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Discussion Starter #1
My 2012 S5 just threw a couple of codes. P0308, P0300 and P0305. So basically random misfire on cylinders 5 and 8. Runs a little rough until it warms up but then runs perfect. The plugs and coils have been changed within the last 10k miles. Carbon buildup?
 

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Presume you mean S5 coupe with the V8? If so, how many total miles and has carbon buildup ever been cleaned? Might need to switch coils around and see if problem follows. Or reset them and see if it comes back.
 

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It has 70k miles on it. I haven't done the big carbon clean on it but have run the GDI intake cleaner through it. I actually did it the other day and I heard a rattling around in one of the cylinders and a big puff of black smoke went out the back so I guess I jarred loose a big chunk of carbon from the valves.
 

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Sounds like intake needs a look inside for more carbon. I had it done once at 40k + and has been great ever since. Plugs and coils had to be changed then and just again recently at 65k for a misfire that I could actually notice with some hesitation involved. You may have a similar issue but I would have a look at the intake to start since the major one has not been done.
 

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Discussion Starter #6
Just an update I ran a second can of the GDI intake cleaner yesterday and got another big puff of black smoke once I had started it up after the 1 hour heat soak. And today my check engine light went off on its own and the car is running great. My plan had been to do this every 5000 miles buy I may start doing it a little more regularly.
 
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