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Home >> Mazda >> 2002 >> MX-5 Miata Base, Standard >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self-Diagnostics >> Diagnostic Tests >> DTC P0300: Random Misfire Detected >> Diagnostic Procedure

Diagnostic Procedure

  1. Turn ignition off. Connect New Generation Star (NGS) tester or equivalent to OBD-II Data Link Connector-2 (DLC-2). See Figure. Turn ignition on. Monitor for FREEZE FRAME DATA and ensure data has been recorded. Check for service bulletins and/or other line of repair information related to this DTC. If repair information is available, perform diagnosis and repair as necessary. After repair, go to step  21. If repair information is not available, go to next step.
  2. Monitor for DTC or pending coderelated to this DTC. If other DTC is present, follow appropriate DTC diagnostic procedure. If no other DTC is present, go to next step.
  3. Start engine and let it idle until it reaches normal operation temperature. Using NGS tester or equivalent, access ECT, MAF, TP and VSS PIDs. For system PID specifications. See SYSTEMS INPUT SIGNAL SPECIFICATIONS  table. If all systems input voltages are as specified, go to next step. If any system input voltage is not as specified, inspect system sensor and system wiring harness, repair or replace as necessary. For circuit identification, see MIATA in WIRING DIAGRAMS article. After repair, go to step  21
  4. Inspect same PIDs while simulating recorded FREEZE FRAME DATA conditions, If no input signal drastically change, go to next step. If any input signal drastically change, inspect system sensor and system wiring harness, repair or replace as necessary and go to step  21.
  5. Turn ignition off. Perform CMP sensor inspection. See CAMSHAFT POSITION SENSOR under ENGINE SENSOR & SWITCHES in SYSTEM & COMPONENT TESTING - MIATA, MILLENIA, MPV, PROTEGE, PROTEGE5 & 626 article. If CMP sensor is okay, go to next step. If CMP is faulty, inspect for installation condition and damage on timing belt or gears, if problem is found repair or replace as necessary. If no problem is found, replace CMP sensor. After repair, go to step  21.
  6. Verify CKP sensor installation and condition (looseness or damage). If CKP sensor is properly installed and condition is okay, go to next step. If CKP sensor is found loose or damaged, secure or replace CKP sensor. See CRANKSHAFT POSITION SENSOR under ENGINE SENSOR & SWITCHES in REMOVAL, OVERHAUL & INSTALLATION article. After repair go to step  21.
  7. Return vehicle to original condition. Start engine and let it idle. Using a timing light, verify blinking condition on each cylinder. If all cylinder show good blinking conditions, go to step  21. If any cylinder show faulty blinking conditions, go to next step.
  8. Turn ignition off. Inspect high-tension leads for installation, condition, corrosion on terminal, open lead or damaged cover. If high-tension leads are okay, go to next step. If any high-tension leads is faulty, replace high-tension lead and go to step  21.
  9. Disconnect ignition coils 3-pin harness connectors. See Figure. Turn ignition on. Measure voltage between ground and Ignition coils harness connectors terminal "A" (Black/White wire). Voltage should be battery voltage. If voltage is as specified, go to next step. If voltage is not as specified, repair open or short (in Black/White wire) between suspected ignition coil and ignition switch. For circuit identification, see MIATA in WIRING DIAGRAMS article. After repair, go to step  21.
  10. Leave ignition coils harness connectors disconnected and disconnect high tension led wires. Check ignition coils primary and secondary resistances. See IGNITION COIL RESISTANCE - MIATA under IGNITION CHECKS in BASIC DIAGNOSTIC PROCEDURES article. If resistances are as specified, go to next step. If any resistance is not as specified, replace faulty ignition coil. After repair, go to step  21.
  11. Return vehicle to original condition. Start engine and let it idle. Using NGS tester or equivalent, access MAF PID. While racing engine RPM, verify that MAF PID changes quickly according to engine RPM. If MAF PID changes as specified, go to next step. If MAF PID does not changes as specified, replace MAF sensor. After repair, go to step  21.
  12. Leave engine running at idle speed. While spraying rust penetrating agent in short spurts, inspect for air suction (vacuum leak) due to cracks, damages, or loose parts between following components:
    CAUTION: DO NOT spray rust penetrating agent on or near coils or plug wires, an arcing wire or coil could start a fire. Allow enough time for rust penetrating agent to dissipate.
    • From MAF sensor to throttle body.
    • From throttle body to dynamic chamber.
    • From dynamic chamber to intake manifold.
    • Vacuum hoses.

