Land Rover Repair: LR3 will not stay at a constant idle, mass airflow sensor, throttle position sensor

Question
Hello, my 2005 LR3 HSE while in part idles from 500-750 rpm. when coming to a stop sign, on occation it will stall or lunge forward when pressing on the gas.I suspect this is due to the idle not being at a constant rpm. At highway or city traffic, on occation the "reduced engine performance" indicator will come on. I have changed the mass airflow sensor, did nothing. I changed the spark plugs, did nothing. I added a K&N air filter, did nothing. I can't find any vacuum leaks. Could it be the EGR valve? How would i test it? I'm an engineer and still can't figure this one out. What could be causing this? Thanks, Nathan

Answer
Hi Nathan,

a common problem with LR's is the throttle position sensor (TPS).  Could also be idle air control valve sensor, it's harness connector or wiring.  LR electrics are known to corrode, so check out the various wiring looms and connectors.  It could be ignition related and there are many sensors that have an impact on the engine management system. Could also fuel delivery fault - maybe the pump?  Do you have any trapped fault codes? OBDII reader would help narrow your search.

Here's some data I have from the older Series II Disco's (2003) that should still apply to your LR3.  If you prefer, go to www.discoweb.org and check out the forum there for TPS and IACV threads.  

TPS Input/Output
The TP sensor has electrical input and output. Input is a 5 volt supply via pin 10 of connector C0636 of the ECM. The signal output is via pin 24 of connector C0636 and is a varying voltage, less than 0.5V (closed throttle) and greater than 4.5V (wide open throttle) depending on throttle plate position. The TP sensor earth is via pin 25 of connector C0636 of the ECM, this acts as a screen to protect the integrity of the TP sensor signal.

The connector and sensor terminals are gold plated for corrosion and temperature resistance, care must be exercised while probing the connector and sensor terminals.

If the TP sensor signal fails, the ECM uses a default value derived from engine load and speed.

The TP sensor can fail the following ways or supply incorrect signal:
. Sensor open circuit.
. Short circuit to vehicle supply.
. Short circuit to vehicle earth.
. Signal out of parameters.
. Blocked air filter (load monitoring, ratio of the TP sensor to air flow).
. Restriction in air inlet (load monitoring, ratio of the TP sensor to air flow).
. Vacuum leak

IACV Input/Output:
The input to the IACV is a 12 volt signal from fuse 2 located in the engine compartment fuse box. The output earth signal to open and close the actuator is controlled by the ECM as follows:
. IACV (open signal) - via pin 42 of connector C0636 of the ECM
. IACV (closed signal) - via pin 43 of connector C0636 of the ECM

The IACV can fail the following ways or supply incorrect signal:
. Actuator faulty.
. Rotary valve seized.
. Wiring loom fault.
. Connector fault.
. Intake system air leak.
. Blocked actuator port or hoses.
. Restricted or crimped actuator port or hoses.

Best of luck,
JohnMc