In this video I will show you how to use Idle Air Control (IAC) position to identify a vacuum leak. It is important to understand that speed density engines (MAP engine with no MAF sensor) can have normal fuel trim numbers with a vacuum leak. This is why understanding how to read and interpret idle air control position on a scan tool it critical. The type of IAC used on this car is called a stepper motor and it's position is displayed on the scan tool in steps or counts. A "0" count means the IAC valve is fully closed.

Engine Performance Diagnostics chapter 20

symptoms 50

  • check engine light (P0300, P0171, P0507)
  • high idle speed
  • poor gas mileage

fix 50

  • replace power brake booster (leaking vacuum)

test shown enh 50

  • how to interpret IAC motor position on the scan tool
  • how to locate a vacuum leak
  • why it is important to remember the one way check valve in the power brake booster
  • how to locate a leaking power brake booster
  • why fuel trim numbers look normal with a vacuum leak

Tools used

  • scan tool
  • smoke machine

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(Chapter 20) Idle Speed Problems and Testing

Related videos:

Vacuum leak testing without a smoke machine (a SD Premium video):


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


How to find a vacuum leak with water:


P0506 IAC System RPM below expected (vacuum leak) - Ford:


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


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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.

Want even more diagnostic training? Whether you are a DIY trying to fix your own car, someone looking to become an auto technician, or a current auto technician that wants to get more into diagnostics, subscribe to ScannerDanner Premium. There is a 14 day free trial.

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!