Hello, I hope the following is of assistance.
Thee valve you refer to is classed as either a suction control valve or inlet metering valve, it's not an idle control valve. This valve is controlled by the engine management (PCM) and is opened and closed to control the fuel pressure in the high pressure fuel system.
When you disconnect the valve it will set to a default value, the engine management light will come on as the PCM detects the circuit fault, if this valve is replaced the high pressure fuel system learned values need to be reset using a suitable diagnostic tool.
Before going any further rather than relying on the RAC report I would have a diagnostic scan carried out on the PCM, this will retrieve any stored fault codes and help pinpoint the sensor or actuator giving the concern. On this engine there are problems with the EGR valves failing causing exhaust gases to enter the engine at the wrong time.
Going back to the valve if this fails it can cause the fuel pressure relief valve to activate, if this happens then this valve needs replacing and it's counter resetting otherwise the fuel pressure becomes uncontrolled.
Start with confirming where the fault actually is, disconnecting various sensors or actuators won't really help to pinpoint the fault as the PCM will try to compensate for the disconnected component.
Let me know if I can assist further or if you have more questions regarding this and I'll reply as soon as I'm able to.