Diagnostic Method: Notes

Fig 1: Engine Control Module (ECM) Circuit Diagram

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When the misfire occurred, the engine speed is decreased and the crankshaft position speed will change. Calculate the interval difference value (diagnostic value) from crankshaft position speed by the following formula, and judge whether the misfire occurs or not comparing the calculated result with judgment value. Counting the number of misfire up, and if the misfire ratio is higher during 1000 rev. or 2000 rev., judge NG for the corresponding cylinder.

Calculate the diagnostic value (from crankshaft position speed) -->

Misfire detection every single ignition (Compare diagnostic value with judgment value)

• 180° Interval Difference Method
• 360° Interval Difference Method
• 720° Interval Difference Method

--> NG judgment (Judge misfire occurrence required by the law) (Compare number of misfire with judgment)

• FTP1.5 times misfire NG judgment
• Catalyst damage misfire NG judgment

As the following figure, pick out a random cylinder as the standard and name it omg 0. And the former crankshaft position speed is named omg 1, the second former crankshaft position speed is named omg 2, the third is named omg 3, and the following is the same.

Fig 2: Crankshaft Position Speed Graph

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• 180° Interval Difference Method 

  Diagnosis value domg 180 = (omg -1 omg 0) - (omg 5 - omg 1)/4

  Judge misfire occurs in the following cases.

  • domg 180 > judgment value of positive side
  • domg 180 > judgment value of negative side

  (judgment value before 180"CA)

  Fig 3: Interval Difference Method Graph (1 Of 3)

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• 360° Interval Difference Method 

  Diagnosis value domg 360 = (omg 1 - omg 0) - (omg 3 - omg 2)

  Misfire judgment domg 360 > judgment value --> Misfire occurs

  Fig 4: Interval Difference Method (2 Of 3)

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• 720° Interval Difference Method 

  Diagnosis value domg 720 = (omg 1 - omg 0) - (omg 5 - omg 4)

  Misfire judgment domg 720 > judgment value --> Misfire occurs

  Fig 5: Interval Difference Method (3 Of 3)

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• FTP 1.5 times misfire (Misfire occurrence level affecting exhaust gas) 
• Judgment Value (Judge that malfunction occurs when the misfire ratio is high in 1000 engine revs.) 
  Fig 6: Threshold Valve Table (1 Of 2)

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  Time Needed for Diagnosis:  1000 engine revs.

  Malfunction Indicator Light Illumination:  Illuminates when malfunction occurs in 2 continuous driving cycles.

• Catalyst damage misfire (Misfire occurrence level damaging catalyst) 
• Judgment Value (Judge that malfunction occurs when the misfire ratio is high in 200 engine revs. (400 ignitions)) 

Fig 7: Threshold Valve Table (2 Of 2)

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