Common Causes:
* Dirty or Faulty Mass Airflow Sensor (MAF): The MAF sensor measures the amount of air entering the engine. A dirty sensor will give inaccurate readings, leading to a rough idle and poor performance. Cleaning it (carefully, with MAF sensor cleaner) is often the first step. If cleaning doesn't help, it might need replacement.
* Dirty or Faulty Throttle Position Sensor (TPS): The TPS tells the computer the throttle's position. A dirty or failing TPS can cause erratic idle and hesitation. Cleaning (again, carefully) is a starting point, but replacement might be necessary.
* Vacuum Leaks: Small cracks or holes in vacuum hoses can disrupt the engine's air/fuel mixture, resulting in a rough idle. Inspect all vacuum lines for damage and replace any suspect ones. Pay close attention to the lines connected to the intake manifold and PCV system.
* Ignition System Problems: Worn spark plugs, faulty ignition wires, or a failing ignition coil can cause misfires, leading to a rough idle. Check the condition of the spark plugs and wires; replacement might be needed. A failing coil can be diagnosed with a scan tool.
* Dirty or Faulty Idle Air Control Valve (IAC): The IAC regulates airflow at idle. A dirty IAC can stick, causing an erratic idle. Cleaning it is often effective, but replacement may be needed if cleaning doesn't solve the issue.
* PCV Valve: A clogged positive crankcase ventilation (PCV) valve can cause excessive pressure buildup in the crankcase, affecting the engine's operation and leading to a rough idle. This is a relatively inexpensive part to replace.
* Fuel Injectors: While less likely at such low mileage, clogged or faulty fuel injectors can cause a rough idle. This usually requires professional diagnosis and cleaning or replacement.
* Intake Manifold Gasket Leak: A leak in the intake manifold gasket can allow unmetered air to enter the engine, disrupting the air/fuel mixture.
Less Common but Possible Causes:
* Engine Mounts: Worn engine mounts can allow excessive engine vibration, making the idle feel rougher than it actually is. This would likely be accompanied by other noticeable vibrations.
* Transmission Issues (unlikely): Although you mention a manual transmission, extremely rare transmission issues could *indirectly* affect engine idle, but this is less probable.
Troubleshooting Steps:
1. Check engine light: Is the check engine light on? If so, get the codes read using an OBD-II scanner. This will pinpoint potential problems.
2. Visual Inspection: Inspect all vacuum lines, spark plugs, and wires for visible damage.
3. Clean MAF and TPS: This is a simple and inexpensive first step.
4. Check PCV valve: Easy to replace and relatively inexpensive.
5. If the problem persists: Take your Ranger to a qualified mechanic for diagnosis. They have the tools and expertise to identify the root cause more efficiently.
Given the low mileage, it's important to thoroughly investigate the simpler, more common causes first before considering more expensive repairs. A systematic approach, starting with the easiest and most likely culprits, will be the most efficient way to resolve the rough idle.
