P0300

Random combustion misfire detected

Detection Condition

• The ECU determines which cylinder is misfiring by using the crankshaft position sensor to detect variations in the rotational speed of the crankshaft. While the engine is running, the ECU monitors the number of misfires that occur at 200 crankshaft revolution intervals and 1,000 crankshaft revolution intervals. Calculating the misfire ratio for each crankshaft revolution, if the ratio exceeds the preprogrammed threshold, the ECU determines that misfire significant enough to damage the catalyst(s) and/or negatively affect emissions performance has occurred.

Diagnostic Support Notes

• This is a continuous monitor (MISFIRE)
• The check engine indicator illuminates if the ECU detects the above malfunction condition that affects emissions performance, in two consecutive drive cycles
• PENDING CODE is available if ECU detects the above condition that affects emissions performance during the first drive cycle
• Check engine indicator flashes if ECU detects a misfire condition that can damage the exhaust catalysts during the first drive cycle
• PENDING CODE is not available when the check engine indicator flashes
• FREEZE FRAME DATA is available
• DTC is stored in ECU memory

Possible Causes

• Crankshaft position (CKP) sensor malfunction
• Camshaft position (CMP) sensor malfunction
• Spark plug malfunction
• Ignition coil malfunction
• High-tension lead malfunction
• MAF sensor contamination
• Vacuum leak between MAF sensor and dynamic chamber
• Fuel pump malfunction
• Fuel pressure regulator malfunction
• Fuel line restriction
• Fuel filter restriction
• Fuel line leakage
• Purge solenoid valve malfunction
• PCV valve malfunction
• EGR valve malfunction
• Vacuum hose damage or incorrect hose routing
• Related connector or terminal malfunction
• Related wiring harness malfunction

Diagnostic Procedure

1.  Has FREEZE FRAME DATA been recorded?

• Yes - Go to next step.
• No - Record FREEZE FRAME DATA on repair order, then go to next step.

2.  Check for related TSBs and/or online repair information.  Is any repair information available?

• Yes - Perform repair or diagnosis according to available repair information.  If vehicle is not repaired, go to next step.
• No - Go to next step.

3.  Turn ignition key to OFF then ON (engine off).  Any other pending or current DTCs present?

• Yes - Go to appropriate DTC troubleshooting.
• No - Go to next step.

4.  With an OBD interface, access engine coolant temperature (ECT), intake air temperature (IAT), mass airflow (MAF), engine revolutions per minute (RPM), throttle position (TP), and vehicle speed (VS) PIDs.  Is there any signal that is far out of specification when ignition switch is ON and engine is idling?

• Yes - Inspect suspected circuit and/or part according to inspection results, then go to Step 13.
• No - Go to next step.

5.  Inspect same PIDs as in Step 4 while simulating FREEZE FRAME DATA condition. Is there any signal which causes drastic changes?

• Yes - Inspect suspected circuit and/or part according to inspection results, then go to Step 13.
• No - Go to next step.

6.  Turn ignition key to OFF.  Inspect the camshaft position sensor.  Is camshaft position sensor okay?

• Yes - Go to next step.
• No - Inspect for proper timing belt installation and for timing belt damage.  Repair faulty parts and go to Step 23.

7.  Check crankshaft position sensor for looseness.  Is crankshaft position sensor loose?

• Yes - Retighten crankshaft position sensor, then go to Step 23.
• No - Go to next step.

8.  Verify proper ignition system operation using a timing light.  Do all cylinders exhibit proper ignition system operation?

• Yes - Go to Step 12.
• No - Go to next step.

9.  Turn ignition key to OFF and inspect the high tension leads (KL) or coil-on-plug units (KJ) for damage, wear, improper installation, corrosion on terminal, open leads, etc.  Are all items okay?

• Yes - Go to next step.
• No - Replace faulty parts and go to Step 23.