    If air suction (vacuum leak) is not found, go to next step. If air suction (vacuum leak) is found, repair or replace necessary component and go to step  21.

  13. Turn ignition off and perform fuel pressure test. Release fuel pressure from fuel system. Install fuel pressure gauge. For proper fuel pressure test. See FUEL PRESSURE under FUEL SYSTEM in BASIC DIAGNOSTIC PROCEDURES article. Fuel pressure should be 53-61 psi (3.7-4.3 kg/cm2 ). If fuel pressure is as specified, go to step  16. If fuel pressure is not as specified, go to next step.
  14. Turn ignition off. Visually inspect fuel lines for leaks. If fuel leak is found, repair or replace component as necessary and go to step  21. If fuel leak is not found, inspect fuel filter for following condition:
    • Restriction or clogging at fuel filter (high pressure). If restriction or clogging is found, replace fuel filter and go to step  21.
    • Foreign material or stain in fuel filter (low pressure). If foreign material or stain is found inside fuel filter, clean fuel tank and replace fuel filter. After repair, go to step  21.
    • If all items are okay, go to next step.
  15. Turn ignition on. Perform fuel pump maximum pressure test. See FUEL PUMP MAXIMUM PRESSURE INSPECTION under FUEL SYSTEM in BASIC DIAGNOSTIC PROCEDURES article. Fuel pump maximum pressure should be 92 psi (6.5 kg/cm2 ). If fuel pressure is not as specified, check as follow:
    • If fuel pump maximum pressure is less than 92 psi (6.5 kg/cm2 ), check fuel pump circuit for open or poor connection, repair or replace connector or circuit as necessary and go to step  21.
    • If connector and circuits are okay, but fuel pump maximum pressure is less than 92 psi (6.5 kg/cm2 ),replace fuel pump and go to step  21.
    • If connector and circuits are okay, but fuel pump maximum pressure is more than 92 psi (6.5 kg/cm2 ), replace fuel pressure regulator and go to step  21.
    • If fuel pump maximum fuel pressure is as specified, go to next step.
  16. Return vehicle to original condition. Perform engine compression test. For proper engine compression test. See COMPRESSION under PRELIMINARY INSPECTION & ADJUSTMENTS in BASIC DIAGNOSTIC PROCEDURES article. Compression should be about 198 psi (13.9 kg/cm2 ) at 300 RPM. If engine compression is as specified, go to next step. If engine compression is not as specified, determine source of low compression. Repair as necessary and go to step  21.
  17. Turn ignition off. Inspect Purge Control Solenoid Valve (PCSV) operation. See DTC P0443: EVAP CONTROL SYSTEM PURGE CONTROL SOLENOID VALVE, CIRCUIT MALFUNCTION . If PCSV is operating properly, go to next step. If PCSV is not operating properly, replace PCSV. After repair go to step  21.
  18. Inspect Positive Crankcase Ventilation (PCV) valve operation. See POSITIVE CRANKCASE VENTILATION under EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM & COMPONENT TESTING - MIATA, MILLENIA, MPV, PROTEGE, PROTEGE5 & 626 article. If PCV valve is operating properly, go to next step. If PCV valve is not operating properly, replace PCV valve. After repair, go to step  21.
  19. Remove Exhaust Gas Recirculation (EGR) valve for inspection. Visually inspect valve for stuck open condition. If EGR valve is operating properly, go to next step. If EGR valve is not operating properly (stuck open), replace EGR valve. After repair, go to next step
  20. Return vehicle to original condition. Ensure to let engine cool down before performing this test. Remove radiator cap by using thick cloth around it and turning slowly. Implement procedure to bleed air from engine coolant system. Start engine and let it idle. If any small air bubbles which makes engine coolant white at filler opening is found, determine source of air leak into cooling system (possible poor sealing of cylinder head gasket), repair and replace faulty components. After repair, go to next step. If sealing of engine coolant passage is okay, go to next step.
  21. Return vehicle to original condition. Using NGS tester or equivalent, clear DTCs from PCM memory. Start engine and run DRIVE MODE 1. See DRIVE MODES  under SELF-DIAGNOSTIC SYSTEM.Monitor for pending DTCs. If same DTC P0300 is present, replace PCM. For PCM removal and installation, see POWERTRAIN CONTROL MODULE under COMPUTERIZED ENGINE CONTROLS in REMOVAL, OVERHAUL & INSTALLATION article. If any other DTC is present, follow appropriate DTC diagnostic procedure.
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