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Thread: Symptoms of a dying coil? lean miss-fire? (4age 20v blacktop)

  1. #1
    ethanol sniffing Backyard Mechanic adamaw11's Avatar
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    Default Symptoms of a dying coil? lean miss-fire? (4age 20v blacktop)

    I've been having an issue with a miss-fire happening, it was bad enough to be undrivable. It would start easily enough and idle ok when cold, but as it warmed up had a random misfire which would gradually get worse and would then stall.

    Due to the way the wideband kept showing lean when the misfire happened, and also lean on sudden open throttle (despite acceleration enrichment and playing with the tune all the while.), it made me think it was a fuel system problem. So I replaced the fuel pressure regulator (got one with a guage), checked injectors, (and replaced with an upgrade), checked injector wiring and rewired them, checked flow from the fuel pump, checked for contamination in fuel, even checked for
    vacuume leaks.

    Then after running it and reving it for a while despite the missfire, it then died altogether and would no longer start - just didn't seem to fire at all anymore. I then checked for injector pulse, seemed to be no injector pulse, ok, but then I decided to check for spark also, and seemed to be no spark either!

    So then I swapped in a spare coil I had, tested for spark, yes spark, then tried starting it, and it seemed to start ok.
    idle seemed to be better also, and no bad missfire after warming up a bit, so it looks like all along it was a dying ignition coil.

    What I'm confused about now is why was it actually lean on the misfire? if it was only missing spark?
    Why would there be no injector pulse just from a coil not firing?

    Is this the general pattern of death for an ignition coil? gradual worsening misfire for a while, then suddenly no spark.
    I guess I just have to put everything back togethor now and see if it drives ok, as the wideband isn't showing as lean as it was before on sudden open throttle.

    Its been a misleading problem, but hopefully the fuel system and everything else is ok, and its only the coil. It would have saved a lot of time and trouble if I tested for spark earlier and tried doing the coil swap, hence why I'm posting this up.
    http://www.toymods.org.au/forums/showthread.php?t=83259
    '88 A.D.M aw11 '98 4age 20v blacktop
    NA: 14.745@150kmh at willowbank. On E85 with 6psi boost: 13.573@165kmh. ~8psi: 13.187 @169.9kmh. >10psi: 12.9 rod fragments @ 174mm holes in block
    daily: '93 ae101 Levin coupe. Motorbike: '09 Suzuki GSR600: 12.358@179kmh at Willowbank

  2. #2
    Forum Member 1st year Apprentice
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    Default Re: Symptoms of a dying coil? lean miss-fire? (4age 20v blacktop)

    I had trouble with an 18RG all year. The engine ran okay at low revs but sounded like it had a rev limiter fitted at high revs. Everyone said it was running lean and I did everything I could to fix that but I couldn't find the problem. I took it too PBE Dynotech in Newcastle and he found the problem straight away. Coil was dying. Installed a Bosch coil and it runs the way it should.
    RA40 3SGE
    RT132 18RG

  3. #3
    Chookhouse Chooning Automotive Encyclopaedia Hen's Avatar
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    Default Re: Symptoms of a dying coil? lean miss-fire? (4age 20v blacktop)

    This is just what I've read on the internet, so take with a grain of salt, though it makes sense to me.

    Your wideband is measuring the oxygen content in your exhaust. Usually lots of oxygen means lean. But if the engine is misfiring then there will be unburnt fuel and air being pumped into the exhaust. The wideband then senses all the oxygen in the unburnt charge and thinks things are lean. So misfiring can cause lean reading no matter what the AFR really is.
    I need a working 4AGE bottom end. Pref smallport GZE, but all others considered. Also complete motors.
    Drift Volvo. Was fun. 2JZ next time.

  4. #4
    Cressidaspert Carport Converter andrew_mx83's Avatar
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    Default Re: Symptoms of a dying coil? lean miss-fire? (4age 20v blacktop)

    Hen is correct, a miss will show up as lean on the wbo2

    I'd be checking your dwell settings if it's an aftermarket ecu
    There is no substitute for PUBIC inches

    Never late in an x8

  5. #5
    Forum Member 1st year Apprentice
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    Default Re: Symptoms of a dying coil? lean miss-fire? (4age 20v blacktop)

    think of a missfire as a poorfire, rather than a complete miss. A serioualy weak spark is number two cause for dry carbon fouling of a spark plug (after overrich condition). this is because the spark is small enough that fuel remains burning well down the bottom of the power stroke muxh later than it should be and never fully finishes burning before it is ejected out into the exhaust. This shows up as lean on an O2 sensor because there is air still containing oxygen getting through.
    you can confirm all this by inspecting spark plugs.
    a lean O2 reading and white fouled spark plug conductors indicates a true lean condition.
    A lean O2 reading and black sooty spark plugs indicates poor ignition.
    do u remember looking at ur spark plugs by any chance, what did they look like?
    a miserable ignition can make for some pretty sooty carboned up plugs, even disabling them alltogether causing a genuine no fire scenario.

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