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Thread: got a misfire? read all about it here!

  1. #1
    moderator emeritus ...rallyred's Avatar
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    got a misfire? read all about it here!

    THE SYMPTOMS:
    poor idle, part throttle stumble, full throttle stumble, popping noises, solid illumniated "service engine soon" light, flashing "service engine soon" light, lack of power, backfiring

    THE CONDITION:
    a misfire! these can be somewhat difficult to trace but after reading this sticky, you will be well-equipped to track it down.

    THE THEORY:
    your engine needs 3 things for combustion to occur: fuel in the form of atomized gasoline, air from the atmosphere, and a spark from your ignition to light it all off once it is properly compressed. when you develop a misfire, one of these 3 items is missing.

    THE REPAIR:
    fix whatever is taking away one of more of those items! yes, it's that simple
    Ben | Super Mod
    richmondperformancemusclecars
    Quote Originally Posted by Malice View Post
    I really think you should use your $200 for grammar lessons. You've made "an decision". Aren't you embarrassed by that?


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  3. #2
    moderator emeritus ...rallyred's Avatar
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    IGNITION, SPARK, AND TIMING

    while the GM factory ignition is a terrific performer that has carried 3800 II engines deep into the 9s, it can occasionally lose its luster from mileage, water intrusion, or just plain old defective parts. there are 4 main components to our ignition system:

    an ignition control module, the interface between the PCM and ignition coils. the ICM can read inputs from the factory sensor array (crank, cam, and knock sensors) and commands timing during the first few seconds after startup. after that time has elapsed, it switches ignition control over to the PCM, your car's main computer. the ICM is located on the driver's side of the engine compartment, just above the valve cover and is in a sandblasted aluminum tray, covered up by the coil packs.

    3 coil packs,
    which amplify and discharge the electricity sent to them by the ICM. the coil packs sit on the ICM, in the same tray on the driver's side.

    6 plug wires,
    which transmit the electricity to the spark plugs. the plug wires go from the coil towers to the spark plugs.

    and finally 6 spark plugs, which pass electricity between two electrodes to form a spark. the spark plugs are threaded inside the cylinder head, between the exhaust manifold or header primaries.

    any of these items can immediately come into suspicion when a misfire occurs. old or damaged plugs and wires are the most common cause of a misfire, particularly at cylinder #5. if those items check out, it is then time to suspect a coil pack. the location of the coil pack and ICM assembly makes it particularly suceptible to water damage. the slighest amount of water or corrosion can damage or destroy the ICM, a coil, or both.

    HOW TO DIAGNOSE THESE ITEMS:


    if you are lucky enough to set a misfire specific DTC (P0301-P0306), you can start by checking the corresponding cylinder's electronic components. for instance, if you get a P0305 DTC, you should start tracing the misfire at cylinder #5. unfortunately you will often simply get P0300, which is the global "multiple random misfire" DTC.

    spark plugs are very simple and very cheap to replace, but they are commonly overlooked maintenance items. as such, replacement or at a minimum inspection of spark plugs should be considered when a misfire develops.

    here is a nice picture reference that shows different kinds of damage:



    spark plug wires are a little more expensive to replace, but they are also commonly overlooked maintenance items. often a visual inspection is sufficient to locate a damaged or otherwise defective wire. burn marks, charring, or even a broken wire are very easy to spot.
    if the visual inspection yields no results, a multimeter can be used to check resistance and continuity. if one or two wires show a much higher resistance than the group, it can be assumed that those wires are bad, and should be replaced.

    the coils are more expensive to replace, but similarly overlooked. unfortunately not much can be done to test a coil, other than a visual inspection. it is possible to refresh the contacts at the ICM and tower electrodes. in addition, parts stores do have machines that can check for high resistance as well as for problems under load, which is when a problem with this type of electrical component often arises.

    the ICM is the hardest item to diagnose and the most expensive to replace. a DTC of P0300 often points to the ICM. often, when the ICM is damaged by water, it will fail to revert control of ignition timing back to the PCM. as a result your car will operate at static and very low timing advance, which is fine at idle but terrible under load. you will have no power, the engine will stumble, and your SES light will probably flash. there is no repair for the ICM, other than replacement with a new component.

    OTHER ELECTRICAL ITEMS THAT CAN CAUSE A MISFIRE
    MAF sensor: the MAF sensor meters incoming air to tell the computer how much fuel and spark advance to add. if the MAF is dirty, damaged, or otherwise malfunctioning, the engine will not deliver the proper amount of fuel, the proper air/fuel ratio will not be acheived, and combustion can sometimes fail to occur. the easiest way to determine if the MAF is causing your misfire is to simply unplug it. if the misfire goes away, you need to clean, repair, or replace your MAF sensor.

    front (B1S1, B2S1) O2 sensors: the O2 sensors check the exhaust exiting your engine for proper air/fuel mixture and combustion. they are able to control fuel delivery and spark timing, and if they are not functioning properly they can cause much the same problem as a malfunctioning MAF sensor. you can check for proper O2 operation with an engine scanner showing proper switching between the two banks, as well as O2 readings within the acceptable range (500-1100 mv)

    throttle position sensor (TPS)
    : less common, but also a culprit of misfires. symptoms are similar to that of a bad MAF or O2s. the TPS output signal can be checked with a multimeter, at a closed position it should register less than .05v.


    COIL PACK ARRANGEMENT (courtesy of N2OEQIP)
    Ben | Super Mod
    richmondperformancemusclecars
    Quote Originally Posted by Malice View Post
    I really think you should use your $200 for grammar lessons. You've made "an decision". Aren't you embarrassed by that?


  4. #3
    moderator emeritus ...rallyred's Avatar
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    AIR AND AIR FLOW

    AIR INTAKE SYSTEM
    there are 3 main components to your intake sytem:

    the plastic air box, which houses the filter and has a bung for the air intake temperature (AIT) sensor;

    the throttle body, which houses the MAF, TPS, the MAF screen, and the throttle blade;

    and the intake manifold duplex, which holds the injectors, fuel rail, and the runners that go to the individual cylinders.

    HOW DO DIAGNOSE THESE ITEMS

    more often than not, a misfire caused in the intake system will be traced back to an electrical component, as mentioned in the previous post. however, something as simple as an extremely clogged air filter CAN restrict airflow to the point of causing a misfire. it is not common but not unheard of.

    EXHAUST
    there are also 3 main components to the exhaust system:

    the exhaust manifolds, which transport exhaust out of the cylinder;

    the catalytic converter, which chemically alters the exhaust;

    and the tailpipes and muffler, which deflect sound waves and dump the exhaust into the atmosphere.

    HOW DO DIAGNOSE THESE ITEMS

    the catalytic converter, if clogged, can cause a misfire. the exhaust may become very quiet, or barely flow, and the y-pipe going from the exhaust manifolds to the catalyst may even glow red. in this case the clogged catalyst is stopping the proper flow of air into and out of the engine, and creating the misfire.

    the only legal solution to a clogged catalyst is replacement.
    Ben | Super Mod
    richmondperformancemusclecars
    Quote Originally Posted by Malice View Post
    I really think you should use your $200 for grammar lessons. You've made "an decision". Aren't you embarrassed by that?


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