Relearning your vehicle's throttle

There are many causes of erratic engine idling but the most common is usually related to the engine throttle position sensor (TPS).

 A throttle position sensor is a special butterfly flap placed in the airway entering the engine. Its main purpose is to meter the amount of air entering the engine for the control unit to inject fuel that matches the air volume as relayed by the butterfly valve.

 In older models, the butterfly valve was directly connected to the accelerator pedal via a cable through several linkages. The evolution of technology faced out the use of cables and linkages. Most new generation cars utilise sensors and signal cables between the accelerator pedal and the throttle position sensor.

 You may wonder why in ancient cars, a driver would rev up the engine from the engine bay by the use of his hands, something which is not possible in modern cars.

Absence of cables and linkages makes it impossible to rev up the car from the engine side.

  The use of sensors and actuators is a plus for modern technology but has its setbacks especially if one does not have the technical know-how on certain working principles of some components.

How it works

Commonly known as drive by wire system, the butterfly valve inside the throttle housing is operated electronically via a motor.

The vehicles control unit sends a signal to the motor depending on the input from the accelerator pedal sensor. Pressing on the accelerator pedal fully means the throttle must be opened to the fullest. However, when the accelerator pedal is not operated and the vehicle is running at idle speed, the throttle position sensor should be slightly open enough for air passage to the engine for the vehicle not to stall. The control unit knows how to partially open the flap during idling while not depending on the accelerator pedal.

 This should be the case at all idle speeds but sometimes due to other factors, the control unit loses the ability to keep the throttle partially opened when the vehicle is at idle. When this happens, the vehicle will have rough idling or even in some cases it will keep stalling at low speeds.

Once the engine keeps stalling or has rough idling, it is recommended to carry out a throttle position relearn. We shall go through the process of throttle position relearn but first, we have to understand why the vehicles control unit loses memory with the throttle position sensor.

Causes of throttle position sensor memory loss

Depending on vehicle type, the electronic control unit (ECU) may lose memory to the throttle position sensor when the connection to the throttle position sensor is lost or interrupted. Signal interruption may be experienced in so many ways: 

• When the battery dies
• Disconnecting the battery terminals
• During a battery replacement
• Clearing the check engine light via computerised diagnosis
• Throttle body replacement
• Inappropriate cleaning of the throttle body

In some cars, any of the above or both could cause the control unit to forget the positioning of the TPS  while in others none of the above has an impact on the TPS.

If the computer has forgotten the position of the throttle, relearn must take place for the car to idle normally and respond quickly to the accelerator pedal without stalling the car.

Computerised relearn is the most effective way of doing it but due to these machines scarcity, we shall give a simple procedure of accurately relearning the throttle without paying anything.

Relearning the throttle

 It is referred to as relearning because, from the factory, the control unit was taught how to play around with the throttle but along the way it forgot. Perhaps we should make it to learn again. Hence the name "re-learn"

Every car has got different relearn procedures as per manufacturer even though the variations are not so far apart. Today we are mainly dealing with Subaru relearn. we believe it should work for most cars. If it doesn't work for any Subaru car we advise you to check online for your specific model.

Procedure

• Ensure your throttle body is clean and dry from any moisture. clean it with a recommended throttle cleaner if need be
• Before starting the car confirm that your battery voltage is above 12.9V and all other electrical components are switched off. This includes the radio, lights,ac, car charger, brake lights etc. When doing relearn the engine should run devoid of any external load.
• Start the car and let the engine crank up without the aid of an accelerator pedal. Let the car idle for around 8 to 10 min without revving up the engine. The main aim is to let the control unit re-learn the flap idling position.
• After the above time of 8-10min, take a ride on the road. Be very gentle on the throttle without exceeding  2500 revs/min. Drive around up to a distance of around 5 to 10km. ensure that the car shifts through all the gears smoothly.
• Stop the engine then start all over again. At this point, the control unit has fully relearned the throttle position. It should now be responsive to all your throttle desires.

This procedure can only work if your throttle body is healthy. There are no healing procedures for faulty parts in engineering.

The writer is the owner of Boosted Auto car repair shop