There are a number of wire in and plug in interceptor style tuning devices on the market designed to intercept the wiring from the sensors to the ecu so as to trick the ecu, predominantly into changing the fuelling or timing. 

The way this works is to modify the voltages or pulses from the sensors into the ECU in such a way that the ecu is manipulated by being fed a modified signal or voltage.  If for example if the MAF (Mass Air Flow) sensor is intercepted and the output from the MAF is lowered via the interceptor to below what it actually is then the ecu will register less airflow, this in turn will cause it typically to remove fuel and add timing as this is what happens at lower airflow.  The chip will intercept many sensors, in part to be able to adjust them and to satisfy the ECU’s internal checking.  For decades ECU’s have had codes that warn of a fault, as technology improves, and the ECU becomes more sophisticated these internal checks have become increasingly more accurate and reliant on sophisticated modelling within the ECU.  In early electronic ECU’s a sensor would indicate a fault code if the sensor was out of range, if we take the example of a throttle position sensor that has an expected range of 0.3 to 4.6 volts if the voltage falls outside this range, then a fault code will log.  As technology has improved there has become an increasing amount of cross checking.  A current model ECU will check the TPS in multiple ways.  A given TPS voltage represents a throttle angle.  Within the ECU will be a predicted airflow model based on the throttle area at any given angle, the ECU can predict the airflow through the throttle based on its model and compare this the actual airflow measurement from the MAF or speed density calculation.  The throttle model can also predict the manifold pressure at a given amount of throttle opening area, this also is cross checked against the measured air pressure.  This advance system checking occurs for all the vehicle sensors, this needs to be considered if attempting to fit an interceptor style tuning device as it is likely to impact these checks and present fault codes.  

Let’s say for example we have wired in the TPS, MAF and MAP sensors to our interceptor chip and with careful adjustment have been able to prevent fault codes, by lowering the TPS, MAF and MAP outputs we have tricked the ECU into incorrectly calculating the airflow lower, the ecu is now getting enough data that correlates between the sensors to not fault code, however this now has the ECU calculating a different, lower, airflow.  The primary job of an ECU is to accurately calculate airflow so it can as accurately as possible estimate the mass of air in any one cylinder so as to calculate the fuel quantity requirement and output torque.  What has been achieved on the surface may look fine as this was the intention, to lean the vehicle out and to add timing by reducing the airflow calculation, what may not be considered is the impact to other vehicle systems.  By reducing the airflow calculation below what it is in reality the ECU calculates a lower torque output, in doing so other components that rely on the torque output will be given false and lower torque data.  In the case of automatic transmission vehicles this can result in reduced line pressure, early shifting and reduced shift torque reduction.  Over time this can potentially damage the transmission.  Late model transmissions are adaptive and measure internal slippage they have the capacity to adjust the shift pressure, within limits, the transmission may appear to be fine but may deteriorate over time. Another system reliant on the engine output torque calculation is the traction and stability control, if the torque output is calculated low due to the modifications it may also impact the ability for the Traction Control and Stability control to work as effectively.

Ignition timing and fuel quality have a large impact on the output of most late model performance Petrol engines.  In order to maintain the optimal performance without damaging the engine as fuel quality varies all late model ECU’s use a means to detect Knock.  Knock is the uncontrolled spontaneous combustion of fuel under pressure that causes excessive spikes in cylinder pressure that can damage and destroy the engine.  Lower quality, lower octane fuel is more prone to Knock, and the ignition timing needs to be retarded so the combustion process starts later, this results in less power and economy.  There is an ignition timing for any load and RPM that will yield the highest engine torque output, modern ECU’s attempt to get as close to this point without damaging knock occurring.  This point is called MBT, there are multiple interpretations of this, Mean Best Timing, Maximum Brake Torque, but it is the optimal ignition timing for a given load and RPM.  This is important for several reasons, as part of the torque calculation the ECU compares the current ignition timing and compares it to the MBT timing table. The further the actual ignition is away from the MBT timing the lower the engine torque will be, the torque output calculation will be modified relative to the distance (how far the actual timing is form the MBT table) to MBT spark timing. There is a similar torque output scaling relative to the mixture ratio which further contributes to the torque error output if the ECU is not achieving the desired mixture that it requests relative to the internal tables.

An interceptor chip is wired into the crank and camshaft sensors of the engine, these produce a pattern pulse that is used by the ECU to constantly know the location of the crankshaft and camshaft, so it knows where in the combustion cycle each cylinder is. The crank and camshaft sensors are in a fixed location and form the reference point from which all ignition and injection timing occur.  The interceptor chip electronically movers this reference point by shifting the phase of the pattern generated, by adjusting the point of reference back or forward the point of ignition and injection will move similarly.  It is in this way timing advance can be added to make more power, in theory.  The reality is more complex, as mentioned prior the ECU is using knock sensors to monitor knock activity, these sensors will remove timing if knock is present.  In late model ECU’s there is a method to adapt the spark to suit the current fuel quality or octane level.  This is often achieved with multiple sets of ignition tables that are set for high and low octane, increased levels of knock move the source of the ignition timing from the higher spark tables to the lower spark tables.  Due to the high compression ratios or boost used in modern cars the vehicle rarely achieves the highest spark table all the time, they tend to float between.  It is due to this that interceptor chips can create large issues. In tuning the interceptor chip the tuner will try to add timing, as he does this the engine tends to detect knock due to the additional ignition advance, this is unlikely to be audible as the sensors are much more sensitive to low levels of knock than we can hear unaided.  As the knock sensor detects knock the timing will move more from the high octane to low octane ignition maps, resulting in a return to the previous timing, the temporary gain in power that may have occurred is now gone so the tuner adds more timing again resulting in knock further pushing the ignition source to the lower spark map.  This process may be repeated until the ignition timing has hit its point of maximum retard or be on the low octane spark map fully.  At this point the tuner may be able to force more timing into the engine as the ECU has run out of ability to reduce timing due to knock.  Two things have happened here.  The engine is running much more timing than the ECU is commanding because the point of reference has been moved, and the torque calculation has been altered in error. If the Interceptor chip is adding 10 degrees by moving the reference and the maximum timing the ECU can remove is 8 degrees then the engine may be running 2 degrees more timing as measured with a timing light however this is not what the ECU is outputting, the ECU does not know that its point of reference has been moved.  The ECU is only able to see the retarded timing it is outputting and calculating the torque based on the 8 degrees reduced timing, in fact the engine is producing more torque as it is operating with 2 degrees more timing, this will further impact the operation of any systems reliant on the torque output calculation that is now potentially a very long way off and lower.  The tuner will often add timing until knock is audible and stop just prior.  This may sound good as now we have managed to make more power, but what happens the next time you fill up at a different service station or you are in the country and only 95 not 98 octane is available?  The engine will knock and have no more ability to remove timing due to knock as it has been forced back to the minimum learned spark value.  Knock or detonation and engine damage or potential failure can result.  Your sophisticated knock-controlled system designed to produce optimum safe power for varying fuel qualities has been reduced to a 1970’s distributor.

Beware the interceptor chip to tune your late model car, there may be unintended negative and expensive side effects for the small apparent gains achieved.