High-Probability Issues:
* Ignition System Issues (Even After Tune-Up): A tune-up might replace *some* ignition components, but not necessarily all.
* Ignition Control Module (ICM): These are notorious for failing when they get hot. A failing ICM often causes misfires when the engine reaches operating temperature. This is a prime suspect given your description.
* Ignition Coil(s): Similar to the ICM, coils can weaken as they heat up, leading to inconsistent spark. Test each coil individually, or better yet, replace them since they are relatively inexpensive compared to the time spent troubleshooting.
* Worn Distributor Cap and Rotor (if applicable): Even new parts can be faulty. Cracks or carbon buildup can impede spark delivery, especially under stress. Inspect these closely for any issues.
* Spark Plug Wires: Check for cracks, fraying, or poor connections. Even a small crack can lead to misfires when the engine is hot and the wires expand slightly.
* Fuel Delivery Issues:
* Fuel Injectors: A sticking or partially clogged injector might function fine when cold but fail to deliver fuel consistently when hot. A fuel injector cleaning or replacement might be necessary.
* Fuel Pressure Regulator: This could be failing to maintain proper fuel pressure as the engine heats up. A pressure test is necessary to diagnose this.
Medium-Probability Issues:
* EGR Valve: A faulty EGR (Exhaust Gas Recirculation) valve can cause misfires, particularly when the engine is warm. Cleaning or replacing it might help. However, this is less likely if the problem only occurs under light load.
* Vacuum Leaks: Small vacuum leaks might only become significant once the engine components heat up and expand slightly. A thorough inspection of all vacuum lines is necessary.
* Computer Issues (PCM): While less common, a failing PCM could contribute to intermittent misfires that manifest more prominently when the engine is hot. This is usually a last resort diagnostic step and should only be investigated after other possibilities are ruled out.
Troubleshooting Steps:
1. Double-check the tune-up: Make absolutely certain all the spark plugs, wires, cap, rotor (if applicable), and air filter are new and correctly installed.
2. Check for diagnostic trouble codes (DTCs): Use an OBD-II scanner (even a cheap one from an auto parts store) to read any codes stored in the PCM. These codes often pinpoint the problem area.
3. Focus on the ignition system: This is the most likely culprit given your symptoms. Start by visually inspecting all components. Then, consider testing the ICM and coils. A professional mechanic can perform more advanced testing.
4. Perform a fuel pressure test: This will rule out fuel delivery issues.
5. Check for vacuum leaks: Use a vacuum gauge or listen for hissing sounds around vacuum lines and hoses.
6. Clean the EGR valve: It's a relatively easy and inexpensive process that can sometimes solve the problem.
Important Note: If you are not comfortable working on your vehicle's electrical and fuel systems, it is best to take it to a qualified mechanic. Improper diagnosis and repair can lead to further damage or injury. The ICM is a particularly good starting point for professional diagnosis due to its common failure mode in vehicles of this age.
