Range Rover County LWB (LH) V8-4.2L (1993)
Idle Speed: Technical Service Bulletins
Base Idle Speed - Revised Checking/Setting Procedure
Bulletin: P93/19-001
Section: Product
Date: 6/17/93
Model: Range Rover/Defender
Applicable to: USA/CND
Subject:
REVISED BASE IDLE CHECKING/SETTING PROCEDURE
BACKGROUND
Over the last several years, changes in the MFI's Idle Air Control System (IACS) and software have reduced the number of false check engine light (
Malfunction indicator Lamp) illuminations.
The purpose of this Service Bulletin is to update you on changes to the base idle checking and setting procedure which is now possible due to the
improved IACS. Please review the contents of the bulletin carefully and make sure that all Land Rover Technicians are aware of the changes.
WHAT CHANGES BASE IDLE?
Base idle settings are influenced by anything which changes the amount of air entering the engine at idle. These include:
^
plenum bypass bore and throttle plate carboning
^
throttle cable/cruise control actuator adjustment
^
throttle plate stop adjustment
^
mis-adjusted base idle bypass screw
^
vacuum leaks
^
PCV system orifice and filter condition
^
engine internal wear (less volumetric efficiency)
^
ignition timing variations
^
altitude
^
defective IACV/faulty wiring connections to ECM
^
condition of ECM ground on left rear cylinder head
In the past, relatively small changes in the base idle speed could cause an on board diagnostic (OBD) fault code 48. It was necessary to set base idle to a
very tight tolerance to reduce false MIL illumination caused by day to day changes in operating conditions.
With the introductions of the 2665 PROM in November, 1991 the system diagnostic strategy was changed to ignore small base idle changes which
formerly could have caused a false light occurrence. All PROM's since 2665 have been made with this same idle control program.
WHEN SHOULD BASE IDLE BE SET?