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Thread: Misfire under boost (plus misfire diagnosis)

  1. #1
    I'm part of the furniture Moderator Ian C's Avatar
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    Technical Reference Misfire under boost (plus misfire diagnosis)

    Name:
    Misfire under boost

    Also known as:
    Hesitation

    What is it?
    You get a misfire under high boost, usually over 1 bar.

    Usually caused by:
    Running higher than stock boost on stock spark plugs

    Why?
    Spark plugs have different temperature ranges, and 'colder' ones are designed to run in a hotter running engine, if that makes any sense to you. Warmer plugs start to fail under high temperatures, higher boost causes these higher temperatures. As the resistance in the plug increases due to the heat it's exposed to, the spark may not be strong enough to take place. This causes the misfire. This is made worse by the stock plug gap being 1.1mm. Higher boost pressures mean air is more resistant to electrical current. Colder grade plugs are usually pregapped to 0.8mm, and as this is a lot less of a gap for the spark to jump, so the plugs work with higher boost pressures and continue to work as they transmit the extra heat away properly.

    How bad is this problem?
    It ruins the driving experience, but above that the too-hot plug will degrade at an accelerated rate and may eventually disintegrate. Having chunks of spark plug going through your cylinders and turbo is A Bad Thing.

    Remedy:
    Install a colder grade of plug. Denso IK24's are good if expensive, NGK BKR7E's are cheaper but may not be good enough for serious applications and don't last as long. Denso IK22's are slightly colder than stock if you want a long-lasting easy starting plug for mildly upgraded engines.

    Other possible causes:
    One or more of your coil packs are on their way out, and they aren't giving enough spark energy. This is usually a much harsher misfire, and is most often discovered on installing a big single turbo.

    You can get a stumbling problem if you are running mega rich, AFRs of 10:1 or worse.
    Last edited by Ian C; 04-04-05 at 14:28. Reason: Added info about plug gaps
    I love my job. It pays for my Supra.

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    I'm part of the furniture Moderator Ian C's Avatar
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    Arrow Misfire diagnosis

    There are a few ways to get misfires in your car. They act differently, feel different, and are caused by different things - remote diagnosis is difficult when the symptom is simply listed as "misfire" so here are a few of the common descriptions for reference and diagnosis purposes:

    1. Fuel cut.
    A light will light up on the dash and you will lose all powers of acceleration for a couple of seconds. This usually occurs at full boost so you tend to get thrown forwards in your seat and it's very harsh. The stock level of fuel cut is anything 1 bar or above, and the higher the boost the faster cut will occur.

    2. Ignition problems.
    2a - weak coil packs on a single turbo - This will cause a misfire anywhere from a single miss in the rev range to one that is harsh enough to jerk you forwards and backwards in the seat as power comes on and off. Happens on boost, mostly around torque peak but pretty much anywhere in the rev range when on boost, and gets worse rapidly as the coil pack breaks down further - to the point of misfiring at even 0.1bar. If you turn the boost down and the misfires go away, this is another good indication of the coil packs being at fault.

    2b - Spark plug gap not small enough - usually occurs on a BPU car with stock plugs, this causes a more gentle misfire where you feel acceleration fade and reappear at high boost.

    2c - Too low ignition voltage - Usually occurs with an incorrect jumper setting on an aftermarket ECU/ignition system This causes a 2a-like misfire but at a repeatable boost level, below that boost level it never misfires.

    3 - Overfuelling.
    3a - mild overfuelling - You will get a drop in power. The acceleration will fall off and recover once you leave the rich spot. It kind of feels like you've hit a very strong headwind.

    3b - heavy overfuelling - you will get stumbling like 2b as the mixture fails to ignite. This will clear up as soon as you leave the rich load site on the map.

    3c - massive overfuelling - you will get a drop in power harsher than 3b but less harsh than 2a. The engine pretty much won't get past the load site, it will just keep cutting out, so acceleration is lost. If you have a BOV, it will fire rapidly as power comes on and off.

    4 - Air leak.
    4a - Some or all power suddenly drops off due to a popped off hose, and the power is gone until the hose is replaced

    4b - Some or all power intermittently fades because a split hose opens up and vents boost pressure

    4c - There is a problem with your sequential turbo system

    -Ian
    Last edited by Ian C; 01-07-05 at 12:11.
    I love my job. It pays for my Supra.

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