Test H - Fuel Control

Use this test to diagnose only the following components:

• HEGO sensor
• HEGO signal and ground circuit
• HEGO sensor connection
• Vacuum systems
• Fuel injector
• ECA
• Electrical circuits (HEGO GND, HEGO, INJ 1-8, VPWR and SIG RTN)

To prevent replacement of good components, be aware of the following non-EEC areas which may be the cause of driveability concerns:

• Ignition coil, distributor cap or rotor
• Faulty or fouled spark plugs or wires
• Faulty CANP system
• Distributorless Ignition System (DIS)
• EGR valve and gasket
• Air filter
• Fuel or engine oil contamination
• Poor electrical ground
• Faulty fuel pressure
• Intake or exhaust system leaks
• Engine below normal operating temperature

Fig 1: 3.8L PFI & 3.8L SC PFI Test Schematic

Fig 2: Test Schematic For All Other PFI Engines

1. 1) Check Fuel Pressure -  Turn ignition key off and wait 10 seconds. Install fuel pressure gauge. Verify that manifold vacuum supply is connected to fuel pressure regulator (if equipped). Run engine at idle and check fuel pressure. If vehicle will not start, cycle key on and off several times. For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If vehicle is a no start, cycle key on and off several times. If fuel pressure is within specification, go to next step. If pressure is not to specification, remove fuel pressure gauge. Repair electric fuel pump or fuel pressure regulator.
2. 2) Check Fuel Pressure Check Valve -  With KOEO, ensure that fuel pressure remains within specification for approximately 60 seconds. If system holds pressure, but vehicle runs rough or misses, or Codes 41, 42, 91 or 92 are present, go to step 9) for sequential PFI engines, or step 4) for all others. If vehicle is a no start, go to next step. If system does not hold pressure, go to step 6) for all engine applications except sequential PFI. For sequential PFI engines, go to step 9).
3. 3) Fuel Delivery Test -  Ensure fuel is of good quality. With key off, install fuel pressure gauge and pressurize fuel system as in step 1). Disconnect inertia switch. Crank engine for 5 seconds. If fuel pressure drop is greater than 5 psi after 5 seconds of cranking, the EEC-IV system is not the cause of the no start condition. Check additional no start tests in TESTS W/O CODES article. If fuel pressure drop is less than 5 psi, remove fuel pressure gauge, reconnect inertia switch, and go to next step.
4. 4) Check Injector & Harness Resistance -  Turn key off and wait 10 seconds. Disconnect ECA 60-pin connector. Inspect pins for damage. Install breakout box, leaving ECA disconnected. Set DVOM on 200-ohm scale. Do the following:

   PFI ENGINES 

   Measure resistance of injector banks between test pins No. 37 and 58 (Right or rear) or 59 (Left or front) at breakout box. Record resistance. Refer to appropriate INJECTOR BANK RESISTANCE table.

   SEQUENTIAL PFI ENGINES 

   Measure resistance between the suspect cylinder test pin (from the Cylinder Balance Test) and test pin No. 37 at the breakout box. Refer to INDIVIDUAL INJECTOR RESISTANCE  table. For no starts, choose any injector and measure resistance between that injector circuit test pin and test pin No. 37 at breakout box. Record resistance. Refer to INDIVIDUAL INJECTOR RESISTANCE  table.

   PFI ENGINES 

   Go to next step.

   SEQUENTIAL PFI ENGINES 

   Remove breakout box and reconnect ECA. Service open/short in harness/connector. If okay, replace injector. Rerun QUICK TEST and Cylinder Balance Test.

   For No Start: Service open in VPWR circuit.

