Idle Instability At Normal Engine Operating Temperature (P93/19-002)

IDLE INSTABILITY AT NORMAL ENGINE OPERATING TEMPERATURE

TECHNICAL SERVICE BULLETIN

SERVICE INFORMATION

The following information is applicable to 1993 and later model year Range Rover and Defender vehicles with complaints of idle instability (surging when idling) with the engine at normal operating temperature up to the safe VIN detailed below. These complaints may also be accompanied by Malfunction Indicator Lamp (MIL) illumination with an On Board Diagnostic (OBD) fault code of 23, 44, 45 and occasionally 48.

Recent investigation with vehicles displaying the above symptoms has identified a problem with the Mass Airflow Sensor (MAFS) connection pins. The plating on some pins has been shown to be thicker than it should be. This can, under the right conditions, lead to micro-corrosion on the pins which may cause a small resistance build up across the MAFS connections.

When this micro-corrosion occurs on the MAFS ground pin or terminal, the Multiport Fuel Injection (MFI) system adds fuel to compensate for an incorrect (high) mass air flow sensor signal. If this occurs, depending on the severity of the additional circuit resistance, one or more of the above OBD fault codes may be set, in addition to a poorly running engine symptom.

If a vehicle exhibits any of the symptoms described do not replace the heated oxygen sensors, or any fuel system components until the following has been carried out.

1. a.  Remove the mass air flow meter connector. Check the female connectors in the harness plug for damage. Make sure they are all in good condition.

   b.  Spray the supplied deoxidizing cleaner (see note below) onto the sensor terminals, and the connector terminals. Allow the cleaner to penetrate for 5 to 10 seconds and reapply. Use moderate air pressure to remove the excess cleaner from the connector and MAFS terminals. Take care not to dislodge the seal from the harness connector.

   c.  After cleaning the terminals apply a moderate amount of the supplied grease to the mouth of the female connector at each of the pins and push the MAFS harness plug firmly back in place.

   NOTE: DO NOT USE ANY GREASE OR TERMINAL CLEANERS OTHER THAN THOSE SUPPLIED BY LAND ROVER.

   The use of other greases or cleaners may result in connector and/or component damage later in service life.

2. Check the cleanliness and security of the heated oxygen sensor ground strap at the rear of the left hand cylinder head (ETM reference E102). A poor connection here can also cause the problem symptoms described earlier.
   Fig 1: ECU Harness Connector Pins/Terminals

   
3. Check the condition of the 40 way ECU harness connector paying particular attention to pins/terminals 4, 25, and 35. Straighten or clean the terminals as necessary. (See Fig 1 ). Pin 4 is the heated oxygen sensor ground connection, which terminates at the left cylinder head. Pins 25 & 35 are the MAFS ground and signal wires, respectively and terminate at the MAFS.

   If after carrying out the above recommended repairs, the symptoms still exist, call the technical hotline.

VEHICLES INVOLVED

All U.S. and Canadian specification 1993-1994 Range Rover and Defender vehicles up to:

1. Range Rover: ...RA643803
2. Defender: ...RA933971

PARTS INFORMATION

One each of part number LRNA200040 Cleaner/Deoxidizer, and LRNA201325 protective grease will be shipped to each dealer along with this bulletin. Additional supplies are available through the LRNA parts system.

WARRANTY INFORMATION

Claim Type: R

Labor Operation: 192265, .30 hrs.