* Worn or failing ignition components (despite new cap and rotor): While you replaced the cap and rotor, other ignition components could be the culprit:
* Worn spark plug wires: These degrade over time, especially the insulation. High voltage can leak to the engine block, causing misfires, especially under load. Inspect them closely for cracks or damage. A simple visual inspection isn't enough; a spark tester is recommended.
* Failing ignition coil: This provides the high voltage for the spark plugs. A weak coil struggles to deliver enough voltage at higher RPMs.
* Worn or fouled spark plugs: Even new plugs can fail quickly if there's another underlying problem. Check their gap and condition.
* Distributor issues (beyond cap and rotor): The distributor's mechanical components, like the points (if it has them) or the electronic module, could be wearing out.
* Fuel delivery problems (despite new fuel filter):
* Clogged fuel injectors: A clogged injector won't deliver enough fuel to a cylinder, causing a misfire. This is a common problem and often requires cleaning or replacement.
* Fuel pump issues: The fuel pump might be weakening, unable to supply enough pressure at higher speeds. Check fuel pressure.
* Vacuum leaks: Leaks in the vacuum system can disrupt fuel/air mixture, leading to misfires. Inspect all vacuum lines carefully.
* Engine mechanical issues:
* Worn valve lifters or rocker arms: These can cause inconsistent valve timing, leading to misfires.
* Low compression in one or more cylinders: Worn piston rings or valve problems can cause this. A compression test is essential to rule this out.
* Camshaft issues: A worn camshaft can affect valve timing and cause misfires.
Troubleshooting Steps:
1. Check spark plug wires: Visually inspect for cracks and use a spark tester to check for strong, consistent sparks at each plug.
2. Inspect spark plugs: Check their gap and condition. Replace if necessary.
3. Test fuel pressure: This requires a fuel pressure gauge to ensure adequate pressure at idle and under acceleration.
4. Listen for vacuum leaks: Use a propane torch (carefully!) around vacuum lines to detect hissing sounds.
5. Perform a compression test: This will check the compression in each cylinder to identify potential mechanical problems.
The progression of the problem suggests it's worsening under load, implying a component that's struggling to keep up with demand. Start with the simpler checks (wires, plugs, vacuum) and proceed to more involved diagnostics (fuel pressure, compression test) as needed. If you're not comfortable working on the engine yourself, take it to a qualified mechanic for diagnosis and repair.
