Diagnostic Aids
Perform this test only when directed by CIRCUIT TEST AA, CIRCUIT TEST AB or CIRCUIT TEST AC. This test is intended to diagnose:
- IAC system.
- MAP/BARO system.
- EGR system.
1) IAC Check Try to start engine at part throttle. If engine starts and runs smoothly at part throttle, go to CIRCUIT TEST KE, step 4). If engine does not run as described, go to step 3). If engine starts and idles smoothly, go to next step.
2) Check For RPM Drop Turn ignition off. Connect a tachometer to engine. Start engine. Disconnect IAC solenoid. If RPM drops or engine stalls, reconnect IAC solenoid and go to next step. If RPM does not drop, CIRCUIT TEST KE, step 4).
3) Power To MAP/BARO Sensor If vehicle is not equipped with a MAP/BARO sensor, go to step 8). If vehicle is equipped with a MAP/BARO sensor, turn ignition off. Disconnect MAP/BARO sensor. Connect MAP/BARO tester between wiring harness and MAP/BARO sensor. See Fig 1. Connect banana plugs of tester into DVOM. Turn ignition on. If Green light is on, go to next step. If Green light is not on, repair open in VREF circuit. Remove tester and reconnect connectors. Check system operation.
4) MAP/BARO Tester Output Reading Measure several known good MAP/BARO sensors to obtain average voltage for location and date of testing. With MAP/BARO tester and DVOM connected, turn ignition on. Measure MAP/BARO voltage at MAP sensor. If voltage is not within specification for specific altitude, replace MAP/BARO sensor. See MAP VOLTAGE OUTPUT table. If voltage is in within specification for specific altitude, go to step 8) (MAF vehicles) or next step (except MAF vehicles).
| Approximate Elevation (Ft.) | Voltage Output (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 |
5) Check Vacuum Lines Check vacuum hoses for correct routing. Refer to VECI decal. Check MAP sensor vacuum hose for leaks and blockage. If vacuum hoses are okay, go to next step. If vacuum hoses are not okay, repair as necessary. Repeat QUICK TEST.
6) Check MAP Sensor Turn ignition off. Disconnect vacuum hose from manifold vacuum source. Connect vacuum pump to MAP sensor hose. Apply 18 in. Hg vacuum to MAP sensor. If MAP sensor holds vacuum, go to next step. If MAP sensor does not hold vacuum, replace MAP sensor or hose and repeat QUICK TEST.
7) Check Vacuum Manifold Source Install vacuum gauge at manifold vacuum source. Start engine and observe vacuum gauge. If manifold vacuum is present, remove vacuum gauge. Reconnect MAP sensor vacuum hose, and go to next step. If vacuum is not present, remove obstruction and repeat QUICK TEST.
8) Check EGR Vacuum If vehicle is not equipped with EGR system, go to CIRCUIT TEST H, step 2). If vehicle is equipped with EGR, disconnect and plug vacuum line at EGR valve. Attempt to start engine. If vehicle could not start previously and still does not start or if vehicle driveability fault is still present, go to next step. If vehicle did not start previously but starts now or if driveability symptoms are eliminated, go to one of the following:
- Models with Differential Pressure Feedback EGR (DPFE), go to CIRCUIT TEST DL, step 50).
- Models with Pressure Feedback EGR (PFE), go to CIRCUIT TEST DL, step 21).
- On Probe Federal M/T, check EGR system. See EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM/COMPONENT TESTS - EEC-IV (2.0L) article in the ENGINE PERFORMANCE section.
- On Probe except Federal M/T, go to CIRCUIT TEST DD, step 31).
9) Check EGR Valve Inspect EGR valve for leaks. If valve is fully closed, go to CIRCUIT TEST H. If EGR valve is leaking or is not fully seated, repair or replace valve as necessary.