Engine Misfire is a common driveability problem that may or may not be easy to diagnose, depending on the cause. A misfiring cylinder in a four-cylinder engine is, pardon the pun, hard to miss. The loss of 25% of the engine's power output is the equivalent of a horse trying to run on three legs. The engine may shake so badly at idle that it causes vibrations that can be felt in the steering wheel and throughout the vehicle. The engine also may be hard to start and may even stall at idle, depending on the accessory load (air conditioning, headlights and electric rear defroster, for example). When misfire occurs, performance suffers along with fuel economy, emissions and idle quality. And, when a misfiring vehicle is subjected to an emissions test, it will usually fail because of the unusually high levels of hydrocarbons (HC) in the exhaust. CYLINDER MISFIRE CODES The Onboard Diagnostic (OBD II) system on 1996 and newer vehicles monitors misfires and will set a Diagnostic Trouble Code (DTC) if the misfire rate exceeds a certain value that may cause emissions to increase. In cases of severe misfire, the Check Engine light may illuminate or flash while the engine is misfiring. The codes can be read by plugging a scan tool into your vehicle's diagnostic connector located under the dash near the steering column. The last two digits in the misfire code will tell you which cylinder or cylinders are misfiring. The digits correspond to the cylinder number in the engine's firing order: P0300….Random Misfire Code (multiple cylinders involved) P0301….Cylinder 1 Misfire Detected P0302….Cylinder 2 Misfire Detected P0303….Cylinder 3 Misfire Detected P0304….Cylinder 4 Misfire Detected P0305….Cylinder 5 Misfire Detected P0306….Cylinder 6 Misfire Detected P0307….Cylinder 7 Misfire Detected P0308….Cylinder 8 Misfire Detected P0309….Cylinder 9 Misfire Detected P0310….Cylinder 10 Misfire Detected P0311….Cylinder 11 Misfire Detected P0312….Cylinder 12 Misfire Detected CAUSES OF ENGINE MISFIRE What causes a cylinder to misfire? Basically, it's one of three things: loss of spark; the air/fuel mixture is too far out of balance to ignite; or loss of compression. Loss of spark includes anything that prevents coil voltage from jumping the electrode gap at the end of the spark plug. Causes include worn, fouled or damaged spark plugs, bad spark plug wires or even a cracked distributor cap. A weak coil or excessive rotor gas inside a distributor would affect all cylinders, not just a single cylinder. Spark plug fouling is a common cause of ignition-related misfires. Lean Misfire "Lean misfire" can occur when the air/fuel mixture is too lean (not enough gasoline in the mixture) to burn. Depending on the engine, compression ratio and type of fuel delivery system, lean misfire may start to occur when the air/fuel ratio leans out to about 18 to 1. Newer gasoline direct injection engines can often handle leaner air/fuel ratios without misfire under certain driving conditions, but most other engines cannot. Lean misfire can be caused by a dirty, clogged or inoperative fuel injector; air leaks; low fuel pressure due to a weak pump, restricted fuel filter or leaky fuel pressure regulator. Low fuel pressure would affect all cylinders rather than an individual cylinder, as would most air leaks. If you find misfire codes for several cylinders, suspect a low fuel pressure or fuel delivery problem. If only one cylinder is setting a misfire code, suspect the injector for that cylinder, or a spark or compression problem in that cylinder. Lean misfire can also be caused by a leaky EGR valve . Exhaust leaks back into the intake manifold through an EGR valve that is not closing can have the same effect as an air leak. In fact, if a vehicle has one or more misfire codes and a P0401 EGR code, the fault is likely carbon buildup under the EGR valve. Compression Misfire Loss of compression means the cylinder loses most of its air/fuel mixture before it can be ignited. The most likely causes here are a leaky (burned) exhaust valve or a blown head gasket. If two adjacent cylinders are misfiring, the thin area of the head gasket that is between the cylinders has likely burned through or blown out. Also, if the engine is overheating or losing coolant, it's probably a bad head gasket that is leaking coolant as well as compression. Intermittent Misfire Intermittent misfires are the worst kind to diagnose because the misfire comes and goes depending on engine load or operating conditions. They may seem to occur for no apparent reason. The engine may only misfire and run rough when cold but then smooth out as it warms up. Or, it may start and idle fine but then misfire or hesitate when it comes under load. Also, it may run fine most of the time but suddenly misfire or cut out for no apparent reason. The most likely cause in this situation would be an ignition-related fault such as a spark plug or plug wire that is shorting out, possibly a bad coil or coil wiring