Turn ON the ignition.
3. Wait fro the fuel pressure to build.
4. Disconnect the fused jumper.
5. Slowly pinch off the fuel return hose.

Does the fuel pressure indicated by the fuel pressure gauge remain constant?

--

Go to Step 18

Go to Step 19

18 | Replace the fuel pressure regulator. Is the action complete? | -- | Go to Step 29 | --
19 | Locate and replace any leaking fuel injector(s). Is the action complete? | -- | Go to Step 29 | --
20 | Is the fuel pressure indicated by the fuel pressure gauge above the specified limit? | 304 kPa (44 psi) | Go to Step 21 | Go to Step 24

21

1. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
2. Disconnect the fuel return hose from the fuel pressure regulator.
3. Attach a length of flexible hose to the return outlet passage.
4. Place the open end of the flexible hose into an approved gasoline container.
5. Leave the fused jumper connected to the DLC (underhood) terminals.
6. Turn ON the ignition.
7. Observe the fuel pressure indicated by the fuel pressure gauge with the fuel pump running.

Is the fuel pressure within the specified limits?

265-30 kPa (38-44 psi)

Go to Step 22

Go to Step 23

22 | Locate and correct the restriction in the fuel return pipe. Is the action complete? | -- | Go to Step 29 | --
23 | Visually and physically inspect the fuel rail outlet passages for the restriction. Was a restriction found? | -- | Go to Step 6 | Go to Step 13
24 | Is the fuel pressure indicated by the fuel pressure gauge above the specified value? | 0 kPa (0 psi) | Go to Step 25 | Go to Step 26

25

1. Relieve the fuel pressure. Refer to Fuel Pressure Relief .
2. Leave the fused jumper connected to the DLC (underhood) terminals.
3. Turn ON the ignition.
4. Slowly pinch off the fuel return hose.

Do not let the fuel pressure exceed the specified value.

Does the fuel pressure indicated by the fuel pressure gauge rise above the specified value?

305 kPa (45 psi)

Go to Step 13

Go to Step 7

26 | Use Chart A-7A table to diagnose the fuel pump electrical circuit. Was a problem found with the fuel pump electrical circuit? | -- | Go to Step 29 | Go to Step 27

27

Visually and physically inspect the following items:

• | The In-Pipe Fuel Filter for obstructions.

• | The Fuel Feed Pipe for a restriction.

• | The Fuel Pump Strainer for obstructions.

• | The Fuel Pump Flex Pipe for leaks.

Was a problem found in any of these areas?

--

Go to Step 28

Go to Step 9

28 | Repair the problem as necessary. Is the action complete? | -- | Go to Step 29 | --

29

1. Remove the fused jumper.
2. Turn ON the ignition.
3. Observe the fuel pressure indicted by the fuel pressure gauge with the fuel pump running.

Is the fuel pressure within the specified limits?

265-304 kPa (38-44 psi)

Go to Step 30

Go to Step 20

30 | Does the fuel pressure indicated by the fuel pressure gauge remain constant? | -- | Go to Step 31 | Go to Step 14

31

1. Using a scan tool, clear the DTCs.
2. Attempt to start the engine.

Does the engine start and continue to run?

--

Go to Step 32

Go to Step 2

32

1. Allow the engine to idle until normal operating temperature is reached.
2. Select the DTC.

Are any DTCs displayed?

--

Go to Applicable DTC Table

System OK