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Switches & Sensors

Diesel Pre–flap & Throttle Body

Diesel Pre–flap:

  • Designed for diesel engines, used to increase the vacuum in the exhaust manifold and to increase EGR flow
  • Helps to avoid ‘shake off’
  • Helps regenerate catalytic converters by increasing the amount of exhaust gas in the engine

Throttle Body:

  • Key element in control of air/fuel ratio in petrol engines
  • Controls the amount of air flowing into the engine’s combustion chamber

Troubleshooting

  • Symptoms:
  • Poor acceleration
  • Loss of engine power
  • Car falling into limp mode
     
  • Possible causes for failure:
    • Damaged regulating throttle resulting in the valve sticking
    • Contaminants build up on valve causing blockages to the main airway

Testing:

While it is difficult to test a throttle body/diesel pre-flap there are a few intermediate steps that can be taken before using a scan tool.

  1. Check all fusing connected to the part. If the fuse has blown this will need to be replaced. If defective, ensure that the new fuse does not have a higher fuse rating than the one in the vehicle previously.
  2. Next, ensure all wiring connected is secure and not damaged. At this stage you can test to ensure that an electrical current is present using a voltmeter. If no current is present this suggests that wiring looms will be the problem as the part is receiving power to actuate when it receives a signal from the OBD.
  3. Once this has been checked, inspect hosing connected to the part. If there are any cracks or weak seals this will affect the airflow entering the part. If there are leaks this in itself can result in the same symptoms that a defective throttle body/pre-flap would give.
  4. The last check is to inspect the part itself. If the inside walls appear to be oily or carbonised this will restrict airflow. If carbonised, the part will need to be replaced. Cleaning of the part is not recommended as this will damage the seal and affect the flow rate.

Other components to check:

  • EGR valve – a faulty EGR valve may cause an inconsistent flow rate to the throttle body
  • Idle air control valve – on older vehicles a malfunctioning valve may result in irregular idling
  • Solenoid valve – if faulty this may fail to actuate pneumatically controlled components such as the EGR valve

Common Fault Codes

  • P0505 Idle Control System Malfunction

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