2004 m3, did a full rebuild last year, and up until this point I've had no issues. Slowly a cylinder 1 misfire has developed, but strangely it's only under certain load. It developed somewhat progressively, a few days after first feeling it slightly, it got bad.
Usually the issue shows itself at low rpm/high load, or at any really high throttle. I can feel when it happens because there's a complete cutout of power, and the vibration is noticable. A few times, I've had it happen immediately after startup, at idle it's misfiring. Turning the car off and back on fixes it somehow. It typically doesn't throw a CEL, but occasionally it will and the only code is for a cylinder 1 misfire. When it does throw a code, it'll disable cylinder 1 completely. I had a scanner hooked up and was looking at the "number of deactived cylinders" under the fuel system live data, and when I could feel the bogging it showed that all 6 cylinders were deactivated. Not sure how accurate that is, but what it tells me is that there's a fuel cut happening due to something, possibly the misfire in cylinder 1.
So far I've replaced fuel filter, pump, pressure regulator, checked the spark plug (looks perfect, only 3k miles on it), ran a vanos test (the adjustment range is good but the time to adjust is slow). I've also checked compression and it was at 180psi. I've tried resetting fuel trims and reseating the intake boot, to no avail. Typing this now I realize it could be something wrong with the ITB or the intake boot could cause this. That'll be what I check next.
If anyone has ideas as to what I should try and diagnose next I'd love some pointers.
Sometimes missfires will show up when you have a vacuum leak oddly enough. I would wonder if the intake gasket on cylinder #1 is pinched, or perhaps the boot from the plenum isn't seated properly. Do a visual check first of course, and if you don't see anything I would see about getting the intake smoke tested.
Darin
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05 325it Electric Red(438)/Gray(N6TT) ZCW, ZSP 5sp Manual Back set cover, trunk mat, Euro Infra-Rot front windshield, and mud flaps! Mr. Wagon My new Winter car.
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95 318is Montreal Blue Met (297)/Beige(K1SN) RIP, killed by an Idiot.
84 M535i gray market Burgandy Rot Met/Black Leather Lowered by Intrax on bilies, poly everywhere, B&B cat back system, K&N, and a hitch! Da Beast - Still running w/400k+!
91 316i euro Tizianrot/Gray cloth - E-36 w/M-40 RIP, but great on gas! Best was 38 MPG
82 528i euro Saphire Blue Met/Blue Leather RIP
79 525 euro Green/Green RIP
79 318 Silver/Black - The first one that got it all started
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