Fuel System Issues:
* Fuel Injectors: Worn or clogged injectors can cause a rough running condition, especially under load. They might be delivering inconsistent fuel amounts at higher demands. This is a very common issue with older vehicles.
* Fuel Pump: A weak fuel pump might not be able to supply enough fuel at higher throttle positions. Check fuel pressure. Low fuel pressure at higher RPM is a strong indicator.
* Fuel Filter: A clogged fuel filter restricts fuel flow, leading to similar symptoms. This is a cheap and easy part to replace, so do it first.
* Fuel Pressure Regulator: This regulates fuel pressure in the system. A faulty regulator can lead to inconsistent fuel pressure.
Ignition System Issues:
* Worn Spark Plugs and Wires: Old spark plugs and wires can misfire under load, resulting in rough running. Check the gap on the plugs and replace if necessary. Look for cracks or damage on the wires.
* Ignition Coil(s): A failing ignition coil can cause misfires, especially under stress. These often require specialized testing to diagnose.
* Distributor Cap and Rotor (if applicable): If your engine has a distributor, inspect the cap and rotor for cracks, carbon buildup, or wear.
Air Intake System:
* Mass Airflow Sensor (MAF): A dirty or faulty MAF sensor will provide incorrect air/fuel mixture readings to the computer, particularly noticeable under load. Cleaning it (with MAF sensor cleaner) is often worth a try.
* Vacuum Leaks: Leaks in the vacuum lines can disrupt the engine's timing and air/fuel mixture, causing rough running. Inspect all vacuum hoses for cracks or disconnections.
Other Possibilities:
* Catalytic Converter: A clogged catalytic converter can restrict exhaust flow, causing backpressure and affecting engine performance, especially under load.
* Oxygen Sensor(s): A faulty oxygen sensor can result in a poor air/fuel mixture, although this is more likely to cause a persistent rough running condition, not one that's only present under load.
Troubleshooting Steps:
1. Check for trouble codes: Use an OBD-II scanner to read any diagnostic trouble codes (DTCs) stored in the engine control module (ECM). This will often point you towards the most likely culprit.
2. Fuel pressure test: Measure fuel pressure at the fuel rail at idle and at higher RPMs. This is crucial for ruling out fuel delivery problems.
3. Visual Inspection: Carefully inspect all hoses, wires, and components for obvious damage.
4. Check spark: Use a spark tester or timing light to check spark at each cylinder, especially under load.
5. Start with the simple and cheap: Change the fuel filter, check and clean spark plugs and wires, and inspect vacuum lines. These are low-effort solutions that often solve the problem.
If you're not comfortable performing these checks yourself, take it to a mechanic. Providing them with the information you've already investigated (like the TPS replacement) will help them diagnose the problem more efficiently. Be sure to mention the specific condition: rough running only at half or full throttle.
