Engine W11: diagnostic fault code P1688 stored

Complaint

If fault P1688 is stored, the following complaint can exist:

Loss in power output of the engine at speeds above 4000 rpm and no throttle response. After switching off the ignition and restarting the engine, the fault is no longer noticeable; the throttle response is normal again. However, the EML light is still lit.

Causes

This complaint has one of the following causes:

• Leaking air intake system (throttle-valve housing, compressor, charge air cooler, intake manifold)
  
• Pressure sensor intake pipe or pressure sensor intake pipe before compressor delivers incorrect values
  
• Bypass valve defective
  

Troubleshooting

• Check air intake system for leaks
  
• Check both pressure sensors using the test module
  
• If there is no leak and both pressure sensor deliver plausible values, replace the bypass valve
  

Notes on test drive

The success of the repair must be verified by a test drive. The following procedure must be used here:

• Accelerate the vehicle in succession in 1st and 2nd gear sharply to the start of the red engine speed range.
  
• When the red line is reached in 2nd gear, release the accelerator pedal until the engine has fallen back to 4000 rpm. Then accelerate, increase the engine speed up to the red line and let it fall back once again to 4000 rpm.
  
• Then shift into 3rd gear and accelerate sharply to 5500 rpm to 6000 rpm. Then release the accelerator pedal until the engine drops to 3500 rpm. Then accelerate again to 5500/6000 rpm and again let the engine drop to 3500 rpm.
  
• Repeat the operation in 3rd gear 4 to 5 times.
  

If no EML light lit up during the test drive, the repair measure was successful and the fault has been remedied.

Important

During the test drive, observe speed limits. Only perform the test drive described when the traffic conditions permit! Make sure that no other road users are endangered or inconvenienced!