Fuel Delivery System:
* Fuel Pump: A weak fuel pump can't deliver enough fuel at idle or under acceleration. The pressure might be okay when cold, but decrease as the engine warms. This is a strong possibility since the problem worsens after the car has been running. Testing fuel pressure is crucial.
* Fuel Filter: A clogged fuel filter restricts fuel flow, leading to similar symptoms. It's inexpensive and easy to replace, so it's worth checking.
* Throttle Body: Buildup of carbon deposits on the throttle body can disrupt the airflow and cause rough idling and hesitation. Cleaning the throttle body is a relatively simple procedure.
Ignition System (Even though you replaced plugs and wires):
* Ignition Coil: While less likely if the plugs are new, a failing ignition coil can cause misfires, particularly when the engine is under load or has been running for a while.
* Distributor (if applicable): Older Toyotas sometimes use distributors. If yours does, worn components inside could be causing inconsistent spark delivery. This would affect idle and acceleration.
* Crankshaft Position Sensor (CKP) or Cam Position Sensor (CMP): These sensors tell the computer the engine's rotational position, crucial for proper timing and fuel injection. A faulty sensor can cause all sorts of erratic behavior.
Vacuum Leaks:
* Vacuum Hoses: Small cracks or disconnections in vacuum hoses can disrupt the air/fuel mixture, leading to rough idling and hesitation. Inspect all hoses carefully, paying particular attention to those near the intake manifold.
* Intake Manifold Gasket: A leak in the intake manifold gasket can also cause similar problems.
Other Possibilities:
* EGR Valve: The Exhaust Gas Recirculation valve helps reduce emissions. A sticking or faulty EGR valve can cause rough running, especially at idle.
* Mass Air Flow Sensor (MAF): A dirty or faulty MAF sensor provides inaccurate air flow readings to the engine computer, leading to poor fuel delivery.
* Engine Computer (ECU): While less common, a failing ECU can cause intermittent and unpredictable problems. This is usually a last resort diagnosis.
Troubleshooting Steps:
1. Check and replace the fuel filter: This is the cheapest and easiest thing to try.
2. Check fuel pressure: This requires a fuel pressure gauge. Compare your reading to the specification in your repair manual.
3. Inspect vacuum hoses: Look for cracks, disconnections, or loose clamps.
4. Clean the throttle body: Use a throttle body cleaner and follow instructions carefully.
5. Have a mechanic check for diagnostic trouble codes (DTCs): Your car's computer may have stored codes related to the problem.
Important Note: Trying to diagnose this yourself without the proper tools and knowledge can be frustrating and time-consuming. If you're not comfortable working on your car, taking it to a qualified mechanic is strongly recommended. They have the tools and expertise to diagnose and fix the problem efficiently. Describe the symptoms clearly to them—the worsening after the car has run and the hesitation are key pieces of information.
