How does the ECU know a misfire occurred at a specific cylinder? My impreza 1.6 was missing a lot in the morning. I know the sparkplug cables are getting old because it only happens once in a while (1 week, sometimes overnight) it has been standing for some time in the rain. When it heats up (5 km) it is running as it should. It is very occasionally. However it is hard to get hands on a set of original subaru spark leads because dealers = stealers over here (well most of them, they charge 25 euros/ a cable).

However until now I never ever got a CEL with a misfire code, only once the knock sensor. So what does the ECU thinks is a misfire?

 

(additionally, can I use NGk wires those i can easily get?)

The ECU detects misfires when it notices a change in crankshaft speed. At any given RPM the ECU knows when to expect the next tooth on the crankshaft sprocket to pass by the crank sensor. When that doesn't happen within a the certain time that the ECU is looking for that next tooth, the ECU basically records that as a pattern that it compares to several different variables to figure out which cylinder is misfiring. It can take some time before it determines exactly which cylinder is misfiring. But it keeps track of the changes that occur, and based on the programming of the ECU it will eventually figure out a cylinder and set a code.

 

I've read that NGK wires are just as good as Subaru wires, and some think NGK are the OE supplier for Subaru.