Exhaust flap

The purpose of the exhaust flap is to minimise exhaust noise at idling and low engine speeds.

Brief description of components

Exhaust flap and components

The following components are involved in the function of the exhaust flap:

• Exhaust flap with vacuum-operated diaphragm canister
  
• Vacuum pump and vacuum hose
  
• Electromagnetic changeover valve 1 for controlling the diaphragm canister
  
• Electromagnetic changeover valve 2 for charging/venting the diaphragm canister
  
• Vacuum pipes for connecting the diaphragm canister to the electromagnetic changeover valves
  

Supply:

• Changeover valve 1 is controlled by the digital engine module (DME).
  
• Changeover valve 2 is permanently energised while the ignition is on.
  

Main functions

Noise optimisation

The exhaust flap is fitted on the tail pipe of a rear silencer box. It is closed when the engine is running at idling speed or low revs. When the engine speed exceeds the threshold speed, the exhaust flap is opened. This cuts in additional silencer capacity. The exhaust flap is also open when the engine is switched off.

The necessary depression for controlling the diaphragm canister is generated by the vacuum pump used for the power assisted brakes. Vacuum hoses connect it to electromagnetic changeover valve 1 and the diaphragm canister. When the engine is running at low revs, changeover valve 1 is opened by the DME. The depression applied closes the exhaust flap by means of the diaphragm canister.

At higher engine speeds, changeover valve 1 is closed. This pressurises the diaphragm canister via the route changeover valve 2 -> changeover valve 1. As a result, the exhaust flap opens.

Electromagnetic changeover valve 2 is used to control opening of the exhaust flap after the engine is switched off. A special jet ensures that the diaphragm canister is pressurised slowly. As a result, the exhaust flap opens smoothly.

Failsafe characteristics

A defective vacuum hose to the diaphragm canister causes the exhaust flap to remain permanently open. If changeover valve 1 is not operated by the DME, the exhaust flap similarly remains open as a result of the absence of vacuum.

If there is a fault on changeover valve 2, the opening operation after the engine is switched off is uncontrolled. As a result, a brief clap may be heard.

When the exhaust flap is open there is no restriction of vehicle performance under any conditions.

If the exhaust flap sticks partially open, mechanical faults on the flap or the diaphragm canister are generally to blame. A partially open exhaust flap can bring about a reduction in performance and noise problems due to the increased exhaust back-pressure.

Diagnosis

If there is a fault in the electrical operation of electromagnetic changeover valve 1, the following fault is recorded in the DME fault memory:

• 279E ... Exhaust flap activation
  

Mechanical malfunctions or faults on changeover valve 2 can only be detected by the relevant test module.