GM Service Manual Online
For 1990-2009 cars only
Makes 🢒 Chevrolet 🢒 1998 🢒 Cavalier (CNG) 🢒 Engine 🢒 Engine Controls - 2.2L 🢒 Fuel System Diagnosis

Fuel System Description
Object Number: 401962 Size: MF
(1)LPR Secondary Pressure Test Port
(2)HPR Pressure Test Port
(3)LPR Primary Pressure Test Port

The alternative fuel engine control unit (AF ECU) receives a signal from the fuel pressure sensor (FPS). If there is sufficient CNG in the fuel tank, the vehicle will start and operate on CNG. A high pressure lock-off (HPL) solenoid in the tank prevents fuel flow unless the engine is cranking or running. When the key is ON and the engine is running, CNG flows from the tank through the high pressure fuel lines at up to 24 820 kPa (3,600 psi) to the high pressure regulator (HPR), where the pressure is reduced to approximately 1 172 kPa (170 psi). CNG continues to flow through the medium pressure fuel lines to the low pressure lock-off (LPL) solenoid. The LPL also prevents fuel flow unless the engine is cranking or running. From the LPL, fuel flows to the low pressure regulator (LPR) where the pressure is reduced to approximately 1.5 kPa (0.21 psi or 3.5" w.c.). Fuel then flows to the gas mass sensor/mixture control valve (GMS/MCV), which measures and controls the amount of CNG delivered to the gas distribution adapter (GDA). The GDA introduces the CNG to the air intake stream above the throttle body. This air/fuel mixture is then drawn into the engine for combustion.

Diagnostic Aids

This procedure diagnoses a vehicle operating on CNG. If the vehicle will not operate on CNG, refer to Fuel System Unwanted Switch to Gasoline for preliminary checks. While performing this diagnostic procedure, monitor the fuel indicator lamp (FIL) operation. If the lamp is illuminated, the vehicle is operating on gasoline. The vehicle will always attempt to start on CNG if enough fuel if available.

Certain DTCs (33, 34, 41, 51, 52, 53 and 58) will initiate a switch from CNG to gasoline if the fault is present during key on or crank. If these DTCs set while monitoring fuel pressure, a momentary fuel pressure reading will be observed and then quickly drop to zero. This does not necessarily indicate a problem with the AF fuel system.

Tools Required

    • J 7333-6 Slack Tube Manometer
    • J 28552 0-15 PSI Gauge
    • J 39200 High Impedance Multimeter
    • J 39970 CNG Electronic Pressure Gauge
    • J 42580 Low Pressure Test Kit
    • J 41416 Ultrasonic Leak Detector

Test Description

The number(s) below refer to the step number(s) on the diagnostic table.

  1. If there is not enough fuel in the tank, the vehicle may not operate on CNG. Also, if the CNG lines are vented to under 350 psi and the tank pressure is below 1200 psi, the vehicle will not attempt to run on CNG until the vehicle is refilled.

    2.

  2. The power brake procedure simulates a partially loaded condition. It is normal for the primary pressure to drop below specification but then quickly recover and maintain pressure.

    3.

  3. If the primary pressure is greater than 4 psi, the primary stage of the LPR is not correctly regulating pressure.

    4.

  4. This step determines if the resistance of the LPL coil is within specification. An LPL coil out of specification may not open properly and cause a fuel flow restriction.

    5.

  5. Low LPR secondary pressure may indicate a leak in the AF fuel system after the LPR. Inspect for damaged or leaking low pressure fuel hoses or a damaged or leaking GDA.

    6.

  6. This step determines if the resistance of the HPL coil is within specification. An HPL coil out of specification may not open properly and cause a fuel flow restriction.

    7.

  7. When installing the test port plugs and the PRD, coat the threads with pipe thread sealant. Do not use Teflon™ tape. Tighten the test port plugs finger-tight plus 1/2 to 1 -1/2 turns. Tighten the PRD finger-tight plus 3/4 to 2 turns.

Step

Action

Value(s)

Yes

No

1

Were you referred to this procedure by a DTC diagnostic chart?

--

Go to Step 3

Go to Step 2

2

Perform the AF Powertrain OBD System Check.

Are any DTCs present in the AF ECU?

--

Go to the applicable DTC table

Go to Step 3

3

Verify that the CNG fuel tank has a minimum of 1/4 tank of fuel.

Does the vehicle have fuel?

--

Go to Step 4

--

4

  1. Connect the J 28552 using the J 42580 in the primary test port of the low pressure regulator (LPR).
  2. Start the engine. Allow it to reach operating temperature.
  3. Record the pressure reading at idle.
  4. Apply the parking brake.

    5. Notice: Do not load the engine for more than 10 seconds or the transmission may be damaged.

  5. Place the vehicle in gear. While firmly holding the brake pedal, open the throttle to 80 % or greater while observing the pressure gauge.
  6. Record the minimum and maximum readings.

    7. Notice: To prevent transmission damage, idle the engine for two minutes to cool the transmission fluid.

  7. Allow the engine to idle for two minutes.

Was the LPR primary pressure within the specified range?

2-4 psi

Go to Step 10

Go to Step 5

5

Was the LPR primary pressure greater than the specified value?

4 psi

Go to Step 21

Go to Step 6

6

  1. Turn OFF the ignition.
  2. Disconnect the low pressure lock-off (LPL) connector C014.
  3. Install a test light between the pins of the LPL connector.
  4. Crank the engine. The test light should illuminate.

Does the test light illuminate?

--

Go to Step 7

Go to Step 9

7

Measure the resistance of the low pressure lock-off (LPL) using a DVOM.

Is the resistance within the specified range?