5. 5) Isolate Faulty Injector Circuit -  Turn key off. Install breakout box and disconnect ECA. Disconnect all injectors on suspect bank. With DVOM set on 200-ohm scale, connect one injector and measure resistance between test pins No. 37 and 58 or 59. Disconnect injector and repeat process for all other injectors. See INDIVIDUAL INJECTOR RESISTANCE  table for specifications. If all injectors are within specifications, go to step 6). If injectors are not to specification, remove breakout box, reconnect ECA and injectors. Repair open or short in injector harness. If circuit is okay, replace injector. Rerun QUICK TEST.
6. 6) Check Injector Drive Signal -  With key off and breakout box installed, connect ECA to breakout box. Use a non-powered 12-volt test light and the following connection procedure:

   PFI ENGINES 

   Connect test light between test pins No. 37 and 58 at the breakout box. Connect light between test pins No. 37 and 59 at breakout box.

   SEQUENTIAL PFI ENGINES 

   Connect test light between test pin No. 37 and the suspect injector(s) test pin at the breakout box.

   Crank or start engine. A properly operating system is indicated by a test light that glows dimly. If light glows dimly, go to next step; except on sequential PFI engines. On sequential PFI engine, remove breakout box and reconnect ECA. Clean and test injectors. Rerun QUICK TEST and CYLINDER BALANCE TEST. If test light does not glow dimly, perform the following procedure:

   • No Light - Check for 12 volts at test pins No. 37 and 57.
   • Bright Light - Check injector circuit for shorts to ground. If system is okay, remove breakout box and replace ECA. Remove breakout box, reconnect all components and repeat QUICK TEST.
7. 7) Fuel Pressure Problems: External Sources -  Pressurize fuel system as per procedure in step 1) of this test. Examine fuel system using the following procedures as applicable:

   PFI ENGINES 

   Visually inspect, fuel injector "O" rings, fuel pressure regulator, and fuel rails. If there is a visible leak, remove pressure gauge. Repair leak as necessary. Rerun QUICK TEST. If there is no leak, go to next step. Fuel delivery system is okay. Problem is in common area for all cylinders, such as, vacuum leak, fuel contamination, or EGR leak.

8. 8) Injector Balance Test -  Connect tachometer to engine. Start and run engine at idle. Disconnect and reconnect injectors one at a time. Note RPM drop for each injector. Each injector should produce a momentary RPM drop of about 100 RPM. Note that ISC will attempt to re-establish correct RPM. If each injector does not produce at least 100 RPM drop when disconnected, service or replace faulty injector(s) and repeat QUICK TEST. If RPM drop is correct for all injectors, fuel delivery is okay. Problem is in area common to all cylinders such as a vacuum leak, fuel contamination, EGR, or HEGO ground. Check MAP frequency, etc.
9. 9) Cylinder Balance Test: Sequential PFI Engines ONLY -  Perform KOER Self-Test. After last code is displayed, wait 5-10 seconds. Enter CYLINDER BALANCE TEST by briefly opening throttle for one second (not WOT). Test time is approximately 2-3 minutes. A Code 77 indicates that the throttle was touched when the test was running and that the test was not completed. Use table to interpret codes received from test. If balance test does not display Code 90, go to step 4). If balance test displays Code 90 (Pass), fuel delivery is okay, fault is in an area common to all cylinders (except vehicles with Codes 41/91 or 42/92). Vehicles with Code 41 or 91 displayed, go to next step. If a Code 42 or 92 is displayed, go to step 23)
   CYLINDER BALANCE TEST SERVICE CODES
   
   

   Service Code	Cylinder or Injector No.	Breakout Box Test Pin No.
   90	(Pass)	(Pass)
   10	1	58
   20	2	59
   30	3	12
   40	4	13
   50	5	14
   60	6	15
   70	7	42
   80	8	52
   77	(Retest)	(Retest)

