GM 3100 engine, knock sensor test.

This engine had a slight rod knock or upper engine noise that was causing the knock sensor to record a false signal. The computer's reaction was to retard ignition timing in an attempt to quiet the knock sensor down. The result was a severe low power condition at times. The main take away with this case study is for low power complaints, don't forget to look at the knock sensors activity as a possible cause.

Engine Performance Diagnostics chapter 10 pages 6-7

symptoms 50

  • intermittent low power
  • runs good cold and at WOT
  • knock retard degrees on scan data goes very negative

fix 50

  • internal engine noise was causing false knock sensor activity
  • left knock sensor disconnected (car was not worth repair cost to fix the engine)

test shown enh 50

  • how to test a knock sensor
  • how to determine the cause of a low power condition

Tools used

  • scan tool
  • labscope
  • back-probing tools

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(Chapters 9, 10 and 11) The 5v Reference and Signal Circuit Quick Tests

Related videos:

How to test a Knock Sensor (P0328) - Subaru:


How to test a Knock Sensor (P0328) - Nissan:


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For more information on this topic, I have written a “field manual” called Engine Performance Diagnostics which is available as an eBook or paper book.

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