1. Fuel Delivery Issues (Beyond the Pump & Regulator):
* Fuel Filter: A clogged fuel filter restricts fuel flow, especially under higher demands (acceleration). This is a cheap and easy check.
* Fuel Injectors: Clogged or failing injectors might not deliver enough fuel at higher RPMs. They'll need testing (flow test or dynamic resistance test) to diagnose.
* Fuel Pressure (under load): You've checked pressure at idle, but the pressure needs to maintain itself under load. A pressure gauge that can measure fuel pressure *while* accelerating is needed here. A pressure *drop* during acceleration points to fuel delivery issues.
2. Ignition System Issues (Despite replacing components):
* Distributor Cap and Rotor: Even with a new distributor, these parts can degrade over time. Inspect them for cracks, carbon tracking, or wear.
* Worn or Damaged Ignition Components (Internal): While you've replaced the coil and distributor, internal components within the distributor itself *could* be failing under stress (high RPMs). This is less likely but possible.
* Timing Belt/Chain: Incorrect timing will cause a significant stumble, especially at higher RPMs. Verify the timing belt/chain is correctly installed and hasn't skipped a tooth. This requires experience and the correct tools.
3. Air Intake System Issues:
* Air Flow Meter (AFM) / Mass Air Flow Sensor (MAF): If your car has an AFM (older models) or MAF sensor, a dirty or failing sensor will provide incorrect readings to the computer, leading to a lean fuel mixture and stumbling at higher RPMs. Clean it (AFM) or replace (MAF) it if necessary.
* Vacuum Leaks: Any leaks in the vacuum lines can disrupt fuel/air mixture. A thorough inspection of all vacuum lines and connections is critical. Use a vacuum gauge or smoke test to detect leaks.
* Catalytic Converter: A severely clogged catalytic converter can restrict exhaust flow, leading to back pressure and a stumble, especially during acceleration.
4. Engine Management System (ECU):
* ECU Issues (Less Likely): While less probable, a malfunctioning ECU could be causing the problem. This usually requires professional diagnosis with a scan tool.
Troubleshooting Steps:
1. Check Fuel Filter: This is the easiest and cheapest first step.
2. Inspect Distributor Cap and Rotor: Look for any signs of damage or wear.
3. Check for Vacuum Leaks: Carefully inspect all vacuum lines and connections. A smoke test is highly recommended.
4. Test Fuel Pressure Under Load: This is crucial to rule out fuel delivery issues during acceleration.
5. Clean or Replace AFM/MAF Sensor: This is a relatively straightforward procedure.
6. Consider a Professional Diagnosis: If the above steps don't solve the problem, a professional mechanic with a scan tool can diagnose problems within the engine management system. They can also perform more advanced diagnostics, such as fuel injector testing and dynamic pressure testing.
Remember safety first! Always disconnect the battery negative terminal before working on any electrical components. If you're not comfortable performing these checks yourself, it's best to consult a qualified mechanic. The symptoms point to something beyond the components you've already replaced, and a professional can help pinpoint the problem efficiently.