   NOTE: There is a break in the step numbering sequence at this point, skipping from step 9) to step 11). Break is due to previous chart references to these testing procedures. No test procedures have been omitted.
10. 11) Code 41 or 91: Fuel Control Always Lean -  If vehicle is equipped with dual HEGOs, Code 41 refers to the Right (rear) HEGO and Code 91 refers to the Left (front) HEGO. Run engine at 2000 RPM for 2 minutes. Turn key off and wait 10 seconds. Rerun KOER Self-Test. Check for Code 41 or 91. If Code 41 or 91 is not displayed, go to step 20). If Code 41 or 91 is displayed, go to appropriate step number as listed below.
    • Engines equipped with MAP sensors: Go to next step.
    • Engines equipped with MA meters: Go to step 14).
11. 12) Checking HEGO Sensor on Engines With MAP Sensors -  Turn key off and set DVOM on 20-volt scale. Disconnect appropriate HEGO sensor from vehicle harness. Connect DVOM to HEGO SIGNAL lead at sensor and engine ground. Apply 10-14 in Hg to Manifold Absolute Pressure (MAP) sensor and start engine. Run engine at 2000 RPM for 2 minutes. If reading is greater than 0.5 volt, go to step 15). If reading is less than 0.5 volt, replace HEGO sensor, reconnect MAP, and rerun QUICK TEST.
    NOTE: Vacuum or air leaks in non-EEC-IV related areas may cause Code 41 or 91. Check the following areas: leaking vacuum actuator (A/C control motor, etc.) engine seals, EGR system, PCV system and lead contamination of HEGO sensor.
    Fig 3: Checking HEGO Sensor

    
    NOTE: Vacuum or air leaks in non-EEC-IV related areas may cause Code 41 or 91. Check the following areas: leaking vacuum actuator (A/C control motor, etc.) engine seals, EGR system, PCV system and lead contamination of HEGO sensor.
    Fig 4: Checking HEGO Sensor & Circuit

    
    NOTE: There is a break in the step numbering sequence at this point, skipping from step 12) to step 14). Break is due to previous chart references to these testing procedures. No test procedures have been omitted.
12. 14) Check HEGO Sensor on Engines With MA Sensor -  The purpose of this test is to verify HEGO sensor can generate a voltage signal of greater than 0.5 volt during KOER Self-Test. With key off and DVOM set on 20-volt scale and disconnect HEGO sensor from harness.

    4-WIRE HEGO SENSORS 

    Connect DVOM between HEGO SIGNAL and HEGO GND at the HEGO sensor connector.

    ALL OTHER HEGO SENSORS 

    Connect DVOM to HEGO SIGNAL at the sensor and battery negative post.

    Rerun KOER Self-Test and monitor HEGO sensor voltage. If voltage is greater than 0.5 volt, go to next step. If voltage is less than 0.5 volt, replace HEGO sensor and rerun QUICK TEST.

    NOTE: Vacuum or air leaks in non-EEC-IV related areas may cause Code 41 or 91. Check the following areas: leaking vacuum actuator (A/C control motor, etc.) engine seals, EGR system, PCV system and lead contamination of HEGO sensor.
    Fig 5: Checking HEGO Sensor

    
13. 15) Check Continuity of HEGO SIGNAL & POWER GROUND Circuits -  Turn key off. Install breakout box, leaving ECA and HEGO disconnected. Set DVOM on 200-ohm scale. Measure resistance between test pin No. 49 at the breakout box and battery negative terminal. Measure resistance between test pin No. 29 at the breakout box and HEGO SIGNAL at the vehicle harness connector.

    ALL VEHICLES WITH DUAL HEGO SENSORS 

    Measure resistance between test pin No. 43 at the breakout box and HEGO SIGNAL at the vehicle harness connector. If all resistances are less than 5 ohms, go to next step. If not, repair open circuit. Remove breakout box, reconnect all components and repeat QUICK TEST.

