Air mass system test for diesel engines

Air mass system

The air mass system consists of several component groups that ensure the optimum fresh air supply to the engine at every operating point. Malfunctions in these component groups cause a variety of disturbances in driving operation, for example lack of power, black smoke, etc.

The air mass system test runs an overall check of the air mass system and thus allows clear pinpointing of the cause of the fault to the defective component group(s).

Component groups of the air mass system

The air mass system consists of:

• Charge-air pressure sensor
  
• Intake air temperature sensor
  
• Charge-air-temperature sensor
  
• Air-mass flow sensor
  
• The exhaust-gas recirculation controller comprises the EGR valve with position sensor, pressure converter, pneumatic and electrical activation of the pressure converter
  
• Charge-air pressure actuator, comprises turbocharger with adjustment, electrical activation of the charge-air pressure actuator
  
• Swirl flaps with electrical actuator (not on N47uL)
  
• Air filter, air hoses, pipes, charge air cooler, exhaust system, etc.
  

Test procedure

1. The charge-air pressure sensor is checked for indication of plausible values.
   
2. The intake air temperature sensor and charge-air temperature sensor are checked for signal plausibility
   
3. The mass air flow meter is checked at output speed and idling by a comparison between measured and theoretically calculated air mass (exact description, see test module and function test instructions ”Mass air flow meter”).
   At the same time, there is a check as to whether the system builds up the charge-air pressure quickly enough and to a high enough level.
   
4. Function check of the exhaust recirculation control valve and the charge-air pressure actuator/turbocharger by means of the following automatic test procedure:
   
   • Activation of the charge-air pressure actuator at minimum pulse duty factor
     
   • Gradual opening of the exhaust-gas recirculation controller, measurement of the air-mass change
     
   • Closure of the exhaust-gas recirculation controller
     
   • Activation of the charge-air pressure actuator at maximum pulse duty factor
     
   • Gradual opening of the exhaust-gas recirculation controller, measurement of the air-mass change
     
   The idling speed is automatically increased for this test.
   
5. The measured changes in air mass flow are automatically analysed. They are compared with one another in such a way that conclusions can be drawn as to the function of the exhaust-gas recirculation controller, the charge-air pressure actuator and the turbocharger.
   

Evaluation of the test

The program automatically evaluates the test and specifies the procedure for any further troubleshooting.

NB: If incorrect measured values are specified for the mass air flow meter and at the same time for another component group, the fault on the mass air flow meter is likely to be a secondary fault. For this reason, fault elimination must be started at the other component group. In cases of doubt, the function of the throttle valve and swirl flaps should be checked.

The air mass system test must then be repeated in order to check the mass air flow meter once again.

Evaluated and displayed measured values

The measured charge-air values are also displayed.

Influence of the swirl flaps

A malfunction on the part of the swirl flaps has only a slight effect on the air mass flow system test.

However, a malfunction of the swirl flaps can lead to fault code memory entries with regard to plausibility of the air mass.

When driving, a malfunction of the swirl flaps has the following effects:

• Swirl flaps stick in closed position: lack of power at high engine speeds
  
• Swirl flaps stick in closed position: deterioration of exhaust emissions
  

Requirements for the air mass system test

In order to ensure that the air mass system test delivers a meaningful result, the following requirements must be met:

• The engine must be at operating temperature
  
• All additional loads, e.g. air conditioning system or seat heating, must be switched off
  
• No electrical faults in the following systems may be stored in the DDE fault memory:
  
  • Air-mass flow sensor
    
  • Exhaust-gas recirculation controller
    
  • Charge-air pressure actuator
    
  • Charge-air pressure sensor
    
  • Intake air temperature sensor
    
  • Charge-air-temperature sensor
    
  • Ambient-pressure sensor
    
  • throttle valve
    
  • Swirl flaps
    
• The air intake guide and air filter must be OK
  
• The throttle valve and swirl flaps must be functional