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Home >> Mazda >> 2003 >> MX-5 Miata Base, Automatic >> Repair and Diagnosis >> Engine Performance >> System >> Engine Controls - Self-Diagnostics >> Diagnostic Tests >> DTC P0300: Random Misfire Detected >> Diagnostic Procedure

Diagnostic Procedure

  1. Verify Related Repair Information Availability -  Turn ignition off. Connect scan tool to DLC-2. See Figure. Turn ignition on. Monitor for FREEZE FRAME DATA and ensure data has been recorded. Check service bulletins for repair information related to this DTC. If repair information is not available, go to next step. If repair information is available, perform diagnosis and repair as necessary. After repair, go to step  21.
  2. Verify Related Pending Code Or Stored DTCs -  Turn ignition on, engine off. Monitor for other DTC or pending coderelated to this DTC. If no other DTC is present, go to next step. If other DTC is present, follow appropriate DTC diagnostic procedure. See DIAGNOSTIC TROUBLE CODE DEFINITIONS .
  3. Verify Current Input Signal Status (Ignition Key At On/Idle) -  Start engine and let it idle until normal operating temperature is reached. Using scan tool, access ECT, MAF, TP and VSS PIDs. For system PID specifications. See SYSTEM INPUT SIGNAL SPECIFICATIONS  table. If all system input voltages are as specified, go to next step. If any system input voltage is not as specified, inspect suspect sensor and system wiring harness, repair or replace as necessary. See appropriate WIRING DIAGRAM under ENGINE PERFORMANCE in SYSTEM WIRING DIAGRAMS. After repair, go to step  21.
  4. Verify Current Input Signal Status Under Trouble Condition -  Inspect same PIDs while simulating recorded FREEZE FRAME DATA conditions. If no input signal drastically changed, go to next step. If any input signal drastically changed, inspect suspect sensor and system wiring harness, repair or replace as necessary and go to step  21.
  5. Inspect CMP Sensor -  Turn ignition off. Perform CMP sensor inspection. See CAMSHAFT POSITION SENSOR under ENGINE SENSOR & SWITCHES in SYSTEM & COMPONENT TESTING - MAZDA6, MIATA, MPV, PROTEGE & PROTEGE5 article. If CMP sensor is okay, go to next step. If CMP sensor is faulty, inspect for installation condition and damage to 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 Condition -  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. Check Ignition Coil Operation & High-Tension Lead With Timing Light -  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. Check High-Tension Leads Of No Blinking Cylinder -  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. Inspect Power Supply Terminal At Ignition Coil Connector -  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 reading should be battery voltage. If voltage reading is as specified, go to next step. If voltage reading is not as specified, repair open or short in Black/White wire between suspect ignition coil 3-pin harness connector terminal "A" and ignition switch terminal "IG1". See appropriate WIRING DIAGRAM under ENGINE PERFORMANCE in SYSTEM WIRING DIAGRAMS. After repair, go to step  21.
  10. Inspect Ignition Coil Resistance -  Leave ignition coils harness connectors disconnected and disconnect high tension led wires. Check ignition coils primary and secondary resistances. See IGNITION CHECKS in BASIC DIAGNOSTIC PROCEDURES article. If resistance readings are as specified, go to next step. If any resistance reading is not as specified, replace faulty ignition coil. After repair, go to step  21.
  11. Inspect MAF V PID -  Return vehicle to original condition. Start engine and let it idle. Using scan tool, 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.
    NOTE: During next procedure, engine speed may change when rust penetrating agent is sprayed on air suction area.
    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.
  12. Inspect Excessive Air Suction In Intake-Air System -  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:
    • 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 faulty component. After repair, go to step  21.

  13. Inspect Fuel Line Pressure -  Turn ignition off. Release fuel pressure from fuel system. Install fuel pressure gauge. For proper fuel pressure test. See FUEL PRESSURE (EXCEPT B2300, B3000, B4000 & TRIBUTE) under FUEL SYSTEM in BASIC DIAGNOSTIC PROCEDURES article. Fuel pressure should be 53-61 psi (370-420 kPa). If fuel line pressure is too low, go to next step. If fuel line pressure is as specified, go to step  16. If fuel line pressure is excessively high, check following:
    • Fuel pump maximum pressure. See FUEL PUMP OPERATION under FUEL SYSTEM in BASIC DIAGNOSTIC PROCEDURES article.
    • Fuel return line for restriction.