    Fig 6: HEGO Sensor Connector

    
14. 16) Check HEGO Circuit For Short to Ground -  Turn key off. Leave breakout box installed and ECA disconnected. Disconnect HEGO sensor and set DVOM on 200-k/ohm scale. Measure resistance between test pin No. 29 and test pins No. 40 and 49 at the breakout box. For vehicles with dual sensors, measure resistance between test pin No. 43 and test pin No. 40. If any reading is less than 10 k/ohms, repair short to ground. Remove breakout box, reconnect all components and repeat QUICK TEST. If reading is 10 k/ohms or more, go to next step.
15. 17) Check HEGO Sensor For Short to Ground -  With key off, breakout box installed, and ECA disconnected, set DVOM to 200-k/ohm scale. Disconnect HEGO sensor. Measure resistance between PWR GND and HEGO SIGNAL at the HEGO sensor connector. If resistance is less than 10 k/ohms, replace HEGO sensor, remove breakout box, reconnect all components and repeat QUICK TEST. If resistance is greater than 10 k/ohms, go to the appropriate following procedure.
    • Engines equipped with MAP sensor: Go to next step.
    • Engines equipped with MA (Mass Air Sensor): Remove breakout box, reconnect HEGO sensor, replace ECA and rerun QUICK TEST. On 3.8L SC PFI engine, check supercharger system components before replacing ECA.
    Fig 7: Checking HEGO Sensor

    
16. 18) Attempt to Eliminate Code 41: Engines With MAP Sensor -  Turn key off. Install breakout box and reconnect ECA and HEGO sensor. Ensure MAP sensor vacuum hose is disconnected and plugged. Start and run engine at 2000 RPM for 2 minutes. Allow engine to return to idle. Apply 10-14 in. Hg to MAP sensor. Perform KOER Self-Test. If Code 41 is still present, remove breakout box. Reconnect ECA and MAP sensor vacuum line. If engine still runs rough, go to TEST S, step 2). If not, replace ECA and repeat QUICK TEST. If Code 41 is not present, remove breakout box and reconnect ECA and MAP sensor vacuum hose. HEGO input circuit is okay. On sequential PFI engines, fuel delivery is okay, fault is in an area common to all areas. Service as necessary. On all other engines return to step 1) of this test.
    NOTE: There is a break in the step numbering sequence at this point, skipping from step 18) to step 20). Break is due to previous chart references to these testing procedures. No test procedures have been omitted.
17. 20) Check Resistance of HEGO Heating Element -  Turn key off, disconnect HEGO sensor, and set DVOM on 200-ohm scale. Measure resistance between KPWR circuit and PWR GND circuit at HEGO sensor connector. Hot to warm resistance should be 5.0-25.0 ohms. Room temperature resistance is 2.0-5.0 ohms. If resistance is within specification, go to next step. If not, replace HEGO sensor, and repeat QUICK TEST.
    Fig 8: Checking HEGO Sensor

    
18. 21) Check For Power at HEGO Harness Connector -  With KOEO, and HEGO disconnected, set DVOM on 20-volt scale. Measure voltage between KEY POWER circuit and PWR GND circuit at HEGO vehicle harness connector. If voltage is greater than 10.5 volts, HEGO system is okay, go to step 1) of this test (except sequential PFI engines). On vehicles equipped with sequential PFI engines, fuel delivery is okay, HEGO sensor may have cooled prior to KOER Self-Test. If a driveability symptom persists, fault is in an area common to all cylinders such as air or vacuum leaks, fuel contamination, EGR, MAP, etc. For all vehicles, if voltage is less than 10.5 volts, go to next step.
    Fig 9: HEGO Sensor Connector

    
19. 22) Check Continuity of Power Ground Circuit -  Turn key off and wait 10 seconds. Disconnect HEGO sensor and set DVOM on 200-ohm scale. Measure resistance between PWR GND circuit at HEGO vehicle harness connector and battery negative post. If resistance is less than 5 ohms, repair open in KEY POWER circuit. Reconnect HEGO and rerun QUICK TEST. If resistance is more than 5 ohms, reconnect HEGO sensor and repair open in PWR GND circuit. Reconnect HEGO sensor and rerun QUICK TEST.
20. 23) Code 42 or 92: Fuel Control Always Rich, Check HEGO SIGNAL For Short To Power -  On dual HEGO systems, Code 42 refers to right (or rear) side HEGO, Code 92 refers to left (or front) side HEGO. Turn key off and wait 10 seconds. Disconnect appropriate HEGO sensor. Set DVOM on 20-volt scale. With KOEO, measure voltage between HEGO SIG and PWR GND at HEGO vehicle harness connector. If reading is more than .5 volt, repair short to power in HEGO circuit. Reconnect HEGO sensor and repeat QUICK TEST. If reading is less than 0.5 volt, go to next step.
    Fig 10: HEGO Sensor Connector

