Engine - Diagnosis of AJ6 Driveability Symptoms
303-47DATE 10/01
V8 XJ Series/XK
MODEL
1997-98 MY XK8 Range
1998 MY V8 XJ Series
1998-99 MY XJR
VIN
Hesitation, Surging, Detonation, Poor Throttle Response/Acceleration - Diagnostic Flowcharts - AJ 26 Engines
Issue:
This Technical Bulletin provides diagnostic information relating to various engine driveability concerns such as hesitation, backfiring, surging, knocking, poor throttle response, and cruise control inhibited. The diagnosis is organized by the symptoms that the customer experiences.
Action:
Before using the guide, check the symptom Matrix for the diagnostic flowchart sequence. For example, if the cruise control is inhibited or disabled, work through flowcharts P28, P30 and P27.
Following the flowcharts in the sequence identified will help rectify any concerns while optimizing the utilization of workshop time.
Note:
When working on any vehicle ensure that exposed paintwork is protected with the appropriate fender protection covers.
RELAY CODES AND DATE STAMPS
^ Relay date codes come in two forms:
^ Relays prior to December 1998 have the format - Letter - Number - Letter i.e. R6 K1
^ Relays after December 1998 have the format Number - Letter - Letter i.e. 2AB
All date codes are printed in white on the top face of the relay, adjacent to the part number.
Warning:
Working on the fuel system can result in fuel vapor being emitted into the atmosphere. Fuel vapor is extremely flammable; hence great care should be taken when working on the fuel system.
Do not smoke in the working area and ensure that there is a C02 fire extinguisher close by. The working area must be well ventilated and extraction equipment used when appropriate. When emptying fuel, use suitable fireproof equipment and an authorized explosion-proof container.
Symptom Matrix
P1 Contaminated Throttle Diagnostic Flowchart
P14 Engine Harness
P15 ECM
P17 Fuel Pump
P19 Mass Air Flow Meter
P20 Fuel Pressure Regulator
P21 Fuel Lines
P22 Oxygen Sensors
P23 Air Leakage
P24 Throttle Sensors
P25 Throttle Motor
P24 and P25 Additional information
Throttle system failure modes
Throttle Control Modes:
1. Normal
2. Cruise
3. Mechanical guard
4. Fixed idle
5. Redundancy
6. Full authority
7. Engine shut down
Normal mode occurs when the ECM uses the mechanical and monitoring arrangement of the throttle valve to control throttle opening. The ECM does not permit driver demand to be exceeded but it can be restricted to allow for such features as stability traction control or engine power limitation. The ECM determines engine idle speed by controlling the throttle valve motor to vary the blade angle between the non-adjustable preset limits of the mechanical guard and the throttle valve motor.
Cruise mode is engaged as a result of the ECM calculating and controlling the required throttle valve. The vacuum system controls the mechanical guard. When the driver releases the throttle pedal the input shaft disengages from the mechanical guard or the vacuum actuator pulls the guard away from the throttle valve. The throttle pedal will feel light should it be pushed again to accelerate (pressing the pedal further will re-engage the input shaft with the mechanical guard and restore normal feel). The ECM utilizes sensors to monitor the relative positions of the mechanical guard and throttle valve and adjusts them to maintain the set cruise speed.
Mechanical guard mode permits full mechanical operation of the throttle if the ECM detects that a problem has been encountered with the throttle valve position sensor DC motor associated harness connectors or the ECM.
Fixed idle mode occurs when any two of the three sensors (two input shaft sensors and the mechanical guard sensor) fail. The ECM will assume values which represent a blade angle of approximately 2.5 degrees and 1200 rpm (unloaded) maximum engine speed.
Redundancy mode occurs when any one of the three sensors (two input shaft sensors and the mechanical guard sensor) fails. The operational pair will be deemed to be safe to continue without intervention but cruise will be inhibited.
Full authority mode is invoked when a mechanical guard failure occurs which indicates that the guard is stuck fully open. The red warning lamp will be lit and road speed will be limited to 120 km/h (75 mph).
Engine shutdown mode (engine stops) will occur following multiple failures such as mechanical guard mode following full authority mode (or vice versa) or the throttle blade sticks.
P26 Spark Plugs
P27 Stop Lamp Switch
P28 Cruise Control Switches
P29 Blocked Part-Load Breather
P30 Mechanical Guard and VSVs
P31 EGR