Rover TSB addressing this very concern.
Recent supplier test results have shown that a high percentage of Idle Air Control Valves (I.A.C.V) returned under warranty have no fault. Vehicle testing by engineers at LRNA of I.A.C.V's have resulted in the same determination. As of June 8, 1994, an I.A.C.V. will not be accepted under warranty by LRNA if it does not have a reproducible fault. To avoid warranty material being returned to your dealership, please consider the points below before - replacing any component.
1) Verifiable Defect
An OBD trouble code (DTC) 48 does not automatically mean a defective I.A.C.V. Code 48 is a DTC relating to the Idle Air Control System and not specifically the I.A.C.V. itself. A software strategy exists within the MFI ECM which, when certain engine running conditions are met, operates the M.I.L. and DTC 48. DO NOT replace an I.A.C.V. for DTC 48 unless the valve checks out as defective by the diagnostic procedure available in the Workshop Manual, Hand Held Tester Manual, or TestBook.
2) DTC 48 - Background
DTC 48 can be flagged as a result of an engine running problem which causes the I.A.C.V. difficulty in maintaining its target range of 665-735 rpm. An engine which has a fuel or ignition problem may suffer with a Code 48 because the stepper motor attempts to maintain the correct idle speed regardless of these other fuel and/or ignition faults. Code 48 occurs because of the idle speed control systems inability to maintain the engine idle speed within the target range. This can happen for two primary of reasons:
a) Air Leaks - Unmetered air passing into the engine resulting in the I.A.C.V. closing to more than 180 steps.
b) Overfueling - As a result of terminals, connections or ground connections problems. In particular ECM, 02 sensor grounds, 02 sensor signal connections or AFM circuit signal and ground wires.
3) Loss of Initialization
Some vehicles may suffer from a symptom of stall immediately following a start. This symptom is more apparent on a hot restart but can occur under any start condition. This stall occurs because the I.A.C.V. pintle "creeps out" from its correct start position prior to starting. As this "creep out" occurs it restricts the volume of air flowing into the engine via the I.A.C.V. system. This leads to the rapid drop in engine speed/stall immediately following a start.
The I.A.C.V. always moves against it's spring to the fully retracted position on vehicle power down. This action leads to the possibility of "creep" occurring. The amount of "creep" that occurs is related directly to the internal mechanical resistance of the I.A.C.V. If a customer complains of the above symptom it is acceptable to remove the I.A.C.V. spring. DO NOT remove the spring if there is no customer complaint. Only a small number of vehicles exhibit this problem.
4) Hand Held Tester Usage
When using the hand held tester to read I.A.C.V. position while setting base idle, on occasions the I.A.C.V. position is seen to:
- Read 180 steps regardless of base idle screw position.
- Please refer to E-mail 19-93/05 for guidance on this problem.
DO NOT replace the I.A.C.V.
5) Base Idle Recap
When checking or setting base idle as described in Service Bulletin P93/19-001, please note the permitted I.A.C.V. position range is anywhere between 135 and 165 steps. It is not necessary to reset the base idle if the I.A.C.V. is anywhere within this range.
Do not adjust base Idle if the I.A.C.V. is within this range.
6) I.A.C.V. Coil Resistance
When checking I.A.C.V. coil resistance, please be aware, DO NOT REPLACE the I.A.C.V. if the coil resistance reads between 40 & 70 ohms. Please amend all reference material which refers to any other resistance range and use this range.
