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This page details some of the more important diagnostic data that may be retrieved from Subaru vehicles via ECUTEK DeltaDash or the Subaru 'Select Monitor'.

Analogue Parameters

Analogue parameters are continuously varying data values that may be retrieved from the ECU. These values generally represent pressures, voltages and temperatures.

Primary & Secondary Control

Wastegate solenoid valve duty cycles. Primary and secondary refers to whether it is the first or second turbo charger. If you only have one, you will only see primary control. The higher the duty cycle the more pressurised air is bled away from the wastegate actuator - more duty encourages higher boost. Low duty restricts boost by allowing the pressurised air to act on the diaphragm, pushing open the wastegate.

If the duty cycle is 0%, do not expect that the boost pressure will be 0 PSI. Under light loads, the boost will be negative (partial vacumm). Also at heavy loads, even if the duty cycle is zero, the boost pressure must overcome the spring tension of the wastegate diaphragm before any exhaust gases can pass around the turbo (through the wastegate). In practise, this means that you may see several PSI of boost (perhaps 8-10PSI) even with no solenoid activity.

Injector Milliseconds

Number of milliseconds that each injector is open for for each cylinder cycle (2 revolutions of the crank). To get injector duty cycle from this take RPM * Injector ms/1200. This gives duty cycle in percent. If you are regularly seeing over 90% duty, you may need bigger injectors. The injectors must have enough 'head room' too cope with unexpectedly high air flows - these may be caused by overboost, faults and particularly cold weather.

O2 Sensors

AKA lambda sensors. Monitor the amount of oxygen present in exhaust gases, in order to attain the air/fuel correct mixture. Values of 0 to 0.9 Volts are normal. 0 being lean, 0.9 being very rich. You will see the sensor voltage oscillate between these extremes when under closed loop control. Under high loads, you should never see the voltage drop below 0.7 Volts. If you do, something needs fixing - quite possibly your air flow sensor or fuel mapping.

The addition of cone style induction kits, whilst improving top end power and throttle response is known to upset air/fuel ratios. Alteration of the fuelling by the ecu is the solution.

AFC - Air Fuel Correction

When the fuelling is under closed loop control by the lambda sensor(s), this refers to the amount of fuel added or subtracted from the value retrieved from the fuel map. -5% would mean that the ecu is fuelling 5% less than the map says in order to achieve the ideal air/fuel ratio. Under high loads, the ECU switches off closed loop control, and uses values from the map. At this point, you will see AFC drop to 0%. This is why it is important that fuelling mapping is accurate (or at least rich) at high loads - the ECU does not compensate for errors here.

Knock Correction

The number of degrees added or subtracted from the ignition timing based on the amount of knock detected. Positive values are ignition advance (due to the absence of knock). Negative values are ignition retard (due to the presence of knock). These ECUs run active knock correction, and it is quite normal to see +2 to -3 degrees of correction. Maximum power is produced on the point of knock beginning, and the sensor is there to keep the timing 'on the edge'.

Battery Voltage

Measured voltage from the car battery. May be as low as 10 Volts when ignition is off. Should rise to around 14-15 when engine is running.

Coolant Temperature

Temperature in Centigrade of the cooling water. Expect to see around 85 - 95 from a warmed up engine. Don't work the engine too hard until the temperature is at least 80 degrees.

Atmospheric Pressure

Just what it says. Generally not updated often, since the value is obtained by switching the MAP sensor to atmosphere at intervals - reading the boost pressure is a lot more important !

Manifold Absolute & Relative Pressure

This is boost pressure, and may be represented as absolute or relative, depending on the ECU. Absolute pressure in the manifold is relative to a vacuum. Subtract approx 14.7 PSI to get relative pressure. When boost pressure in the manifold is shown as relative to atmospheric pressure, negative values represent partial vacuums in the manifold.

As an example... If a car is said to be running 16 PSI of boost, this would be a 'manifold relative pressure' of 16 PSI, or a 'manifold absolute pressure' of 16 + 14.7 = 30.7 PSI. That's 16 PSI relative to the atmosphere, or 30.7 PSI relative to a complete vacuum.

1 atmosphere = 1 Bar = 14.7 PSI.

Ignition Timing

Ignition timing that the engine is currently running.

Engine Speed

Speed of rotation of the engine in revolutions per minute.

Vehicle Speed

Speed of wheel rotation in kilometres per hour for standard wheel circumference. Data value may not be updated as frequently as engine speed, hence acceleration times may be more accurately determined from engine speed. This value may not be accurate if the car wheels or tyres have been changed.

Intake Air Temperature

Temperature of air drawn into the engine for combustion. Generally measured at the point of entry to the air filter. This will not give an indication of charge temperature, and so is not particularly useful.

Mass Air Flow / Air Flow Sensor Voltage

The rate of flow of air into the engine. Some ECUs report air flow voltage, whilst others report calculated flow rate. The voltage from which the ecu calculates mass air flow is non-linear, with smaller changes in output voltage being seen for flow changes at high rates compared with low flow rates.

Throttle Opening Angle/Sensor Voltage

Displays state of the throttle. High voltage or % value means a more open throttle. When logging engine activity (especially on the dyno), it is useful to log throttle position. This makes it easy to see when a power runs begins and ends i.e. when the driver's foot is depressing the accelerator fully.

Exhaust Gas Temperature

Temperature of exhaust gases on more recent cars. Some sensors are not capable of low temperature readings, so it is normal to see a value of 200 degrees with the engine off. This is not a fault.

VVT Advance Angle

Number of degrees of cam advance that the variable valve timing system is applying.

Tumble Valve Position

Sensor monitors the tumble generator valve (TGV) positions. These valves are generally active during starting only.

Idle Speed Control Valve

Controls the amount of air let into the manifold when the engine is idling. It is normal to see this value fluctuating slightly with lambda on idle. Switching on the air-conditioning or headlights will also cause this value to change slightly.

Alternator Duty

On some STI cars, the alternator duty is actually reduced at high engine loads, in order to reduce the power drawn from the engine. This results in slightly higher power at the wheels than for standard cars.

Digital Data

The digital data available from the ECU varies from car to car. These parameters are generally less useful for tuning and fault diagnosis since they are on/off values. The data generally shows the state of the relays and switches in the car, such as ignition switch, air-con switch, cooling fan & power steering relays.


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