DTC P0300 Engine Misfire Detected (1 Of 2)
DTC P0300 Engine Misfire Detected (2 Of 2)
Engine Schematic
Circuit Description
The crankshaft position sensor and camshaft position sensor detects a misfire, The Vehicle Control Module (VCM) monitors the speed of the crankshaft. The VCM detects a deceleration of the crankshaft that is not associated with normal engine speed reduction. In order to determine if a misfire occurred, the VCM compares the deceleration information to the engine speed and engine load. If a misfire event is determined, the VCM compares the crankshaft position to the cam sensor signal in order to determine which cylinder misfired. The VCM stores the information in separate accumulators for each cylinder. Upon completion (or failure) of the test, the VCM evaluates the number of misfires in each accumulator. If the accumulators are somewhat even or it 3 or more cylinders are misfiring, then this determines that a random misfire has occurred. The VCM also utilizes the input from the ABS wheel speed sensor in order to determine if a rough road condition exists which could cause a crankshaft acceleration and deceleration. If a rough road condition exists, the diagnostic will not run. This DTC is a type B DTC.
Conditions For Setting The DTC
No Throttle Position (TP) sensor DTCs.
No Mass Air Flow (MAF) sensor DTCs.
No Ignition Control (IC) sensor DTCs.
No Vehicle Speed (VS) sensor DTCs.
No Crankshaft Position sensor DTCs.
Rough road is not detected.
The Engine Coolant Temperature (ECT) is between -6.75°C and 95.2°C.
The engine speed between 300 RPM and 5600 RPM.
The system voltage between 9 volts and 16 volts.
The positive throttle position change is less than 4.9% for 100 msec.
The negative throttle position change is less than 2.9% for 100 msec.
A misfire is detected.
Action Taken When The DTC Sets
If the VCM determines that the misfire is significant enough to have a negative impact on emissions, the VCM turns on the Malfunction Indicator Lamp (MIL) after the misfire has been detected on 2 non-consecutive trips under the same operating conditions. If the misfire is severe enough that catalytic converter damage could result, the MIL flashes while the misfire is present.
Conditions For Clearing The MIL/DTC
The VCM turns the MIL off after 3 consecutive driving trips without a fault condition present. A history DTC will clear if no fault conditions have been detected for 40 warm-up cycles (coolant temperature has risen 40°F from the start-up coolant temperature and the engine coolant temperature exceeds 160°F during that same ignition cycle) or the scan tool clearing feature has been used.
Diagnostic Aids
If the Misfire Index indicates that one bank of the engine is experiencing misfire, the HO2S for that bank may be faulty (i.e. Bank 1 Sensor 1 or Bank 2 Sensor 1). If the Misfire Index indicates that paired cylinders (i.e. cylinders 1-4, 6-3 are experiencing misfire, inspect the Crankshaft Position (CKP) sensor reluctor ring for damage.
The Misfire Index counts the number of misfires. The scan tool can monitor the Misfire Index. There is a current and history misfire counter for each cylinder. Use the current misfire counter in order to determine which cylinder is misfiring. Many different condition could cause an intermittent misfire.
Check for the following conditions:
Check the spark plug wires and the coil wire for the following conditions:
Ensure that the spark plug wires are securely attached to the spark plugs and the distributor cap.
Check the wire routing in order to ensure that cross-firing is not occurring.
If the misfire occurs when the weather is damp, the problem could be due to worn plug wires.
NOTE : In order to test for this condition, spray the wires with water and with the engine running, watch for spark to jump from the wires. If a spark is visible, replace the wires.
Check for contaminated and a low fuel level and the following conditions:
Check the fuel condition and quality. Dirty or contaminated fuel could cause a misfire condition.
If the fuel level is low, contaminants in the bottom of the fuel tank could enter into the fuel metering system.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
If DTCs P0337 (Crankshaft Position Sensor Circuit Low Input) or P0338 (Crankshaft Position Sensor Circuit High Input) are set, this could result in a misfire condition.
In order to duplicate the conditions under which the misfire occurred, it may be necessary to drive the vehicle and monitor the scan tool DTC Set This Ignition Cycle variable.
When checking the spark at the spark plug wires, the spark should be consistent. A few sparks then nothing is no spark.
At this point, the ignition system is OK and the problem may be in the fuel system. Fuel System Diagnosis must be performed in order to determine the cause of the problem.