10.  Disconnect ignition coil connector (KJ).  Turn ignition key to ON (engine OFF) and measure the voltage between ignition coil terminal D (harness-side) and body ground.  Is voltage reading B+?

• Yes - Go to next step.
• No - Check for open circuit between ignition coil connector and ignition switch.  Repair or replace wiring harness and go to Step 23.

11.  Check ignition coil resistance.  Is coil resistance okay?

• Yes - Go to Step 23.
• No - Replace ignition coil (KJ) or coil/distributor unit (KL) then go to Step 23.

12.  Start engine and access MAF PID using an OBD interface.  Verify that MAF PID changes quickly as the engine is raced in Park or Neutral.  Is MAF PID response okay?

• Yes - Go to next step.
• No - Replace MAF sensor and go to Step 23.

13.  Inspect for air leakage between MAF sensor and throttle body or between throttle body and dynamic chamber (KJ).  Are there any leaks present?

• Yes - Repair or replace suspected part and go to Step 23.
• No - Go to next step.

14.  Inspect fuel line pressure.  Is fuel line pressure okay?

• Yes - Go to Step 18.
• No - If pressure is too high, go to next step.  If pressure is too low, go to Step 16.

15.  Disconnect vacuum hose from pressure regulator and start engine.  Is vacuum felt at the open end of the vacuum hose?

• Yes - Inspect fuel pump maximum pressure and for fuel return line restriction or deformation.  If all items are okay, replace the fuel pressure regulator and go to Step 23.
• No - Verify vacuum hoses are connected correctly.  If okay, replace fuel pressure regulator control (PRC) solenoid valve.  If not okay route hoses to their correct positions and go to Step 23.

16.  Inspect fuel pump maximum pressure.  Is fuel pump maximum pressure within 630-800 kPa (92-116 psi)?

• Yes - Go to next step.
• No - Inspect fuel pump circuit for open or poor connection.  Repair or replace suspected circuit then go to Step 23.  If okay, replace fuel pump and go to Step 23.

17.  Visually inspect for fuel leakage from fuel line.  Is any fuel leakage found?

• Yes - Replace suspected fuel line and go to Step 23.
• No - Inspect high-pressure and low-pressure fuel filters for restriction and clean or replace if necessary then go to Step 23.  If all items are okay, replace the fuel pressure regulator and go to Step 23.

18.  Inspect engine compression.  Is engine compression okay?

• Yes - Go to next step.
• No - Implement engine overhaul procedure then go to Step 23.

19.  Perform purge solenoid valve inspection.  Is the purge solenoid valve okay?

• Yes - Go to next step.
• No - Replace purge solenoid valve then go to Step 23.

20.  Turn ignition key to OFF, remove the PCV valve and check for proper operation.  Is PCV valve operation okay?

• Yes - Go to next step.
• No - Replace PCV valve and go to next Step 23.

21.  Remove EGR valve and perform a visual check to ensure the valve is not stuck open.  Is the EGR valve stuck open?

• Yes - Repair or replace the EGR valve and go to Step 23.
• No - Go to next step

22.  With the engine sufficiently cool, remove the radiator cap, implement coolant system bleed procedure, then idle the engine.  Are there any small bubbles, which make the engine coolant appear white at the filler opening?

• Yes - Air from combustion chamber is entering cooling system.  Repair or replace faulty parts, then go to next step.
• No - Go to next step.

23.  Make sure to reconnect all disconnected connections.  Turn ignition key to ON (engine OFF).  Clear DTC from memory using OBD interface.  Start engine and perform DRIVE MODE 1.  Is PENDING CODE for P0300 present?

• Yes - Replace ECU then go to next step.
• No - Go to next step.

24. Perform "After Repair Procedure" and scan for stored codes.  Is there a DTC present?

• Yes - Go to appropriate DTC troubleshooting.
• No - Troubleshooting completed.

[P0300 - From 2002 Mazda Millenia factory service manual]