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Circuit Description
Engine Control Module (ECM) uses input signals provided by crankshaft and camshaft position sensors to monitor the changes in crankshaft rotation. If engine speed rate changes enough to equal a preset number, a misfire is detected and Malfunction Indicator Light (MIL) is illuminated. If misfire rate is great enough and driving conditions will cause catalytic converter damage or overheating, the MIL will blink when misfire is occurring. DTC is set when misfiring cylinders is detected during any particular 200 or 1000 revolutions. Possible causes are
- Open or short in engine wiring harness.
- Defective connections at ECM or component.
- Disconnected, restricted or damaged vacuum hose(s).
- Ignition system malfunction.
- Defective fuel injector(s).
- Improper fuel pressure.
- Defective Mass Airflow (MAF) meter.
- Defective Engine Coolant Temperature (ECT) sensor.
- Improper engine compression.
- Improper valve clearance.
- Improper valve timing.
Diagnostic Aids
When 2 or more DTCs for misfiring cylinder are recorded repeatedly and DTC P0300 does not exist, it indicates misfires were detected and recorded at different times. If misfire cannot be reproduced, the reason may be because vehicle was driven with a lack of fuel, improper fuel, fouled spark plug or etc. Using Toyota hand-held tester or scan tool, read FREEZE FRAME data. Freeze frame data records engine conditions when malfunction is detected. If COOLANT TEMP displayed in freeze frame data is less than 176°F (80°C), there is a possibility that misfire only exists during warm-up.
If oscilloscope is available, fuel injector operation on misfiring cylinder may be checked by reading fuel injector signal waveform pattern and fuel injector duration signal waveform pattern by connecting oscilloscope between terminal No. 21 (White/Black wire) at ECM electrical connector E7 and fuel injector driver terminal at ECM with engine idling. Terminal No. 21 is the E01 terminal at ECM. For ECM electrical connector terminal identification (Схема №7) For fuel injector driver identification, see IDENTIFYING ECM FUEL INJECTOR DRIVER TERMINALS table. Fuel injector signal waveform pattern and fuel injector duration signal waveform pattern should be properly displayed within 100 millisecond intervals. (Схема №13)