    
21. 24) Check HEGO Sensor For Short to KPWR Circuit -  Turn key off. Disconnect HEGO sensor. Set DVOM on 200-k/ohm scale. Measure resistance from KPWR circuit to HEGO SIG circuit at HEGO sensor connector. If reading is 10 k/ohms or less, replace HEGO sensor and repeat QUICK TEST. If reading is greater than 10 k/ohms, go to next step.
22. 25) Attempt to Generate Code 41 or 91 -  The following non-EEC areas may cause these codes: fuel contaminated engine oil, ignition misfire and faulty CANP system. Turn key off and wait 10 seconds. Disconnect appropriate HEGO sensor. Using jumper wire, connect HEGO SIG circuit at HEGO vehicle harness connector to battery negative terminal. Repeat KOER Self-Test. If Code 41 or 91 is present, remove jumper wire and go to next step for engines with MAP. For all other engines, go to step 28). If Codes 41 and 91 are not present, remove jumper wire. Disconnect ECA connector and inspect for damage. If connector is okay, replace ECA. Repeat QUICK TEST.
    NOTE: Because MAP sensor has a large influence on fuel control, a Code 42 or 92 may be the result of a faulty MAP sensor, even though a Code 22 has not been set. The next 2 test steps will verify the MAP vacuum circuit.
23. 26) Check MAP Sensor For Vacuum Leaks -  Turn key off and wait 10 seconds. Disconnect vacuum hose from MAP sensor and connect vacuum pump to sensor. Apply 18 in Hg to MAP sensor. If sensor does not hold vacuum, replace MAP sensor. Remove vacuum pump. Connect vacuum hose and reconnect HEGO. Repeat QUICK TEST. If MAP holds vacuum, release vacuum and go to next step.
24. 27) Check For Loss of Vacuum to MAP Sensor -  Using vacuum "T", connect vacuum gauge in intake manifold vacuum hose at MAP sensor. Start engine and note vacuum reading at a stable idle. Turn key off and wait 10 seconds. Remove "T" and vacuum gauge. Reconnect hose to MAP sensor. Connect vacuum gauge at a different intake manifold location. Start engine and note vacuum reading. If readings differ by more than 1 in. Hg, inspect vacuum hoses for leaks, kinks, or blockage. Repair as necessary. If readings are within 1 in. Hg of each other, go to next step.
25. 28) HEGO Sensor Check -  Turn key off and wait 10 seconds. Disconnect HEGO sensor. Set DVOM on 20-volt scale and connect to HEGO SIGNAL at HEGO sensor connector and battery negative post. Remove a vacuum hose to create a vacuum leak, which will cause the HEGO sensor to go lean.

    ALL MA PFI ENGINES 

    Disconnect any vacuum hose from manifold vacuum tree fitting.

    ALL OTHER ENGINES 

    Disconnect the PCV valve hose from the PCV valve.

    Start engine and run at approximately 2000 RPM. DVOM should indicate less than 0.4 volt within 30 seconds. If not, replace HEGO sensor. Reconnect vacuum hoses and repeat QUICK TEST. If voltage is to specification, HEGO sensor is okay. If engine has sequential PFI, fuel delivery is okay, problem is in an area common to all cylinders, such as a vacuum leak, fuel contamination, EGR system, MAP frequency, etc. Service as necessary. For all other engines, go to step 1) of this test.

