You can follow up with a power balance test, a compression test and/or a leak down test to further isolate the fault. A flickering vacuum reading typically indicates a cylinder with a compression problem. These types of problems also can be found with a vacuum gauge. Low compression also can cause misfires if a cam lobe has rounded off. When it reaches the converter, the fuel ignites and is burned up.Ĭompression misfires will occur if a cylinder fails to hold compression because of a leaky head gasket, bent valve, burned exhaust valve or broken valve spring. When a cylinder misfires, that cylinder’s dose of air and fuel passes unburned into the exhaust. HC emissions are a factor in urban smog, but can also be very damaging to the vehicle’s catalytic converter. But a frequent or steady misfire will cause a sharp rise in unburned hydrocarbon (HC) emissions, a noticeable drop in power and a significant decrease in fuel economy. As long as the misfires are fairly random and spaced far enough apart, it causes no harm. OBD II can give you an exact count of the misfires cylinder by cylinder (which you can find in Mode $06 with a scan tool), but it can’t tell you what’s causing the misfires.Įvery engine will experience an occasional misfire. One very important point to keep in mind about OBD II misfire detection is that the system does not tell you why the engine is misfiring. The OBD II misfire monitor is normally active when a warm engine is running under normal loads, but it may not track misfires if other fault codes are present or codes are pending for the fuel system, evaporative emission control (EVAP) system or EGR system. This helps avoid the setting of false misfire codes, and also allows the system to spot pattern misfires that may cause an increase in emissions or damage to the catalytic converter. By averaging the pass/fail ratings of all 16 blocks, the OBD II system gets the big picture of what’s actually going on. But if the number of misfires in that block is too high, the block receives a “fail” rating. The tally is stored in 16 memory blocks, so every 3,200 rpm the misfire count starts over again.Īs long as the number of misfires in any given memory block remains below a certain value, there is no problem and OBD II gives that block a “pass” rating. Any misfires that occur are recorded every 200 revolutions of the crankshaft. On most applications, OBD II uses a “block learn” strategy to track misfires. And if a misfire is random and jumps from cylinder to cylinder, you may find a P0300 random misfire code. If more than one cylinder is misfiring, there will be additional codes for those cylinders as well.
Andrew Markel, Director of Content, ShopOwnerĪ code P0302, for example, would tell you cylinder number two is misfiring.Robert Whitehurst, Service Dealer Solutions, Epicor.Alda Rodriguez, Service Dealer Solutions, Epicor.Tim Hardin, Senior Vice President & General Manager, Automotive, Epicor.
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