6.2-10 ohms

Go to Step 8

Go to Step 22

8
  1. Connect the fuel pressure gauge, the transducer and the adapter from the J 39970 CNG Electronic Pressure Gauge to the underhood HPR pressure test port.
  2. Reconnect the LPL connector.
  3. Start the engine. Allow it to reach operating temperature.
  4. Record the pressure reading at idle.
  5. Momentarily open the throttle to 75 % or greater, noting the pressure readings.
  6. Record the minimum and maximum readings obtained.
  7. Allow the engine to idle for two minutes.

Was the HPR pressure within the specified range?

115-220 psi

Go to Step 21

Go to Step 13

9
  1. Turn OFF the ignition.
  2. Connect the test light to the chassis ground. Probe pin A of the LPL connector.
  3. Crank the engine. The test light should illuminate.

Does the test light illuminate?

--

Go to Step 19

Go to Step 20

10

  1. Turn OFF the engine.
  2. Connect the Fuel Pressure Gauge J 7333-6 using the J 42580 in the secondary test port of the low pressure regulator (LPR).

    3. Important: Keep the vent port of the water column gauge closed until after the engine is started.

  3. Start the engine. Allow it to reach operating temperature.
  4. Record the pressure reading at idle.
  5. Apply the parking brake.

    6. Notice: Do not load the engine for more than 10 seconds or the transmission may be damaged.

    Important: Rhythmic fluctuations in pressure are normal and do not indicate a defective LPR unless the fluctuations become excessive.

  6. Place the vehicle in gear. While firmly holding the brake pedal, open the throttle to 80 % or greater while observing the pressure gauge.
  7. Record the minimum and the maximum readings.

    8. Notice: To prevent transmission damage, idle the engine for two minutes to cool the transmission fluid.

  8. Allow the engine to idle for two minutes.

Was the LPR secondary pressure within the specified range?

2"-5" w.c.

Go to Step 12

Go to Step 11

11
  1. Inspect the vapor hose between the LPR and the GMS/MCV for loose clamps, cracks, or deterioration.
  2. Inspect the fuel hose between the GMS/MCV and the gas distribution adapter (GDA) for loose clamps, cracks, or deterioration.
  3. Remove and inspect the GDA for physical damage and loose or missing components. Refer to Gas Distribution Adapter Replacement .
  4. Inspect the butterfly in the bore of the GMS/MCV. Verify that the butterfly moves freely and does not drag when rotated.

Was a problem found?

--

Go to Step 23

Go to Step 21

12
  1. Connect the fuel pressure gauge, the transducer, and the adapter from the J 39970 to the underhood CNG fuel line pressure test port.
  2. Reconnect the LPL connector.
  3. Start the engine. Allow it to reach operating temperature.
  4. Record the pressure reading at idle.
  5. Momentarily open the throttle to 75 % or greater, noting the pressure readings.
  6. Record the minimum and the maximum readings obtained.
  7. Allow the engine to idle for two minutes.

Was the HPR pressure within the specified range?

115-220 psi

Go to Step 25

Go to Step 13

13

Was the HPR pressure greater than the specified value?

220 psi

Go to Step 24

Go to Step 14

14
  1. Replace the HPR fuel filter. Refer to High Pressure Regulator Filter Replacement .
  2. Start the engine. Allow it to reach normal operating temperature.
  3. Record the pressure reading at idle.
  4. Momentarily open the throttle to 75 % or greater, noting the pressure readings.
  5. Record the minimum and the maximum readings obtained.
  6. Allow the engine to idle for two minutes.

Was the HPR pressure within the specified range?

115-220 psi

Go to Step 25

Go to Step 15

15

Inspect for the following conditions:

    • Kinks, bends, or damage to the CNG fuel lines.
    • Obstructions (frosting or cool to the touch during running) in the CNG fuel lines.
    • A partially closed CNG manual shut-off valve (MSV).
    • Verify that the manual lock down screw in the HPL is fully open.

Was a problem found?

--

Go to Step 23

Go to Step 16

16

  1. Turn OFF the ignition.
  2. Disconnect the high pressure lock-off (HPL) connector C453.
  3. Install a test light between the pins of the HPL connector.
  4. Crank the engine. The test light should illuminate?

Does the test light illuminate?

--

Go to Step 17

Go to Step 18

17

Measure the resistance of the HPL using a DVOM.

Is the resistance within the specified range?

16.3-20.6 ohms

Go to Step 24

Go to Step 22

18
  1. Turn OFF the ignition.
  2. Connect the test light to the chassis ground. Probe pin A of the HPL connector.
  3. Crank the engine. The test light should illuminate.

Does the test light illuminate?

--

Go to Step 19

Go to Step 20

19

Repair the open in the lock-off ground circuit. Refer to Wiring Repairs in Engine Controls - 2.2L in the J Platform Service Manual.

Is the action complete?

--

Go to Step 25

--

20

Repair the open in the lock-off power circuit. Refer to Wiring Repairs in Engine Controls - 2.2L in the J Platform Service Manual.

Is the action complete?

--

Go to Step 25

--

21

Replace the low pressure regulator (LPR). Refer to Compressed Natural Gas Low Pressure Regulator Replacement .

Is the action complete?

--

Go to Step 25

--

22

Replace the lock-off. Refer to Low Pressure Lock-Off Solenoid Replacement or High Pressure Lock-Off Solenoid Replacement .

Is the action complete?

--

Go to Step 25

--

23

Replace the component as necessary.

Is the action complete?

--

Go to Step 25

--

24

Replace the high pressure regulator. Refer to High Pressure Regulator Replacement .

Is the action complete?

--

Go to Step 25

--

25
  1. Disconnect all the test equipment.
  2. Install the primary and the secondary test port plugs.
  3. Start the engine.
  4. Leak check the test port plugs using the J 41416 or the equivalent.

Is the action complete?

--

System OK

--