1. Vacuum leaks: A vacuum leak can cause the engine to run lean, which can lead to a rough idle. Check all vacuum lines and connections for leaks, and replace any that are cracked or damaged.
2. Fuel injector issues: Clogged or dirty fuel injectors can cause the engine to run rich or lean, which can lead to a rough idle. Try cleaning the fuel injectors with a fuel injector cleaner, or replace them if necessary.
3. EGR valve issues: A faulty EGR (exhaust gas recirculation) valve can cause the engine to run rough, especially at idle. Try cleaning the EGR valve, or replace it if necessary.
4. Idle air control valve issues: A faulty idle air control valve can cause the engine to idle too high or too low, or to surge. Try cleaning the idle air control valve, or replace it if necessary.
5. PCV valve issues: A faulty PCV (positive crankcase ventilation) valve can cause the engine to run rough at idle, and can also lead to oil leaks. Try cleaning the PCV valve, or replace it if necessary.
6. Spark plugs: If your spark plugs are old or worn out, they can cause the engine to run rough at idle. Replace the spark plugs if necessary.
7. Ignition timing: If your ignition timing is off, it can cause the engine to run rough at idle. Check the ignition timing and adjust it if necessary.
8. Compression issues: Low compression in one or more cylinders can cause the engine to run rough at idle. Check the compression and have any necessary repairs done.
9. Clogged catalytic converter: A clogged catalytic converter could also be responsible for a rough idle. Take your 4Runner into a mechanic to have it replaced if it is clogged.
If you have checked all of these things and your 4Runner is still idling rough, it is best to take it to a qualified mechanic for further diagnosis.
