High on the suspect list:
* Vacuum Leak: A broken or loose vacuum line can disrupt the carefully balanced air/fuel mixture, especially at idle when the engine is under less load. At higher RPMs, the engine's inherent power can often overcome the leak's effects, making it run smoother. The sputtering points towards an inconsistent fuel/air mixture caused by the fluctuating vacuum.
* Dirty or Failing Mass Airflow Sensor (MAF): The MAF sensor measures the amount of air entering the engine. A dirty sensor can provide inaccurate readings, leading to a poor air/fuel mixture and rough running, especially at idle.
* Failing Idle Air Control Valve (IACV): The IACV controls the amount of air bypassing the throttle plate at idle. A malfunctioning IACV can cause erratic idle speeds and stalling.
Other possibilities (less likely given your description):
* Spark Plug Issues: Worn, fouled, or improperly gapped spark plugs can cause misfires, leading to rough running. However, this is less likely to improve at higher RPMs.
* Ignition Coil Problems: A failing ignition coil can cause misfires in one or more cylinders.
* Fuel Delivery Problem: While you mentioned bad fuel, it's less likely to cause a problem that's *only* at idle and low RPM. If the fuel was genuinely bad, you'd likely experience problems across the RPM range. However, a clogged fuel filter or a weak fuel pump *could* contribute.
What to do:
1. Inspect Vacuum Lines: Start by visually inspecting all vacuum lines connected to the intake manifold, throttle body, and other components. Look for cracks, loose connections, or any signs of damage.
2. Clean the MAF Sensor: Carefully clean the MAF sensor with MAF sensor cleaner (available at auto parts stores). Do not touch the sensor element itself.
3. Check Spark Plugs: Remove and inspect the spark plugs for wear, fouling, or damage. Replace them if necessary.
4. Consider the IACV: Testing the IACV often requires specialized tools or knowledge. If you suspect this, it's best to consult a mechanic.
Important Note: Before making any major repairs, it's best to get a diagnostic scan from a mechanic or using an OBD-II scanner. This can help pinpoint the exact problem more quickly and accurately. Ignoring vacuum leaks can cause other issues down the line.
