TRACKING MISFIRES<Analyzing Ignition Misfires>DTC:ATS:MIL:警告灯:P0304:
Analyzing Ignition 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 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 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.