Common Causes:
* Spark Plugs & Wires: Worn spark plugs or faulty wires can misfire, causing a rough idle.
* Fuel System:
* Fuel Filter: A clogged fuel filter restricts fuel flow, leading to uneven fuel delivery.
* Fuel Injectors: Dirty or faulty injectors can cause inconsistent fuel delivery.
* Fuel Pressure Regulator: A malfunctioning fuel pressure regulator can lead to incorrect fuel pressure, affecting idle.
* Intake System:
* Air Filter: A clogged air filter restricts airflow, affecting the air-fuel mixture.
* Intake Manifold Leak: A leak in the intake manifold can cause a vacuum leak, disrupting the idle.
* Vacuum System:
* Vacuum Hoses: Cracked or disconnected vacuum hoses can lead to leaks, affecting engine operation.
* EGR Valve: A faulty EGR valve can cause excessive exhaust gas recirculation, affecting idle.
* Engine Sensors:
* Oxygen Sensor: A failing oxygen sensor can provide inaccurate readings to the engine control unit, causing an uneven idle.
* Mass Airflow Sensor: A faulty mass airflow sensor can provide incorrect airflow readings, affecting the air-fuel mixture.
* Throttle Position Sensor: A faulty throttle position sensor can provide incorrect throttle position readings, leading to idle problems.
* Idle Air Control Valve: A faulty idle air control valve can disrupt the amount of air entering the engine at idle.
* Timing Belt: A stretched or broken timing belt can affect the engine's timing, leading to a rough idle.
* Engine Compression: Low engine compression can result in a rough idle.
Troubleshooting Steps:
1. Start with the Basics:
* Check engine light: See if there are any codes stored in the engine control unit. This can help narrow down the potential causes.
* Inspect spark plugs and wires: Look for signs of wear or damage. Replace them if necessary.
* Check the air filter: Replace it if it's dirty.
2. Inspect Fuel System:
* Replace the fuel filter.
* Check fuel pressure: Ensure it's within the manufacturer's specifications.
* Clean or replace fuel injectors.
3. Examine Vacuum System:
* Inspect vacuum hoses for leaks or damage.
* Check the EGR valve operation.
4. Check Engine Sensors:
* Inspect and test the oxygen sensor, mass airflow sensor, and throttle position sensor.
5. Inspect Idle Air Control Valve:
* Clean or replace the idle air control valve.
6. Timing Belt Inspection:
* Inspect the timing belt for wear or damage.
Important Note:
* The Miller cycle engine was unique to Mazda and can be more complex than standard engines. If you're not comfortable working with the engine, it's best to take it to a qualified mechanic for diagnosis and repair.
Remember, these are just some common causes of a rough idle. There could be other factors specific to your car's model and year. Always consult a repair manual for your specific vehicle for more detailed troubleshooting information.
