Your Malfunction Indicator Lamp (MIL) is on, and there is a misfire code. It is a code P0304, which tells you cylinder number four is misfiring. There are no other codes and the engine has a steady miss. Now what? Misfire diagnosis in this kind of situation should be fairly easy. You have a code, you know which cylinder is misbehaving, and you can hear and feel the misfire. The cause of the misfire can only be one of three things: Ignition, fuel or compression. The hard-to-diagnose misfires are the ones that come and go or do not set any codes. These are the ghosts that can drive you crazy. Fortunately, the supernatural has nothing to do with the problem. The underlying cause is still ignition, fuel or compression related. The challenge is pinpointing the cause and correcting it. Intermittent misfires are almost always caused by a weak spark or a lean fuel mixture. That piece of knowledge may not tell you what exactly is causing the misfire, but it should help you plot a diagnostic course in one of two directions. Random misfires (which may or may not set a code P0300) are another type of misfire that can be hard to nail down. Random misfires that jump around from one cylinder to another may also be caused by a lean fuel condition or a weak spark. The challenge here is figuring out what is upsetting the fuel mixture or robbing the spark. The underlying cause is often a vacuum leak in the intake manifold or one behind the throttle body that allows unmetered air to bypass the air flow sensor. A lean misfire can also be caused by an EGR valve that is leaking or not fully closing due to carbon deposits under the EGR valve seat. TRACKING ENGINE MISFIRES No engine fires every cylinder 100 percent of the time. Misfires can occur at idle, when the engine is pulling hard under load, at high rpm and during throttle transitions as the air/fuel mixture changes. A few misfires are to be expected under these conditions and should cause no major performance problems or significant increase in emissions. But if the misfires get out of hand and occur too often, they can make the engine idle or run rough, stumble when accelerating, waste gas and fail an emissions test. On 1996 and newer vehicles that are Onboard Diagnostics II (OBD II) compliant and have misfire detection, the OBD II system tracks and counts misfires. The misfire monitor runs continuously on vehicles that have it anytime the engine is running. On most applications, the OBD II system uses the crankshaft position sensor (CKP) to look for subtle changes in the speed of the crankshaft between cylinder firings. If the crank suddenly slows a bit, it indicates a misfire. The only problem with this approach to detecting misfires is that the misfire monitor can sometimes be fooled when the vehicle is driven on a rough road. Consequently, some OBD II systems are programmed to temporarily ignore variations in crank speed under rough-road conditions. On some vehicles, the amperage of the spark current is analyzed when each spark plug fires to determine if the mixture burned or not. When the rate of misfire in any cylinder exceeds a certain threshold (typically around two percent), it will usually set a misfire code for that cylinder and turn on the Check Engine light. If the misfire rate is less than about two percent, it usually will NOT set a code. Even so, the misfire may still be noticeable enough that you can feel it when the engine is under load or accelerating. When the OBD II system detects a misfire, it stores operating data such as engine speed, load and warm-up status. While the misfire is happening, the OBD II system is supposed to flash the MIL lamp once a second to alert the driver. Because this might distract the driver from her cell phone conversation, sipping her Starbucks coffee or yelling at her kids in the back seat, the OBD II system will set a temporary misfire code after the second such occurrence. From that point on, the MIL lamp should flash every time the misfire returns. If the same thing happens on the next trip, the MIL lamp should blink as before and remain on even when the misfire ceases. If the misfire problem has gone away and does not recur on the second or following trips, the OBD II system may erase the temporary misfire code and forget the entire episode. The code may also be erased if no misfires are encountered under similar driving conditions during the next 40 drive cycles. Knowing this, you should always use a scan tool to look at the history freeze-frame data when diagnosing a misfire code. If the code was set when the engine was cold, chances are the OBD II system is being overly sensitive and there is no real misfire problem. Check for any technical service bulletins (TSBs) that may be out on the vehicle for false misfire codes. The cure, in many instances, is to flash reprogram the PCM so the OBD II system will be less sensitive to misfires. On some cars (Volkswagen, for example), it is possible to set false random or individual misfire