DTC P0300 Engine Misfire Detected. Buick Century 1996

For 1990-2009 cars only

Makes 🢒 Buick 🢒 1996 🢒 Century 🢒 DTC 🢒 DTC P0300 Engine Misfire Detected

Circuit Description

The PCM has the ability to detect a misfire by monitoring the 3X reference and camshaft position input signals from the Ignition Control Module. The PCM monitors crankshaft speed variations (reference period differences) to determine if a misfire is occurring.

If 2% or more of all cylinder firing events are misfires, emission levels may exceed mandated standards. The PCM determines misfire level based on the number of misfire events monitored during a 200 engine revolution test sample. The PCM continuously tracks 16 consecutive 200 revolution test samples. If 11 or more misfires are detected during any 5 of the 16 samples, DTC P0300 will set. If the misfire is large enough to cause possible three-way catalytic converter damage, DTC P0300 may set during the first 200 revolution sample in which the misfire was detected. In the case of a catalyst damaging misfire, the MIL will flash to alert the vehicle operator of the potential of catalyst damage.

Conditions for Setting the DTC

•	No VSS, Transmission, TP sensor, Fuel trim, Fuel injector, MAP sensor, EVAP, HO2S, IC/Bypass line monitor, EGR flow, EGR pintle position, ECT sensor, CKP sensor CMP sensor, or MAF sensor DTC(s) set.

•	Engine speed between 450 and 5800 RPM.

•	System voltage between 9 and 16 volts.

•	The ECT indicates an engine temperature between -7°C (19°F) and 120°C (248°F).

•	Throttle angle steady.

•	The PCM is detecting a crankshaft RPM variation indicating a misfire sufficient to cause three-way catalytic converter damage or emissions levels to exceed mandated standard.

Action Taken When the DTC Sets

•	The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed.

•	The PCM will store conditions which were present when the DTC set as Freeze Frame and Failure Records data.

Conditions for Clearing the MIL/DTC

•	The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been run and passed.

•	The History DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction.

•	The DTC can be cleared by using the scan tool.

Diagnostic Aids

The scan tool provides information that can be useful in identifying the misfiring cylinder. If the DTC P0300 is currently stored as Test failed since code clear, the misfire history counters (Misfire Hist #;1 - #;6) will still contain a value that represents the level of misfire for each cylinder.

The scan tool displayed misfire counter values (Misfire Hist. #; 1 through #;6) can be useful in determining whether the misfire is isolated to a single cylinder or to a cylinder pair (cylinders that share an ignition coil-1/4, 2/5, 3/6.) If the largest amount of activity is isolated to a cylinder pair, check for the following conditions:

•	Secondary Ignition Wires: Check wires for affected cylinder pair for disconnected ignition wires or for excessive resistance (the wires should measure under 30 Kohms

•	Damaged Or Faulty Ignition Coil: Check for cracks, carbon tracking or other damage. Also check coil secondary resistance. Secondary resistance should be between 5000 ohms and 8000 ohms (5K ohms and 8K ohms).

•	Substitute a Known Good Coil: Swap ignition coils and retest. If the misfire follows the coil, replace the ignition coil.

If the misfire is random, check for the following conditions:

•	System Grounds: Ensure all connections are clean and properly tightened.

•	MAF: A Mass Air Flow (MAF) sensor output that causes the PCM to sense a lower than normal air flow will cause a lean condition.

•

Air Induction System: Air leaks into the induction system which bypass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, incorrectly installed or faulty crankcase ventilation valve, or for vacuum leaks at the throttle body, EGR valve, and intake manifold mounting surfaces.

•	Fuel Pressure: Perform a fuel system pressure test. A faulty fuel pump, plugged filter, or faulty fuel system pressure regulator will contribute to a lean condition.

•	Injector(s): Perform injector coil/balance test to locate faulty injector(s) contributing to a lean or flooding condition. In addition to the above test, check the condition of the injector O rings.

•	EGR: Check for leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow.

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

Test Description

Number(s) below refer to the step number(s) on the Diagnostic Table.

A faulty injector or injector circuit will cause a misfire DTC to be set. The DTC P1200 table diagnoses any problems with the injector circuits.

The Misfire Cur # display may normally display a small amount of activity (0 - 10 counts) but should not steadily increment during an entire 200 revolution test sample period.

Depending on the cause of the misfire, the misfire history counter will display a very large number for the misfiring cylinder(s); values for the non-misfiring cylinders will be less than 1/2 as great as the misfiring cylinder(s). When investigating a misfire, always start with items associated with the cylinder(s) that has the largest number of counts stored in the misfire history counter.

Check for conditions that can cause a random cylinder misfire.

Checks for injector faults using J 34730-380. While using this tool DTC P0107, P0117, P1107 and P1114 may be stored as active. This is from disconnecting the 10-way injector connector to install the tool (the MAP sensor and ECT sensor circuits also pass through this connector). Disregard these DTCs when using this tool.

Checks for conditions that can cause a non-random or single cylinder misfire.

This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed. Refer to EEPROM Programming procedures.

Step

Action

Value(s)

Yes

Was the Powertrain On-Board Diagnostic (OBD) System Check performed?

Go to Step 2

Go to the Powertrain OBD System Check

Is DTC P1200 also set?

Go to DTC P1200 Injector Control Circuit

Go to Step 3

Start and idle the engine.
Review and record scan tool Freeze Frame data.
Operate the vehicle to duplicate the conditions present when the DTC was set (as defined by the Freeze Frame data).
Monitor the scan tool Misfire Cur. # display for each cylinder.

Does Misfire Cur. # display increment for any cylinder (indicating a misfire currently occurring)?

Go to Step 4

Refer to Diagnostic Aids

Does Misfire Hist. # display a very large value for more than one cylinder?

Go to Step 5

Go to Step 14

Visually and physically inspect the vacuum hoses for splits, kinks, and improper connections.
If a problem is found, repair as necessary.

Was problem found?