Engine Performance - Poor Idle Quality

05.2-08

DATE:
11/95

Sedan Range

MODEL:
1995-96 MY
XJ12 Sedan

VIN:
720001-ON

Evaporative Emission Control (EVAP) - OBD II Monitoring - Idle Quality

ISSUE:

The performance of the Evaporative Emission Control (EVAP) is one of several functions that the On-Board Diagnostic II (OBD II) system monitors on 1995-96 MY XJ12 vehicles.

When the engine is started at operating temperature and allowed to idle for 90 to 120 seconds, the Engine Control Module (ECM) automatically performs an EVAP diagnostic routine.

During the EVAP diagnostic routine, the ECM opens each Evaporative Emission Control Valve (EVAPP) individually at a predetermined rate, which richens the mixture. The ECM then checks for a corresponding change in air/fuel ratio closed loop control value, idle speed control feedback, and engine RPM.

If the EVAPP wiring or hoses are reversed, these changes will occur on the wrong bank. As a result, the ECM will not recognize these changes on the bank being monitored at that moment. The ECM will continue to open the EVAPP, which will make the mixture overly rich, causing intermittently poor idle quality.

The ECM will then perform the diagnostic routine on the opposite engine bank with the same results.

ACTION:

If the EVAP lines or wiring must be disconnected during servicing, use care to ensure that all components are reconnected properly. In case of a customer complaint of erratic idle quality on 1995-96 MY XJ12 vehicles, check the following in addition to the more common causes of erratic idle:

1. Start the engine when at operating temperature and observe idle quality after 90 to 120 seconds. Listen for a clicking noise in the area behind the left headlamp. If the rough idle occurs when the clicking noise is heard, proceed to step 2.

2. Refer to Section 5.2 in the Jaguar XJ6 - XJ12 Vehicle Service Manual to gain access to the EVAPPs.





3. Verify the proper positioning of the EVAPP harness connectors PI18 (Purple/Green and White/Orange and PI19 (Purple/Brown and White/Orange (Illustration 1). Refer to the Sedan Range 1995 Electrical Guide Fig. 04.5 (VIN up to 739427) or Fig. 04.9 (VIN from 739427) for additional information.

4. In addition, check that each EVAPP hose leads to the correct intake manifold (A-Bank - right bank; B-Bank - left bank, as seen from the driver's seat).

5. Correct wiring and/or hose connections, if necessary.

6. Reinstall components removed for access.

7. If the complaint still exists, refer to Section 5.2 in Volume 2 of the 1995 MY XJ6-XJ12 Electrical Diagnostic Manual.





WARRANTY INFORMATION: