Ignition System Issues (Despite the Tune-Up):
* Worn Distributor Cap and Rotor: Even new ones can be faulty. Check for cracks, carbon tracking, or corrosion. A simple visual inspection is crucial.
* Worn or Damaged Spark Plug Wires: High resistance in the wires can lead to misfires. Inspect for cracks, fraying, or damage to the insulation.
* Faulty Ignition Coil: A weak or failing coil won't provide sufficient voltage to the plugs. Testing with a multimeter is recommended.
* Incorrect Spark Plug Gap: Even with new plugs, the gap might not be correctly set. Check and adjust if necessary. Incorrect gap can cause misfires and rough running.
* Ignition Module (Electronic Ignition Control): This component can fail and cause erratic firing. These are prone to failure in older vehicles. Testing is needed to rule this out.
Fuel System Problems:
* Fuel Injectors: Clogged or leaking injectors can cause a rough idle and poor performance. A fuel pressure test and injector cleaning or replacement might be needed.
* Fuel Filter: A clogged filter restricts fuel flow, leading to lean conditions and rough running. Replace it as part of your troubleshooting.
* Fuel Pump: A weak or failing fuel pump may not deliver sufficient fuel pressure. Check fuel pressure.
* Vacuum Leaks: Leaks in the vacuum lines can disrupt the fuel mixture and cause rough running. Inspect all vacuum hoses for cracks, splits, or disconnections.
Other Potential Problems:
* Incorrect Timing: Double-check the timing even after the replacement. A single tooth off can significantly affect performance. Use a timing light for accurate verification.
* Engine Sensors: Various sensors (e.g., MAP sensor, coolant temperature sensor, oxygen sensor) can provide incorrect readings to the engine control unit (ECU), leading to poor fuel mixture and rough running. These require testing with a multimeter or a scan tool.
* Crank Position Sensor (CPS): A faulty CPS will prevent the engine from starting or running smoothly. It tells the ECU where the crankshaft is.
* Exhaust Leaks: A leak before the oxygen sensor can upset the fuel/air mixture.
* Compression Issues: Low compression in one or more cylinders can cause a rough running engine. A compression test should be performed to rule this out.
Troubleshooting Steps:
1. Verify Timing: Absolutely critical after a timing chain replacement.
2. Check for Vacuum Leaks: A visual inspection is relatively quick.
3. Inspect Ignition Components: Cap, rotor, wires, and coil.
4. Test Fuel Pressure: Check that the fuel pump is delivering the correct pressure.
5. Check Fuel Filter: Easy and inexpensive to replace.
6. Perform a Compression Test: To rule out mechanical problems.
If you've already performed a tune-up, many of the initial steps might seem redundant, but a thorough check for even small issues is necessary. If you are uncomfortable working on these systems, taking it to a mechanic might be the safest and most efficient route. Remember to start with the simplest and most probable causes first before moving on to more complex diagnostics.
