* Fuel Delivery Problems:
* Fuel Pump: A weak fuel pump might supply enough fuel for idle, but struggle to keep up with the increased demand under load. The pump may be failing, clogged, or simply not providing sufficient pressure.
* Fuel Filter: A clogged fuel filter restricts fuel flow, causing similar problems to a weak fuel pump.
* Fuel Injectors (if fuel injected): Clogged or malfunctioning injectors might deliver enough fuel at idle but fail to provide adequate fuel at higher RPMs. This could be due to dirt, wear, or electrical issues. If it's a carburetor, similar problems may occur with the carburetor jets and their passages.
* Fuel Pressure Regulator (if fuel injected): This regulates the fuel pressure. A malfunctioning regulator can cause erratic fuel delivery.
* Ignition System Problems:
* Distributor Cap and Rotor (if distributor ignition): Worn or cracked components can cause poor spark distribution at higher RPMs when the distributor spins faster.
* Ignition Coil: A weak coil might provide enough spark at idle but not have the power needed under load.
* Spark Plugs and Wires: Worn or damaged spark plugs or wires can lead to misfires, especially under stress. This is a very common cause.
* Ignition Module (if electronic ignition): The module controls the spark; a faulty module can prevent proper spark delivery.
* Other Potential Issues:
* Catalytic Converter: A severely clogged catalytic converter can restrict exhaust flow, causing the engine to struggle under load.
* Vacuum Leaks: Leaks in the vacuum system can disrupt the engine's timing or fuel delivery, especially at higher RPMs.
* Transmission Issues (Automatic): While less likely to be the *primary* cause of the problem, a slipping transmission or torque converter can cause the engine to rev without the corresponding increase in speed, making it seem like the engine is struggling.
* Intake Manifold Gasket Leak: A vacuum leak that gets worse under load can cause a similar symptom.
Troubleshooting Steps:
1. Check the basics: Start by inspecting spark plugs, wires, distributor cap and rotor (if applicable), and fuel filter. Look for obvious signs of wear or damage.
2. Listen for unusual noises: Listen carefully to the engine while it's running at idle and under load. Unusual sounds can point towards specific problems.
3. Check fuel pressure: If you have a fuel pressure gauge, measure the fuel pressure at idle and under load. A significant drop under load indicates a fuel delivery problem.
4. Test the ignition system: You might need a multimeter or a specialized ignition tester to diagnose problems with the coil, ignition module, or other components.
Without more information, it's impossible to pinpoint the exact cause. Systematic checking of the above areas is necessary to find the faulty component. If you're not comfortable working on your car yourself, take it to a qualified mechanic.
