Subaru 2009 IMPREZA Service Manual page 804

Diagnostic Trouble Code (DTC) Detecting Criteria
GENERAL DESCRIPTION
4. DIAGNOSTIC METHOD
When a misfire occurs, the engine speed will decrease and the crankshaft position speed will change. Cal-
culate the interval difference value (diagnostic value) from crankshaft position speed by the following formu-
la, and judge whether a misfire is occurring or not comparing the calculated result with judgement value.
Count the number of misfires, if the misfire ratio is higher during 1000 revs. or 200 revs., judge corresponding
cylinders as NG.
Diagnostic value calculation
(Calculate from angle speed)
As shown in the following figure, pick a cylinder as the standard and name it omg 0. And the former crank-
shaft position speed is named omg 1, the second former crankshaft position speed is named omg 2, the third
is named omg 3, etc.
(A)
(B)
(A) Ignition order
Misfire detection every single ignition
(Compare diagnostic value with
judgement value)
• 180° Interval Difference Method
• 360° Interval Difference Method
• 720° Interval Difference Method
#3 #2 #4 #1
omg7 omg6 omg5
(B) Crankshaft position speed
GD(H4SO)-80
NG judgement (Misfire occurrence
judgement required by the law) (Com-
pare number of misfire with judgement)
• FTP 1.5 times misfire NG judgement
• Catalyst damage misfire NG judge-
ment
#3 #2 #4
omg4 omg3 omg2 omg1
#1 #3 #2
omg0
EN-01774