This page details some of the more important diagnostic data that may be retrieved from Subaru vehicles via DeltaDash or the Subaru 'Select Monitor'. It also explains the symptoms of some common problems.
Analogue parameters are continuously varying data values that may be retrieved from the ECU. These values generally represent pressures, voltages and temperatures.
Wastegate solenoid valve duty cycles. Primary and secondary refers to whether it is the wastegate duty for the first or second turbo charger. If the vehicle only has one turbo, this will be primary control. The higher the duty cycle the more pressurised air is bled away from the diaphragm of the wastegate actuator. The spring opposing the diaphragm forces the wastegate to close. This forces more exhaust gases to pass through the turbine - more duty encourages higher boost pressures. Low duty restricts boost by allowing the pressurised air to act on the diaphragm, pushing open the wastegate, allowing exhaust gases to bypass the turbine.
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 actin on 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.
At sea level, this should be around a bar or 14.5 PSI. On some vehicles, the value is not updated continuously, since a single pressure sensor is shared for reading both manifold and atmospheric pressures, a solenoid being using to switch the input to the sensor.
This is boost pressure, and may be represented as absolute or relative, depending on the ECU - some ECUs report both parameters, whilst some only report one. Absolute pressure in the manifold is relative to a vacuum. Subtract approx 14.5 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.
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. However intake temperature is useful to the ECU for determination of the wastegate duty cycle required to produce a given boost pressure. High boost pressures may be attained with lower wastegate duty cycles when the IAT is low.
When people discuss boost pressures, they are generally referring to manifold relative pressure. For vehicles running high boost, it is better to view manifold absolute pressure due to the way in which the data is reported: The manifold relative pressure parameter can only report pressures up to around 19 PSI. Beyond this pressure, the ECU will just report 19 PSI. To get around this limitation, read manifold absolute pressure instead. This parameter will read up to approx 37PSI. Subtracting 14.5 PSI for atmospheric pressure shows that this parameter can convey boost pressures of up to approx. 22 PSI.
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 + 14.5 = 30.7 PSI. That's 16 PSI relative to the atmosphere, or 30.7 PSI relative to a complete vacuum.
1 atmosphere = 1 Bar = 14.503 PSI.
Number of milliseconds that each injector is open for for each cylinder cycle (2 revolutions of the crank). To calculate injector duty cycle: Duty Cycle % = RPM * 'Injector ms' / 1200. DeltaDash will also do this conversion for you. 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.
These parameters show the current passing through the front air/fuel sensor and the sensor's resistance. These are inputs used to calculate front sensor air/fuel ratio.
Displays the air fuel ratio as determined by the front air/fuel sensor. This sensor is in close proximity to the engine exhaust ports and is before any catalytic converters. When running on closed loop fuelling control, this sensor provides the main feedback for optimising fuelling. This parameter reports an air/fuel ratio as opposed to a simple rich/lean signal.
Short term correction percentage applied to fuelling based on the output of the front air/fuel sensor.
Short term correction percentage applied to fuelling based on the output of the rear O2 sensor. This sensor is after any catalytic converters and helps to fine tune the fuel mixture to minimise emissions.
Long term correction percentage applied to fuelling based on feedback from front and rear sensors.
These parameters report the output voltage of the O2 sensors. Early vehicles tend to have a single 'Front O2 Sensor', whereas newer vehicles have both a 'Front A/F Sensor' and a 'Rear O2 Sensor'. These sensors do not report an accurate air/fuel ratio, but instead provide a rich/lean signal to the ECU. Their output voltages switches sharply as the AFR crosses the stoichiometric ratio. Values of approx 0 to 0.9 Volts are normal. 0 being lean, 0.9 being very rich. The sensor voltage will oscillate between these extremes when under closed loop control. Under high loads, the voltage should never drop below 0.7 Volts. If it does, this means that the fuel mixture is too lean when on boost. Quite possibly there is a fault with the air flow sensor.
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 ecu calibration (AKA a remap) is the solution.
Reports the 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. The EGT sensor is placed after the up-pipe catalytic converter and allows the ECU to monitor the temperature of this 'cat'. It is important for the ECU to regulate the temperature of the cat: If the temperature is too low, the cat will not perform efficiently. If the temperature is too high, the cat may be damaged, pieces may break away potentially destroying the turbo in the process. This is the reason for the EGT sensor and trouble code display.
Reports the output voltage of the fuel level sensor.
Reports the pressure present in the fuel tank.
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.
Overall ignition timing that the engine is currently running, incorporating the knock correction component described below.
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 -3 to + 12 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'. Some ECUs run more aggressive knock correction than others. E.g. 1999/2000 model year turbo ECUs only run 1-2 degree positive values, whereas 2001-2003 ECUs may run much more than this.
Reports the amount of intake cam advance applied. There are two parameters - left & right - because two separate mechanical systems control the valve timing for the left & right sides of the flat-four engine. The higher the advance angle, the earlier the intake valves open - this causes more intake/exhaust valve overlap which can help the engine to breath more efficiently at particular RPMs and loads. It is normal to see small differences between left and right sides of the engine. Large, continuous differences may indicate a fault.
The oil control valves control the VVT advance angles. The values are a duty percentage, indicating the proportion of time that the valves are energised.
Reports the current passing through the oil control valve solenoids.
Report the output voltage of the tumble generator valve position sensors.
Measured voltage from the car battery. May be as low as 10 Volts when ignition is off. Should rise to around 14-15 Volts when the engine is running.
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.
Speed of rotation of the engine in revolutions per minute.
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 from standard, since this changes the rolling circumference.
The rate of flow of air into the engine. Some ECUs report air flow voltage, whilst others report a 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. It is from mass air flow that the ECU calculates engine load which has a big influence on ignition timing and fuelling.
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 run begins and ends i.e. when the driver's foot is depressing the accelerator fully. There may be more than one throttle parameter.
Present on fly-by-wire engines where there is no direct link between the accelerator pedal and throttle butterfly. This parameter reports the voltage measured on the accelerator pedal sensor. There may be more then one accelerator sensor parameter.
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.
On some cars, the alternator duty is may be controlled by the ECU. This allows the load on the engine from the alternator to be controlled. E.g. The ECU may reduce the alternator load at high engine loads, in order to reduce the power drawn from the engine. This results in slightly more power being put down on the road.
Symptoms:
Cause: MAF sensor reads too high an air flow on idle, and too low an air flow at high loads. Since the ECU thinks there is more air flow at idle than there really is, it puts in too much fuel, causing an overrich mixture which the ECU cannot compensate for sufficiently. Since the ECU thinks there is less air flow as high loads than there really is, it puts in too little fuel, causing a lean mixture. Since the ECU runs open loop air/fuel ratios at high loads, AFR compensation is not even attempted.
Remedy: Replace the air flow sensor ASAP. It is not possible to clean these delicate hot-film sensors effectively. As the sensor deteriorates, mixtures become leaner, eventually causing damage to the engine due to det and high temperatures. As a short term fix, raise the idle speed, use a high octane fuel and keep engine revs as low as possible while driving.