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Thread: Strange intermittent misfire - please help

  1. #1
    Join Date
    Jul 2008
    Location
    Medina, Ohio
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    My Cars
    1999 M3, 2000 323it

    Strange intermittent misfire - please help

    Here's a real head scratcher for all of you E46 mechanical gurus.

    The car = 2000 323ita, 105k miles.
    First the background - I had a bad O2 sensor (band2/#2) and was also intermittently getting a p0303 misfire. The car would shutter noticeably with the misfire until I turned it off and restarted it. Once restarted the misfire would go away. I replaced all of the 02 sensors and cleared the codes. I drove the car again and the p0303 misfire came back - same as before. I then switched the cylinder 2 sparkplug and coil with cylinder 3 and cleared the codes to see if it moved the code/misfire.

    Second, the problem. I am now VERY intermittently getting a misfire again, but now the check engine light comes on then goes off by itself and the misfire will go away on its own. It will also go away if I restart as before.
    The real problem is now when I scan for codes I get the MILS light = ON, but no codes or pending codes. The only time the misfire and check engine light happens now is on a stretch of highway that has a long downhill. Basically it only happens when the car is coasting for an extended period of time (1/4 mile+). Fortunately northern Ohio is pretty flat and I only drive this highway on the weekends when I go skiing.

    Any ideas before I start throwing parts it it?

    More background info just in case. I have replaced the following in the last 7k miles (all of your typical E46 issues )
    Spark plugs and coils
    EGR
    Waterpump and overflow tank
    Rebuilt the vanos
    All four 02 sensors
    PS pump
    K&N replacement airfilter
    All idler pulleys and belts
    Fan clutch

    Thanks for your help - Jon
    Last edited by M3 Jon; 02-15-2010 at 12:08 PM.

  2. #2
    Join Date
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    Williamsburg, VA USA
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    2004 ZHP 6MT
    My first thought is you should stop driving down that road!

    Second thought is to lose the k&n drop in.
    Last edited by danewilson77; 02-15-2010 at 12:25 PM. Reason: Automerged Doublepost

    by Dane Wilson, on Flickr

  3. #3
    nathancarter is offline Stretch Haters Club #1 BMW CCA Member
    Join Date
    Feb 2007
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    Tampa, FL
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    2002 330i 131k
    So, the misfire on #2 stopped after you moved the plug & coil?
    You're not able to "catch" the current codes (if any) because the light is off by the time you plug in your scan tool, or even if the light is on, the tool doesn't show anything?

    In your laundry list, you say you replaced the plugs and coils - all six coils? Did you replace them before this problem started happening, or in an attempt to resolve it?

    How do the coil boots look (the rubber connector between the coils and the plugs)? if those are cracked or decayed, the spark can arc from the coil boot to the head instead of down in the plug where it's supposed to.

  4. #4
    Join Date
    Dec 2009
    Location
    Washington
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    My Cars
    2003 330i
    Maybe it's time to think out of the box a bit. To start, remember, the DME sees a misfire by a oxygen spike corresponding to a specific cylinder as noted by the oxygen sensors.

    Where is the throttle on the long downhill areas? Is it completely closed? Downhill coasting involves:
    1. High manifold vacuum
    2. Some ECU's (depending on the car) will shut the fuel off
    3. Less intake charge into the cylinder

    Because of the higher levels of manifold vacuum, this tends to exaggerate a vacuum leak that normally wouldn't be seen at other times...this could lead to an spike in oxygen levels that would lead the DME to believe a misfire occured. If it happens enough times (crank revolutions), the CEL will turn on. If it goes away, the CEL will turn off.

  5. #5
    Join Date
    Jul 2008
    Location
    Medina, Ohio
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    1999 M3, 2000 323it
    Hey guys - thanks for the responses. The good news (?) is that I finally am getting a code. p1250 and p0302. I am ordering a replacement coil and will see if that takes care of the problem. Maybe they will warranty it since I replaced all 6 last summer. They are not factory coils. I also replaced the intake boot at the same time. I'm going to check those too see of they might have ripped already.
    I will keep you posted.

  6. #6
    Join Date
    Dec 2009
    Location
    Washington
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    2003 330i
    You get a P1250 code and then order an ignition coil? Try to think things through before you start throwings cash at it.

  7. #7
    Join Date
    Jul 2008
    Location
    Medina, Ohio
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    5
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    1999 M3, 2000 323it
    I ordered a coil because the p0303 code moved to a p0302. The miss followed the coil when I moved it. Also, the coil is also under warranty. I agree with you on the p1250. I am still going to look for a vacuum leak.

  8. #8
    Join Date
    Dec 2006
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    Elko,MN
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    94' 325I, 01 X5
    Your vacuum leak is most likely on the underside of your intake boot.

  9. #9
    Join Date
    Jul 2008
    Location
    Medina, Ohio
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    1999 M3, 2000 323it
    Update - The missfire started becoming more common in the last week. The code of P302 kept showing up, but I never got the P1250 again. I replaced the #2 coil last night. So far no CEL or miss. I have my fingers crossed...

    Thanks for your help!

  10. #10
    Join Date
    Oct 2018
    Location
    Glendale
    Posts
    2
    My Cars
    Bmw m3 e46 2003
    It took me a year but i found the problem . It’s the ignition switch. A black piece on the opposite side of the key.

    It was passing the ring voltage. Cost $70 you can install it alone. Check engine didn’t come back. 3 days now with no issues. I’ll keep you posted.

    This has been a reason for my misery. Wasted time going to 4 different mechanics. BMW in Van Nuys recommended this change.
    So far it’s been great.

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