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Old 30th December 2017, 02:13 PM   #1
Goosemann
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Default Bmw e46 M3 flat spots when driving hard

Owned my M3 for around a year now and very rarely use it. So I thought over the Christmas period Ill park up the daily and use it full time.

Went on a spirited drive today and notice when going through the gears it held back at high revs almost like flat spot or fuel cutting.

No lights came on and otherwise was normal. Id probably say that the flat spots were intermittent too.

Anyone have any ideas where I should look first?


The plugs are probably less than 800 miles old so doubt its them.

Only mods on the car (performance wise) is decats, cobra back box and silicone intake elbow.
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Old 30th December 2017, 04:26 PM   #2
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Has it been mapped to suit the decat?
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Old 30th December 2017, 04:51 PM   #3
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No. And it shouldn’t need to be either.
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Old 30th December 2017, 06:25 PM   #4
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Well I fitted decats briefly, and the result was a slightly flat feeling car, which was cured as soon as I refitted my cats.

Apparently these need to be mapped to suit Decats but I couldn't be arsed to go down that route.
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Old 4th January 2018, 07:55 PM   #5
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Anyone got any more ideas?
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Old 14th January 2018, 03:29 AM   #6
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Quote:
Originally Posted by Goosemann View Post
Anyone got any more ideas?
Silicone intake collapsing under vacuum from wide open throttle.
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Old 28th January 2018, 10:55 AM   #7
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Possibly coil packs? They do break down without failing completely and hence you may not get a code. They sit at the top of the engine in the spark plug wells, so get very hot and will eventually fail completely - I change mine every 20,000 miles (which takes me three years or so!).
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Old 28th January 2018, 12:03 PM   #8
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Coilpacks are a common RX8 problem and you can see it happening...

Cos there are only two rotors, and a leading and trailing plug - 4 coils - it'll idle happily with a weak coilpack but increase the rpm (and therefore how hard/quickly the coil has to work) and it will start to flash the eml to indicate a misfire
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Old 28th January 2018, 06:51 PM   #9
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Fuel filter ?
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Old 28th January 2018, 07:25 PM   #10
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Good point - maybe blocked injectors?
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Old 28th January 2018, 07:45 PM   #11
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Tyre pressures ?
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Old 2nd February 2018, 04:18 PM   #12
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How old is the fuel in the tank?
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Old 6th February 2018, 09:34 AM   #13
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Well I did an ecu reset, reset the vanos too. Chucked on the engine management light and brought up the lambda codes. Obviously it’s been decatted and the lambdas have been spaced but still thrown up the errors and management light unlike before I reset it.

Changed the air box back to the k&n filter too and it pulls well again. Not sure why but it appears not to like the oem air box and the silicone intake pipe. I can’t see it collapsing either under full throttle as it’s so stiff you can’t push it in with your hand.

Strange one but all seems ok again now....
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