Any reported sensor fault will cause the ECU to run the engine in limp mode and likely cause over fueling. Do not make any adjustments until the sensor fault is fixed.

Action Plan

• Clear the faults
• Switch off ignition
• Switch on ignition and check the fault codes again
• If the faults persist or after driving the car they reappear, replace the sensor / check the relevant circuit

The ECU will react to conditions based on the values it reads from the sensors fitted to the engine. The following details the behaviour and expected values.

Ignition is ON, Engine not Running

• Engine RPM 0
• MAP 100kPa (this is atmospheric pressure, e.g. no vacuum)
• Air Intake Temperature at general air temperature e.g 20C
• Lambda Voltage 435mV, not oscillating
• O2 System (Lambda Status) is Inactive (0)
• Loop is Open (Closed Loop = 0)

Ignition is ON, Engine COLD : Running and Idle

• Engine RPM 1200 and actively decreasing to 850 as the engine warms
• MAP 40kPa or less (lower is better), higher than 40kPa indicates a possible vacuum fault, 0kPA is a sensor fault
• Coolant Temperature increasing a degree every 10-15 seconds until the thermostat temperature is reached (88C). A static 60C indicates a possible fault
• Air Intake Temperature at general air temperature e.g 20C
• Lambda Voltage will rise and then gradually reduce, then start rapidly oscillating between 100mV and 900mV after 90 seconds
• O2 System (Lambda Status) is Active (1) after 90 seconds. If the lambda fails to oscillate then this value will change 0 and the ECU will enter limp home mode

Ignition is ON, Engine WARM : Running and Idle

• Engine RPM 850 +/- 10
• MAP 40kPa or less (lower is better)
• Coolant Temperature > 80C. Whilst the thermostat is 88C, the ECU determines a coolant temperature > 80C as operating temperature.
• Air Intake Temperature at general air temperature e.g 20C
• Lambda Voltage is oscillating
• O2 System (Lambda Status) is Active (1)

The oxygen monitoring system (O2 System) is critical to the fuel mixture determined by the ECU. The system relies on a working Lambda Sensor (O2 Sensor).
When the ignition is switched on, the ECU Lambda relay is energised to enable the Lambda Sensor heater. A failure in the lambda heater or the circuit will delay the start of the lambda oscillating and a failure in the O2 System.
After approximately 90 seconds the Lambda Sensor should start rapidly oscillating between 100mv and 900mV. The ECU expects this behaviour, if the lambda voltage is a static or oscillations are slow, the ECU will determine a failure in the O2 System. If the lambda is working correctly, the ECU will enter Closed Loop mode where it uses the lambda readings to adjust the fueling accordingly.

A Working O2 System

• Lambda Voltage oscillates rapidly between 100mV and 900mV after 90s of start
• O2 System Status is Active (O2 Lambda Status = 1) after 90s of start
• Loop is Closed (Closed Loop = 1)
• Lambda Heater relay is working, this can be tested

There are several parts to the vacuum system, namely the inlet manifold, the inlet manifold pipes and fuel trap, the vacuum rail and the carbon canister. Pressure is created in the inlet manifold when the engine is running and can be measured to determine the load and performance of the engine. A loss of vacuum pressure will result in the ECU being unable to accurately measure and adapt to engine load and lose the ability to control the rpm of the engine. In the worst case, this will result in erratic engine rpm and massive over-fuelling. The most notorious and troublesome part of this system is the inlet manifold vacuum pipes and fuel trap that run to the ECU. A break in the pipe can lead to complete loss of vacuum, however more common is a split in an elbow connector will give partial loss and over-fuelling will be the most notable symptom rather than erratic running.

Vacuum Pipe Elbows can split causing a high MAP reading

Vacuum Pipe fits behind the inlet manifold

The fuel trap. This prevents vapours entering the ECU MAP sensor

ECU MAP connection

Carbon Filter Solenoid Valve

A Working Vacuum System

• Manifold Absolute Pressure (MAP) is 40kPa or lower. A warm engine is likely to be in the range of 34kPA - 40kPA.
• Carbon Canister Value relay operates after the throttle has been held open at >= 2000 rpm for a period of time (30 seconds or so)

Action Plan for a Failing Vacuum System

• Check vacuum pipes for breaks, in particular check the elbow connectors for splits and are firmly attached to the pipes
• Check vacuum pipes are not pushed so far on to the fuel trap so they block the air flow.
• Check fuel trap by sealing one end with your finger and blowing into it

Action Plan for a Partial Vacuum System

In many cases the Manifold Absolute Pressure (MAP) in a partially failing system maybe just above 40kPa, probably in the range of 41kPA - 50kPA. Over-fuelling will be the most notable symptom rather than erratic running

• Follow steps for a Failing Vacuum System
• Check inlet manifold is tightened correctly
• Check vacuum pipe from inlet manifold to vacuum rail
• Check vacuum pipe to carbon canister
• Optionally replace the vacuum pipes with silicon vacuum pipes (3mm internal diameter).