Honda PGM-FI Main Relay Testing (1998 Honda Prelude) How to test and fix a main relay on a Honda or Acura.

Engine Performance Diagnostics chapter 4 and 5

symptoms 50

  • rough idle
  • higher than normal idle speed
  • lower than normal idle speed
  • run good under load
  • check engine light (P0171, P0174)

fix 50

  • repair vacuum leak at the EGR tube

test shown enh 50

  • how to use freeze frame data to determine type of lean condition
  • how to read and interpret fuel trim numbers (STFT and LTFT)
  • how to locate a vacuum leak with a smoke machine
  • how to use idle speed and desired idle speed data parameters

Tools used

  • incandescent test light
  • soldering gun

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(Chapters 4 and 5) Fuel Trim and Oxygen Sensor Testing

Related videos:

False mass airflow sensor/idle control system codes - vacuum leak - Pontiac


P0507 Idle Control System (RPM Too High) - vacuum leak - GM:


Understanding Short Term and Long Term Fuel Trim (an SD Premium video):


Causes of lean condition trouble codes P0171, P0174 - GM 5 3L:


Ford F-150 P0171 P0174 Trouble Codes - vacuum leak (water test):


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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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On ScannerDanner Premium I will bring you right into my classroom at Rosedale Technical College. You will find page for page lectures taken right from my book as well as exclusive classroom type case studies. What is so special about these classroom case studies? I pull live problem vehicles directly into my classroom and we troubleshoot them in real time, using and applying the theory and testing procedures we learn during the classroom lectures. There is no better on-line training of how to troubleshoot automotive electrical and electronics systems anywhere!