Diagnostic Aids

Perform this test when instructed during QUICK TEST or if directed by other test procedures. To prevent replacement of good components, be aware the following non-EEC related areas may be at fault:

• Excessive blow-by.
• PCV malfunction.
• Vacuum leaks.
• Incorrect fuel pressure.
• Throttle binding.

This test is only intended to diagnose:

• MAP/BARO sensor.
• Throttle Position (TP) sensor.
• Mass Airflow (MAF) sensor.
• Intake Air Temperature (IAT) sensor.
• Fuel injectors.

1) Continuous Memory Code 121, 124 Or 125: TP Sensor Integrity  Code 121 indicates TP sensor is inconsistent with MAF value. Code 124 indicates TP sensor value is higher than expected. Code 125 indicates TP sensor value is lower than expected.

Turn ignition off. Disconnect PCM 60-pin connector. Inspect terminals, and repair if damaged. Install EEC-IV Breakout Box (T83L-50-EEC-IV). Connect PCM to breakout box. Connect DVOM to test pins No. 46 and 47. Slowly apply throttle to WOT, and release to closed position. Voltage should change smoothly from 0.4-4.5 volts. See CIRCUIT TEST DH for schematics and specific engine values. If voltage change is incorrect or erratic, ensure TP sensor is properly installed. If TP sensor is properly installed, replace TP sensor and repeat QUICK TEST. If voltage is okay, go to step 7) for Code 121 or step 2) for all other codes.

2) Check Idle  Ensure idle is correct. If idle is correct, go to step 3). If idle is incorrect, see IDLE SPEED in ADJUSTMENTS article in the ENGINE PERFORMANCE Section.

3) Check Throttle Body  Ensure idle is correct. Check throttle and/or cruise control linkage for binding and rough operation. Inspect throttle body for sludge build-up. Check engine vacuum hoses. Refer to Vehicle Emission Control Information (VECI) decal for proper vacuum hose routing. Check for air leak between ISC solenoid and MAF sensor. If problems are found, repair as necessary and repeat QUICK TEST. If no problems are found, go to step 4).

4) Check MAP/BARO Sensor Output  See CIRCUIT TEST DF for MAP/BARO sensor output check. See Figure for connector terminal identification and MAP/BARO tester hookup. With tester connected and ignition on, measure sensor output voltage. If output voltage is within range for specified altitude, remove MAP/BARO tester and go to step 5). See MAP SENSOR VOLTAGE OUTPUT table. If output voltage is not within range, replace MAP/BARO sensor and repeat QUICK TEST.

If possible, measure several known good MAP/BARO sensors. Average voltage reading will be typical for location and day of testing.

5) Check IAT Sensor  Ensure ambient temperature is more than 50°F (10°C) before performing this test. Check and repair any air leaks in front of IAT sensor. Turn ignition off. Connect breakout box to PCM. Set DVOM to 20-volt scale. Connect DVOM to test pins No. 25 and 46. Start engine, and let it idle. Observe voltage as engine warms. See CIRCUIT TEST DA for voltage specifications. If voltage decreases smoothly and stabilizes when engine reaches operating conditions, system is operating properly at this time. Testing is complete. If voltage does not decrease smoothly and stabilize after engine reaches operating conditions, replace IAT sensor and repeat QUICK TEST.

6) Continuous Memory Code 184 Or 185: Inspect MAF Sensor  Code 184 indicates MAF sensor signal is higher than expected. Code 185 indicates MAF sensor signal is lower than expected. Turn ignition off. Check for air leaks between Idle Air Control (IAC) solenoid and MAF sensor. Inspect MAF sensor for oil contamination caused by excessive blow-by or malfunctioning PCV. If problems are found, repair as necessary. Clear codes, and repeat QUICK TEST. If all checks are okay, go to step 9).

7) Check MAF Sensor Circuit Voltage  Turn ignition off. Disconnect PCM 60-pin connector. Inspect and repair any damaged terminals. Install EEC-IV Breakout Box (T83L-50-EEC-IV). Connect PCM to breakout box. Start engine, and warm it to normal operating temperature. Measure voltage between test pins No. 40 and 60 and MAF test pin at breakout box. See MAF WIRE COLOR & TEST PIN IDENTIFICATION table. If voltage is not within specification, replace MAF sensor and repeat QUICK TEST. See MAF SENSOR DATA table. If voltage is within acceptable range, system is operating normally at this time. See SUMMARY in this article.

8) Continuous Memory Code 186 Or 187: Visual Vacuum Checks  Code 186 indicates pulse width is lower than expected (rich). Code 187 indicates pulse width is higher than expected (lean). Inspect air cleaner and air inlet duct. Replace or repair an necessary. Check for unmetered air leaks between MAF sensor and IAC solenoid. Check all engine vacuum hoses for damage, blockage and improper routing. Make necessary repairs, and repeat QUICK TEST. If all checks are okay, go to step 9).

9) Check Fuel Pressure  Turn ignition off. Install fuel pressure gauge. Ensure vacuum hose is connected to fuel pressure regulator (if applicable). Start engine, and run it at idle. If fuel pressure is within specification, go to step 10). For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If fuel pressure is not within specification, repair as necessary. See FUEL SYSTEM in I - GAS SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

10) Verify Fuel Pressure Retention Ability  Turn ignition on. If fuel pressure remains at specification for 60 seconds, go to step 11) (3.0L, 4.0L Calif. and 5.0L) or go to CIRCUIT TEST H, step 7) (all others). If fuel pressure does not remain at specification, repair fuel delivery system as necessary. See FUEL SYSTEM in I - GAS SYS/COMP TESTS article in the ENGINE PERFORMANCE Section.

11) Cylinder Balance Test  Perform KOER SELF-TEST. After last code, wait 5-10 seconds, and then goose throttle lightly (not wide open throttle). This will activate cylinder balance test. If Code 90 is present after test, go back to step 6). If Code 90 is not present after test, go to CIRCUIT TEST H, step 4).