26. 29) Continuous Memory Code 41 or 91 (With Exhaust Odor or Smoke) -  Codes 41 or 91 indicate that a HEGO circuit has not switched during closed loop fuel control. In this situation a Code 41 or 91 may not indicate a lean condition. Diagnose all other driveability complaints (see SYMPTOMS in TESTS W/O CODES article) before attempting to service a Continuous Memory Code 41 or 91. Check the following areas:

    Circuits:

    • HEGO SIG and GND circuits
    • HEGO harness insulation (near exhaust manifold)

    Unmetered Air/Vacuum Leaks:

    • Canister Purge (CANP) system
    • PCV system
    • Engine sealing
    • Crimped fuel lines
    • Plugged fuel filter
    • Fouled injectors
    • Air leaks between MAF and air outlet tube

    HEGO Fuel Fouled:

    • If an over-rich condition is suspected of fouling the HEGO sensor, run the vehicle at sustained high (legal) speeds followed by a few hard acceleration runs. This will allow the HEGO contamination to be burned off.

    Ignition System:

    • Check for DEFAULT base timing in QUICK TEST procedure.

    Improper Fuel Used:

    • Lead fouled HEGO sensor.

    Fuel Pressure:

    • Check pressure using steps 1) and 2) of this test.

    TP Sensor:

    • With KOEO, move throttle slowly to WOT. Measure voltage between test pins No. 47 and 46 at breakout box. If voltage does not increase with throttle opening, replace TP sensor or linkage as necessary.

    If vehicle still has driveability problems, perform test steps 3) through 6).

27. 30) Continuous Memory Codes 41, 42, 43, 65, 85 or 86 -  Diagnose all other driveability complaints such as rough idle, missing, etc. before servicing these codes:

    CODE 41 

    HEGO indication that the fuel system was lean for more than 15 seconds when the fuel system should have been in closed loop fuel control.

    CODE 42 

    HEGO indication that the fuel system was rich for more than 15 seconds when the fuel system should have been in closed loop fuel control.

    CODE 43 

    HEGO indicated that the fuel system was lean during WOT for more than 3 seconds.

    CODE 65 

    Closed loop not activated on HEGO switching.

    CODE 85 

    Excessive rich mixture condition corrected (made leaner) by adaptive fuel strategy.

    CODE 86 

    Excessive lean fuel mixture condition corrected (made richer) by adaptive fuel strategy.

    If an over-rich fuel condition is suspected or experienced, check ignition system. Burn off fuel deposits on HEGO by operating the vehicle at sustained high (legal) speeds followed by a few hard acceleration runs. This will allow the HEGO contamination to be burned off. Check the following areas:

    Code 41:

    • Intermittent HEGO circuit (SIG or GND circuit).
    • If Code 65 also present, repair faulty HEGO circuit (SIG or GND circuit).
    • If Code 43 also present, service Code 43 first.
    • Low fuel pressure at WOT, check fuel supply.
    • Low fuel flow at WOT (pressure okay), check clogged injectors or low battery voltage.

    Code 42:

    • Intermittent HEGO signal (SIG or GND circuit).
    • Excessive fuel flow, pressure regulator vacuum supply disconnected or blocked fuel return line.
    • Excessive fuel flow, damaged or stuck open injector(s).

    Codes 41 and 42:

    • Intermittent HEGO circuit (SIG or GND circuit).
    • Contaminated HEGO sensor, lead or silicone fouled.

    Code 43:

    • Low fuel pressure at WOT, fuel pump or restricted supply.
    • Low fuel flow at WOT, injectors plugged or low battery voltage (less than 11 volts).

    Code 65:

    • Check for faulty HEGO signal (Signal or Ground circuit).

    Code 85:

    • If Code 42 also displayed, service it first.
    • Excessive fuel pressure, check regulator vacuum line and fuel supply line.
    • Excessive fuel flow, check for damaged injector pintles or injector stuck open.

    Code 86:

    • If Code 41 also displayed, service it first.
    • Low fuel pressure, low pressure pump or restricted fuel supply.
    • Low fuel flow, clogged injectors or low battery voltage.