codes when doing a cranking compression test. If this happens, just clear the codes after the test so the MIL lamp does not come on later. If the engine is misfiring but no codes have yet been set, and you have a scan tool that can access Mode 06 data (the raw misfire counts that are being tracked by the OBD II system), use your scan tool to look at the Mode 06 cylinder misfire counts. If you see one or more cylinders that are showing misfire counts, focus your investigation on those cylinders. QUICK MISFIRE DIAGNOSIS WITH A CODE On OBD II cars, the OBD II system will not only identify misfires but also coil and injector problems. Consequently, if the MIL lamp is on and you find a code for a misfiring cylinder and a second code indicating an injector fault for the same cylinder, bingo – the engine probably has a bad fuel injector. Likewise, if you find a misfire code for a cylinder and also a code indicating a coil fault for a multi-coil distributorless ignition system or coil-on-plug ignition, you can probably bet on a bad ignition coil. In cases where there is a cylinder misfire code but no other codes, the ignition or fuel delivery system may be borderline and not yet bad enough to set a code of its own. A shorted or open fuel injector solenoid, or a shorted or open coil, will usually set a code, but a dirty or weak fuel injector or a weak coil probably will not set a code. Bad spark plug wires are a common cause of misfire codes. After 50,000 or so chilometri, the OEM wires may be leaking current to ground or other wires, shorting the spark before it can reach the plug. Also, internal resistance may be increased, raising the required firing voltage to the point where the engine may misfire under load. Check the resistance and if it exceeds specifications, replace the wire set. SCOPING MISFIRES An oscilloscope is a good tool for identifying and analyzing misfires, but using a scope takes some know-how and experience. Suffice it to say that anybody can see a misfire on a scope, but figuring out whether the cause is ignition or fuel related requires an understanding of ignition waveforms. Once you have identified the problem cylinder, you can look at the primary and secondary patterns for that cylinder to get more diagnostic information. The dwell section of the firing line just ahead of the spark will tell you what is going on in the primary side of the system. The spark line in the firing section after the spark occurs will tell you what is happening in the cylinder and coil. Misfires can be spotted by observing a secondary parade pattern that shows the firing lines for all the cylinders on one trace. The peak firing voltage for a misfiring cylinder with a fouled spark plug, shorted spark plug or plug wire will be lower than the others. But do not forget that low compression and an overly rich fuel condition can also cause the same kind of readings. If a normal, or lower-than-normal, firing voltage drops even lower when the throttle is snapped, it may indicate a plug wire that is shorting to ground. If one of the peak firing voltages is spiking significantly higher than the others, the misfiring cylinder may have a bad plug wire with excessive resistance, a badly worn spark plug or a plug with an excessively wide gap. Lean fuel conditions can also cause firing voltages to jump. Note: If the spark burn lines for all the cylinders are higher than normal (indicating a lean fuel mixture), the underlying cause would point to something that affects all cylinders such as an intake manifold leak, leaky vacuum hose, leaky EGR valve, leaky throttle gasket or low fuel pressure (weak fuel pump or bad pressure regulator). A rich fuel mixture in an individual cylinder is less common, but can occur if the fuel injector leaks. A more common condition would be a rich condition in all cylinders caused by a dead oxygen sensor or coolant sensor that prevents the computer from going into closed loop, or a faulty fuel pressure regulator that feeds too much pressure to the injectors. Something else to look at in the spark burn line is the amount of hash it contains. A good cylinder will show a relatively clean line with little hash. A lot of hash, on the other hand, occurs when ignition misfire or lean misfire conditions are present. The duration or length of the spark burn line can provide more clues about what is going on inside the cylinder. The duration of the spark line for a good cylinder should usually be 1.3 milliseconds or longer at idle with a 0.045- to 0.050-inch plug gap. A shorter spark line would indicate a weak spark, possibly due to a lean fuel condition (dirty injectors or vacuum leaks) or low compression (burned exhaust valve or bad head gasket). If the duration of the spark line is longer than about 2 milliseconds, the air/fuel mixture is running abnormally rich. If the burn line is shorter than about 0.75 milliseconds, the cylinder is running too lean. The last thing you want to look at in the prima…

Fonte: AA1Car.com