• Tiny
  • msamgab
  • 1997 Pontiac Transport
  • 119,000 miles

Whats the cause of code P0300 whats causing

Wednesday, December 12th, 2012 AT 2:33 AM

1 Answer

  • Tiny
  • ASEMaster6371
  • Expert
  • 25,502 posts

Diagnostic Chart

Diagnostic Chart

Diagnostic Chart

Diagnostic Chart

Wiring Diagram

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 Malfunction Indicator Lamp (MIL) will flash to alert the vehicle operator of the potential of catalyst damage.

Conditions for Setting the DTC

No Vehicle Speed Sensor (VSS), Throttle Position (TP) sensor, MAP sensor, ECT sensor, Crankshaft Position (CKP) sensor, Camshaft Position (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 6 C (21.2 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

If the engine misfire is minimal it will not damage the catalyst. The PCM will illuminate the MIL during the second key cycle in which the DTC sets
If the engine misfire is severe it may damage the catalyst. The PCM will flash the MIL while the misfire remains at catalyst damaging levels.
The PCM will disable TCC operation.
The PCM will store conditions which were present when the DTC set as

Freeze Frame and Fail Records data.

Conditions for Clearing the MIL/DTC

The PCM will turn the MIL OFF 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 Clear Info function or by disconnecting the PCM battery feed.

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 detected on 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, is isolated to a cylinder pair (cylinders that share an ignition coil -1/4, 2/5, 3/6), or is random. 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 30,000 Ohm (30 K-Ohm).
Damaged Or Faulty Ignition Coil: Check for cracks, carbon tracking or other damage. Also check coil secondary resistance. Secondary resistance should be between 5000 Ohm and 7000 Ohm (5 K-Ohm and 7 K-Ohm).
Substitute a known good coil: Swap ignition coils on the Ignition Control Module and retest. If the misfire follows the coil, replace the ignition coil. Refer to 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. Try operating the vehicle within the fail records conditions with the MAF sensor disconnected. If the lean or misfiring condition is not present with the MAF sensor disconnected, replace the MAF sensor Refer to MAF Sensor.
Air Induction System: Vacuum leaks that cause intake air to bypass the MAF sensor will cause a lean condition. Check for disconnected or damaged vacuum hoses, incorrectly installed or faulty PCV valve, or for vacuum leaks at the throttle body, EGR valve, and intake manifold mounting surfaces. Refer to Air Induction System, and Crankcase Ventilation System.
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. Refer to Fuel System Pressure Test. See: Powertrain Management Computers and Control Systems Testing and Inspection Component Tests and General Diagnostics System Diagnosis - Fuel System Pressure Test
Injector(s): Perform Fuel Injector Coil Test Fuel Injector 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. See: Powertrain Management Computers and Control Systems Testing and Inspection Component Tests and General Diagnostics Fuel Injector Balance Test
EGR: Check for leaking valve, adapter, or feed pipes which will contribute to a lean condition or excessive EGR flow. Refer to EGR System.
Extended Idle: Excessive open loop operation caused by extended idling or short trip driving may leave deposits on the heated oxygen sensors. The deposits cause oxygen sensors to respond slowly to exhaust oxygen content, affecting fuel control and causing a misfire to be indicated at idle. This condition is not permanent. To determine if this condition is causing the DTC P0300 to be set, review the freeze frame and fail records data for DTC P0300. If the DTC P0300 occurs at high engine speeds, the condition described above did not cause the DTC P0300 to set. If the DTC P0300 occurs at idle or very low engine speeds and at engine coolant temperatures below 80 C (176 F), the condition described above is very likely the cause of the DTC P0300 being set. The deposits on the heated oxygen sensors can be eliminated by operating the vehicle fully warm at mass air flows above 15 gm/s.

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.
Steps 5 through 13 check for conditions that can cause a random cylinder misfire.

Steps 14 through 16 check 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.

Steps 17 through 25 check 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. PCM Replacement/programming.

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Wednesday, December 12th, 2012 AT 2:39 AM

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