Diagnostic Aids

Perform this test when instructed during QUICK TEST, or when directed by other test procedures.

This test is intended to diagnose only the following:

• MAP/BP Sensor
• Throttle Position Sensor (TPS)
• Mass Airflow (MAF) Sensor
• Air Charge Temperature (ACT) Sensor
• Fuel Injectors

To prevent replacement of good components, be aware that the following non-EEC related areas may be at fault: excessive blow-by, PCV malfunction, vacuum leak, fuel pressure or throttle linkage sticking or binding.

1) Throttle Position Sensor Integrity  Code 121 indicates TPS is inconsistent with MAF value. Code 124 indicates TPS value is higher than expected. Code 125 indicates TPS value is lower than expected.

Turn ignition off. Disconnect ECA 60-pin connector. Inspect connector for damaged pins, corrosion, or loose wires. Install breakout box and connect ECA. Set DVOM on 20-volt scale. Connect DVOM to test pins No. 47 and 46. Operate throttle slowly to WOT, and slowly release to closed position. See TEST DH for schematics and specific engine values. If there are any sudden voltage variations, ensure TPS is properly installed on throttle body. If TPS is properly installed, replace TPS; remove breakout box, reconnect all components and rerun QUICK TEST. If there are no sudden voltage variations, go to step 6 (for Continuous Memory Code 121), or go to next step (for all other codes).

2) Check Throttle Body  Check throttle and/or speed control linkage for binding and smooth operation. Inspect throttle body for contamination. Check engine vacuum hoses. Refer to VECI decal for proper vacuum hose routing. Check for air leak between ISC solenoid and MAF sensor. If all checks reveal no problems, go to next step. If problems are found, repair as necessary; remove breakout box, reconnect all components and rerun QUICK TEST.

3) Check MAP/BP Sensor Output  Refer to TEST DF for schematic. With MAP/BP tester connected and ignition on, measure sensor output voltage. If voltage output is within range at specific altitude, remove MAP/BP tester and go to next step. See MAP SENSOR VOLTAGE OUTPUT table for specification. If output reading is outside range, replace MAP/BP sensor; remove tester, reconnect all components and rerun QUICK TEST.

If possible, measure several known good MAP/BP sensors on available vehicles. Average voltage reading will be typical for your location on date of testing.

Elevation (Feet)	Volts
0	1.55-1.63
1000	1.52-1.60
2000	1.49-1.57
3000	1.46-1.54
4000	1.43-1.51
5000	1.40-1.48
6000	1.37-1.45
7000	1.35-1.43

4) Check ACT Sensor  Ensure ambient temperature is greater than 50°F before performing this test. Check and repair any air leaks in front of ACT sensor. Turn ignition off. Install breakout box and connect ECA to breakout box. Set DVOM to 20-volt scale. See TEST DF for schematic. Connect DVOM to test pins No. 25 and 46. Start engine and let idle. Observe voltage as engine warms up. If voltage does not decrease smoothly and stabilize after engine reaches operating conditions, replace ACT sensor; remove breakout box and rerun QUICK TEST. If voltage decreases smoothly and stabilizes when engine reaches operating conditions, system is operating properly at this time. Reconnect all components and rerun QUICK TEST.

5) Visual Inspection Of MAF Sensor  Code 184 indicates MAF sensor value is higher than expected. Code 185 indicates MAF sensor value is lower than expected. Turn ignition off. Check for air leaks between ISC 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 rerun QUICK TEST. If all checks are okay, go to step 8.

6) Check MAF Sensor  Turn ignition off. Disconnect ECA 60-pin connector. Inspect connector for damaged pins, corrosion, or loose wires. Install breakout box and connect ECA. Set DVOM on 20-volt scale. Start engine and allow to idle until engine reaches normal operating temperature. Measure voltage between MAF signal and test pin 40/60 of breakout box.

If voltage is within acceptable range, system is operating normally at this time. See INTERMITTENTS in the TESTS W/O CODES article in this section. If voltage is not within acceptable range, replace MAF sensor. See MAF SENSOR DATA table for specification. Reconnect all components and rerun QUICK TEST.

Engine Condition	(1) Voltage
Idle	.8
20 MPH	1.0
40 MPH	1.7
60 MPH	2.1
(1) Engine at normal operating temperature.

7) Visual Vacuum Checks  Code 186 indicates pulse width is longer than expected (rich). Code 187 indicates pulse width is shorter than expected (lean). Inspect air cleaner and air inlet duct; replace or repair an necessary. Check for unmetered air between MAF sensor and ISC bypass solenoid. Check all engine vacuum hoses for damage, leaks, blockage and proper routing. Make necessary repairs and rerun QUICK TEST. If all items are okay, go to next step.

8) Check Fuel Pressure  Turn ignition off. Install fuel pressure gauge. Ensure fuel pressure regulator is connected to manifold vacuum (if applicable). Start engine and run at idle. If fuel pressure is within specifications, go to next step. For fuel pressure specifications, see FUEL PRESSURE SPECIFICATIONS article. If fuel pressure is not within specifications, repair fuel pump or fuel pressure regulator as necessary.

9) Check Systems Ability To Hold Fuel Pressure  Turn ignition on, leaving engine off. If fuel pressure remains at specification for 60 seconds, go to next step (for SEFI), or go to TEST H, step 7 (for EFI). If fuel pressure does not remain at specification, repair fuel delivery system as necessary.

10) Cylinder Balance Test  Run KOER SELF-TEST. After last repeated code, wait 5-10 seconds 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 5. If code 90 is not present after test, go to TEST H, step 4.