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How many misfires are okay?

 
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Old 12-20-2017, 07:37 AM
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How many misfires are okay?

Occasionally I'll connect my OBD reader to my DB9 to check the number of misfires. My DB9 is a 2010 with about 15K miles and I can't feel any shudder at idle or other symptoms of misfires -- I'm just doing it to tinker. Never had an engine check light.

On average, after driving 15-20 minutes, the average misfire counts range from 0-20 -- but occasionally one or two cylinders will be 80-100. Funny thing is the offending cylinder on one trip might be normal the next, and a previously normal will be high and then go back to normal again. Doesn't make sense to me.

I was thinking maybe the misfire correction wasn't learned -- but it is. Thought the OBD reader was flaky but borrowed one and had same result.

Are this many misfires within normal range? Nothing to worry about? Or should I start preparing myself financially for replacement of the coils and plugs?
 
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Old 12-20-2017, 09:17 AM
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Bad coils would be consistently misfiring, I replaced mine last year and sine you own a 2010, you should have the latest and greatest coils which are not known to go bad. Also being that it is 12 cylinders, a slight misfire would not be very noticeable. Mine misfires all the time, all cars do. When it becomes noticeable and consistent, then it is a problem, think mine bad coil was averaging around 3-400 misfires (i think, don't really remember) and car had a shudder when stopped at a light.
 
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Old 12-20-2017, 08:27 PM
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Put fuel in it and drive! Nothing's wrong with it until you have a light on, unless the misfire corrections have not been learnt. But you stated they have been.
 
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Old 12-21-2017, 03:34 PM
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Thanks, I'll forget about it and enjoy the ride!
 
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Old 12-23-2017, 09:25 AM
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A little confused. So the system learns there are misfires and compensates for them some how? Or learns to ignore them until a certain threshold is met?
 
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Old 12-23-2017, 11:25 AM
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I know my 2010 and earlier DB9s don't have knock sensors so the PCM just monitors the misfires and will shut down a cylindar when it reaches a certain threshold (1.5%?). At that point the engine check light should come on.

I'm not sure when knock sensors were implemented - 2012 or 2013? The knock sensors work with the PCM to advance or retard spark timing.
 
 
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