    If any problem is present, repair as necessary. After repair, go to step  21. If no problem is present, replace fuel pressure regulator.

  14. Inspect Fuel Pump Maximum Pressure -  Perform fuel pump maximum pressure test. See FUEL PUMP OPERATION under FUEL SYSTEM in BASIC DIAGNOSTIC PROCEDURES article. Fuel pump maximum pressure should be 92 psi (640 kPa). If fuel pressure is less than 92 psi (640 kPa), go to next step. If fuel pump maximum pressure is not as specified, check fuel pump circuit for open or poor connection, repair or replace connector or circuit as necessary. After repair, go to step  21. If connector and circuits are okay, replace fuel pump. After repair, go to step  21.
  15. Inspect Fuel Line From Fuel Pump To Fuel Delivery Pipe -  Turn ignition off. Visually inspect fuel lines for leaks. If fuel leak is found, repair or replace component as necessary. After repair, go to step  21. If fuel leak is not found, inspect fuel filters for following condition:
    • Restriction or clogging at fuel filter (high pressure). If restriction or clogging is found, replace fuel filter.
    • Foreign material or stain in fuel filter (low pressure). If foreign material or stain is found inside low pressure fuel filter, clean fuel tank and fuel filter.

    If both fuel filters are okay, replace fuel pressure regulator. Repair fuel system as necessary. After repair, go to step  21.

  16. Inspect Engine Compression -  Return vehicle to original condition. Perform engine compression test. For proper engine compression test, see MECHANICAL INSPECTION 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. After repair, go to step  21.
    NOTE: Purge solenoid valve may also be referred to as a purge control solenoid valve.
  17. Inspect Operation Of Purge Solenoid Valve -  Turn ignition off. Inspect purge solenoid valve operation. See FUEL EVAPORATIVE SYSTEM (MIATA & MPV) under EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM & COMPONENT TESTING - MAZDA6, MIATA, MPV, PROTEGE & PROTEGE5 article. If purge solenoid valve is operating properly, go to next step. If purge solenoid valve is not operating properly, replace purge solenoid valve. After repair, go to step  21.
  18. Inspect PCV Valve Operation -  Inspect PCV valve operation. See POSITIVE CRANKCASE VENTILATION under EMISSION SYSTEMS & SUB-SYSTEMS in SYSTEM & COMPONENT TESTING - MAZDA6, MIATA, MPV, PROTEGE & PROTEGE5 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. Inspect Operation Of EGR Valve -  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.
    WARNING: Removing radiator cap when radiator is hot is dangerous. Scalding coolant and steam may shoot out and cause serious injury. When removing radiator cap, wrap a thick cloth around and turn it slowly.
    NOTE: Large bubbles are normal since they are remaining air coming out from engine coolant passage.
  20. Inspect Sealing Of Engine Coolant Passage -  Ensure to let engine cool down before performing this test. Check cooling system for combustion gasses. Remove radiator cap. Bleed air from cooling system. Start engine and let idle. Check for small bubbles which make coolant white at filler neck. If combustion gasses are not present, go to next step. If combustion gasses are present, repair leak as necessary. After repair, go to next step.
  21. Verify Troubleshooting Of Misfire DTC P0300 Completed -  Return vehicle to original condition. Using scan tool, clear DTCs from PCM memory. See CLEARING DIAGNOSTIC TROUBLE CODES  under SELF-DIAGNOSTIC SYSTEM. Start engine and run DRIVE MODE 1. See DRIVE MODES  under SELF-DIAGNOSTIC SYSTEM.Monitor for pending DTCs. If same DTC is not present, go to next step. 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. After repair, go to next step.
  22. Verify After Repair Procedure -  Using scan tool, perform After Repair Procedure. See AFTER REPAIR PROCEDURE . If no other DTCs are present, testing is complete. If any other DTC is present, go to applicable test and repair.
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