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. A few misfires are to be expected under these conditions and should cause no major performance problems or significant increase in emissions. 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. TRACKING MISFIRESNo engine fires every cylinder 100 percent of the time. 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. 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. The challenge here is figuring out what is upsetting the fuel mixture or robbing the spark. Random misfires that jump around from one cylinder to another may also be caused by a lean fuel condition or a weak spark. Random misfires (which may or may not set a code P0300) are another type of misfire that can be hard to nail down. 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. Intermittent misfires are almost always caused by a weak spark or a lean fuel mixture. The challenge is pinpointing the cause and correcting it. The underlying cause is still ignition, fuel or compression related. Fortunately, the supernatural has nothing to do with the problem. These are the ghosts that can drive you crazy.
The hard-to-diagnose misfires are the ones that come and go or do not set any codes. The cause of the misfire can only be one of three things: Ignition, fuel or compression. You have a code, you know which cylinder is misbehaving, and you can hear and feel the misfire. Misfire diagnosis in this kind of situation should be fairly easy. There are no other codes and the engine has a steady miss. It is a code P0304, which tells you cylinder number four is misfiring. Your Malfunction Indicator Lamp (MIL) is on, and there is a misfire code. Auto Repair Library, Auto Parts, Accessories, Tools, Manuals & Books, Car BLOG, Links, IndexĪnalyzing Ignition Misfires Copyright AA1Car Adapted from an article written by Larry Carley for Import Car magazine
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