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Old 12-13-2008, 11:44 AM   #1
subysouth
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Default What makes a Subaru ECU think there is a misfire?

Like the title says, what does the ECU read to suspect there is a misfire? Is it electrical or what?

Thanks
ss
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Old 12-13-2008, 12:13 PM   #2
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The knock sensor is what picks up the vibrations of a misfire. It is a piezo-electric device that generates a signal when vibrated its resonant frequency.
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Old 12-13-2008, 12:20 PM   #3
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the knock sensor detect knock, nothing to do with misfires.

the ECM calculates crankshaft speed. When there is a misfire the crankshaft slows down in between firing of cylinders. The ECM can detect this. It knows what cylinders should be firing at what time and by measuring the crank speed it can tell what cylinder is not up to par.

Basically it measures any imbalance in the crankshaft. It also uses the AF sensor to detect rich/lean conditions exhaust.
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Old 12-13-2008, 12:28 PM   #4
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Quote:
Originally Posted by munkis View Post
the knock sensor detect knock, nothing to do with misfires.

the ECM calculates crankshaft speed. When there is a misfire the crankshaft slows down in between firing of cylinders. The ECM can detect this. It knows what cylinders should be firing at what time and by measuring the crank speed it can tell what cylinder is not up to par.

Basically it measures any imbalance in the crankshaft. It also uses the AF sensor to detect rich/lean conditions exhaust.
+1 this is why my car was throwing misfire codes as well as crankshaft sensor codes when my crankshaft sensor died on me. once i replaced it, everything was back to normal again.
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Old 12-13-2008, 06:15 PM   #5
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Quote:
Originally Posted by munkis View Post
It also uses the AF sensor to detect rich/lean conditions exhaust.
In theory, ECU could watch AFR sensor for lean spike (slug of unburned air), which might indicate misfire. But, since all cylinders' exhaust are commingled at AFR sensor, I bet this is not what ECU does.
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Old 12-13-2008, 07:14 PM   #6
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Thanks guys for the info.

I also found this old thread and Austin had some helpful posts in it too. He says the computer compares crank and cam position signals(mostly the same as munkis is saying too.)

http://forums.nasioc.com/forums/show...hlight=misfire

I had a persistent misfire in the Legacy and finally gave up trying to track it down and sold it with the CEL to my brother-in-law. Serendipity had a hand in what may have been the problem all along, Ill know for sure in a couple of days. If so Ill add it here and my own thread I had on the problem.

I wanna hit myself in the toe with a hammer if this is/was it.

ss
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Old 12-14-2008, 01:38 AM   #7
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Quote:
Originally Posted by munkis View Post
the knock sensor detect knock, nothing to do with misfires.

the ECM calculates crankshaft speed. When there is a misfire the crankshaft slows down in between firing of cylinders. The ECM can detect this. It knows what cylinders should be firing at what time and by measuring the crank speed it can tell what cylinder is not up to par.

Basically it measures any imbalance in the crankshaft. It also uses the AF sensor to detect rich/lean conditions exhaust.
Thanks for the correction Munkis. I didn't know that. I did a little search for some info about this and here is some info I though may be of interest.




Misfire is detected by analyzing crankshaft speed fluctuations. The crank sensor detects engine speed, and an rpm value is established for each cylinder for comparison. When a sufficient difference in rpm occurs among cylinders, a misfire count is recorded. The rpm varies slightly because of different input variables.

Two types of misfires can be detected. A two-trip misfire, which occurs about 2% of the time, will result in an emissions increase. A one-trip misfire, also called a catalyst-damaging misfire, occurs in about 10% to 20% of the events.

To detect a misfire, the PCM must first learn the spacing of the crank target windows. This is done when the engine is in the Deceleration Fuel Shutoff mode. To learn the crank target spacing, three closed-throttle decelerations from 55 mph to rest must be performed. The misfire monitor will not be enabled until this spacing is learned.

The PCM also checks the machining tolerances within each group of slots on the flywheel. The PCM calculates the variation among each group of slots. This calculation is called rpm error. The variation in rpm readings between cylinder pairs must be less than 5% in order to run the misfire monitor.

Once the PCM has detected a misfire for two consecutive trips, the MIL will be continuously illuminated and a permanent DTC will store. If the PCM detects a catalyst-damaging misfire, the MIL will flash. If the active misfire ends, the MIL will change to continuous illumination. The MIL will remain on for more than one trip, but will go out if the conditions that set the DTC are not found on subsequent trips.
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Old 12-15-2008, 11:54 AM   #8
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Okay I can verify this was the problem on my car:

Loose crankshaft pulley

Ill put some more details in my other thread.

ss
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Old 12-15-2008, 03:07 PM   #9
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Quote:
Originally Posted by subysouth View Post
Okay I can verify this was the problem on my car:

Loose crankshaft pulley

Ill put some more details in my other thread.

ss
that will do it, throws off the balance of the crankshaft
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Old 12-15-2008, 05:00 PM   #10
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Quote:
Originally Posted by munkis View Post
that will do it, throws off the balance of the crankshaft
Not only that it mis-aligned the crank sprocket(which is where the CPS signal originates) which will cause the misread between crank and cam position too I'm thinking.

ss
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Old 12-18-2008, 11:31 PM   #11
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Good reading
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Old 01-01-2009, 12:46 PM   #12
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I am having wierd after 4th gear WOT blinking CEL issue right now.. if I go WOT on 4th gear lets say 2500-4000rpm after that the CEL blinks and sometimes stays on throwing misfire codes on 1,3,4 cylinders. Sometimes all of them. For now I dont know what is causing this because any other gear is just fine.

I dont feel any bogging or anything I was thinking that misfire should be noticed..

Any thoughts?

Last edited by imppu2001wrx; 01-01-2009 at 12:47 PM. Reason: update
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Old 01-01-2009, 05:14 PM   #13
subysouth
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Quote:
Originally Posted by imppu2001wrx View Post

Any thoughts?
Well if you read back through this thread and the linked threads I think youll find most everything that could be causing it short of a faulty ecu. Start with the easier stuff and eliminate things.

ss
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Old 01-01-2009, 05:17 PM   #14
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Here's my other thread where I walked through the attempts I made to cure my own flashing misfire CEL. The resolution was a loose crankshaft pulley which is still almost funny enough to make it worth it.

http://forums.nasioc.com/forums/show....php?t=1472120

ss
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