connection, or maybe even a bad or erratic crank position sensor signal. Engine misfire can be felt at idle as a shaking or vibration. At higher engine speeds, the engine may cutout, stumble or lose power. MISFIRE DIAGNOSIS WITH A SCAN TOOL If you find one or more misfire codes when you check for fault codes with a scan tool, the codes by themselves do NOT tell you WHY the cylinder is misfiring. It could be ignition, compression or fuel related. However, if you find a P0304 misfire code for cylinder number 4, and also a P0204 code (P0200 series codes cover the injectors), you'd know the misfire was probably caused by a bad injector. If there are any EGR codes (P0401 for example), the misfire could be due to carbon buildup under the EGR valve. If there is a lean code (P0171 or P0174), the problem could be a dirty fuel injector. If there are no other codes except for the misfire code, check the ignition components for that cylinder. The cause could be a badly worn or fouled spark plug, a bad plug wire, carbon tracking or moisture inside the boot of a coil-on-plug ignition coil, or a weak or defective coil in a multi-coil distributorless ignition system. If you find a P0300 random misfire code, it means the misfire is random and is moving around from cylinder to cylinder. The cause here would likely be something that upsets the engine's air/fuel mixture, such as a major vacuum leak, a leaky EGR valve or unusually low fuel pressure (weak pump or faulty pressure regulator). If your engine seems to be misfiring, but there are NO codes set (no individual cylinder misfire codes or no random misfire code), and you have a professional grade scan tool that can access Mode $06 data, you can use the scan tool to look at the raw misfire data that is being tabulated for each cylinder. Normally the misfire counts should be zero or close to zero for every cylinder. The OBD II system will usually NOT set a misfire code until the actual misfire count exceeds about two percent for any given cylinder. So by looking at the actual Mode $06 misfire data, you should be able to see any cylinders that are showing an abnormally high misfire rate. For example, if the Mode $06 scan tool data shows zero or close to zero misfires for all cylinders except cylinder number four (which has a count of say 80 or higher), that would tell you cylinder number four has an ignition, fuel or compression problem that will require further diagnosis. FINDING THE CAUSE OF A STEADY OR CONTINUOUS MISFIRE In the case of a steady misfire, isolating the misfiring cylinder is the first step in diagnosing the problem. The old-fashioned method for finding a weak cylinder is to temporarily disconnect each of the spark plug wires, one at a time, while the engine is idling. When there's no change in the idle speed, then you have pinpointed the weak cylinder. A power balance test will tell you the same thing, but this requires some hookups and an engine analyzer. A power balance test is preferable to pulling plug wires, because it keeps you away from the voltage and prevents the voltage from causing any damage to the electronics in the ignition system. When a plug wire is physically disconnected from a spark plug, the high voltage surge from the coil cannot follow its normal path to ground through the plug wire and spark plug, so it passes back through the coil. Most ignition systems are robust enough to withstand such voltage backups intermittently but not on a prolonged basis. If the coil or ignition module is already weak, it may push the component over the brink causing it to fail. ADVANCED MISFIRE DIAGNOSTICS WITH A DIGITAL STORAGE OSCILLOSCOPE A weak cylinder will stick out like a sore thumb on an ignition scope or a digital storage oscilloscope (DSO). The secondary parade pattern will reveal the firing voltages for each cylinder. The number one cylinder will be the first one on the display, followed by each of the other cylinders in their respective firing order, moving across the screen from left to right. If the peak firing voltage for any cylinder is significantly higher or lower than the others, it indicates a problem. An usually low firing voltage would tell you the spark is finding a shortcut to ground. A fouled, shorted or cracked spark plug; arcing past the spark plug boot to ground; and a shorted plug wire would be the most likely causes. An unusually high firing voltage in a cylinder would tell you the spark plug electrode is too wide or too badly worn or that the plug wire is open. If the firing voltages for all the cylinders are about equal with the engine idling, a snap-kV test will help you find a misfire that occurs when the engine is under load. To conduct this test, suddenly open the throttle wide and then let it fall back while observing the firing pattern on the scope. All the firing voltages should increase during the snap-kV acceleration test, but, if any individual cylinder increases significantly more or less than it…

Fonte: AA1Car.com