Ignition System (Even with new parts):
* Distributor Cap and Rotor: While you replaced wires, the cap and rotor are wear items and can cause misfires, especially at idle. Cracks or carbon buildup can disrupt the spark.
* Ignition Coil: A weak coil might provide sufficient spark at higher RPMs but struggle at idle.
* Ignition Module/Control Unit: This controls the spark timing. A faulty module can cause erratic sparking.
Fuel System (Beyond the filter):
* Fuel Pressure Regulator: This regulates fuel pressure to the injectors. A faulty regulator can lead to inconsistent fuel delivery, especially noticeable at idle.
* Fuel Injectors: Clogged or leaking injectors can cause a rough idle and poor starting. A fuel injector cleaner might help, but a professional inspection/cleaning or replacement might be necessary.
* Air Leak in the Intake Manifold: A vacuum leak can dramatically affect the air/fuel mixture, leading to a rough idle. This is common in older cars and can be tricky to find.
Other Possibilities:
* Crankshaft Position Sensor (CKP Sensor): This sensor tells the engine's computer where the crankshaft is. A faulty sensor can cause misfires and starting problems.
* Throttle Position Sensor (TPS): A malfunctioning TPS provides incorrect information to the engine control unit about the throttle position, impacting the air/fuel mixture.
* Vacuum Leaks: As mentioned above, vacuum leaks are a significant possibility and can be difficult to diagnose. Look for cracked vacuum hoses.
* Exhaust System Restrictions: A severely clogged catalytic converter or muffler can restrict exhaust flow, leading to a rough idle.
Troubleshooting Steps:
1. Visual Inspection: Carefully examine all vacuum lines and hoses for cracks or leaks. Inspect the distributor cap and rotor for any signs of damage or wear.
2. Diagnostic Codes (If Equipped): Check for any diagnostic trouble codes (DTCs) stored in the car's computer. A code reader specific to Mercedes-Benz vehicles will be needed.
3. Fuel Pressure Test: Measure the fuel pressure to ensure it's within specifications.
4. Compression Test: A compression test will check the health of your cylinders. Low compression in one or more cylinders indicates potential engine problems.
5. Professional Diagnosis: Given the car's age and the fact you've already replaced several parts, a professional mechanic specializing in Mercedes-Benz vehicles is highly recommended. They have the tools and expertise to properly diagnose the problem.
Don't continue driving the car extensively with a rough idle, as it can cause further damage. A proper diagnosis is crucial to prevent further issues and costly repairs down the road.
