* Fuel Pump Failure: A failing fuel pump can't supply enough fuel to the engine, leading to a sudden stall, especially under load (like driving uphill or at freeway speeds). This is a common cause of sudden shutdowns.
* Clogged Fuel Filter: A severely clogged fuel filter restricts fuel flow, mirroring the symptoms of a failing fuel pump.
* Crankshaft Position Sensor (CKP) Failure: This sensor tells the engine's computer the position of the crankshaft. A faulty CKP sensor prevents the engine from receiving the signals it needs to ignite the fuel, leading to a stall.
* Cam Position Sensor (CMP) Failure: Similar to the CKP sensor, a bad CMP sensor affects the engine's timing and can cause a stall.
* Mass Airflow Sensor (MAF) Sensor Failure: The MAF sensor measures the amount of air entering the engine. A faulty sensor can provide inaccurate readings, leading to a lean or rich fuel mixture and a potential stall.
* Ignition System Problems: Issues with the ignition coil, distributor (if equipped), spark plugs, or ignition wires can prevent proper spark, resulting in engine shutdown.
* Alternator Failure: While less likely to cause an *immediate* shutdown, a failing alternator can suddenly stop charging the battery. If the battery is already low, this can cause the engine to cut out.
* Low Fuel: While seemingly obvious, running out of gas is a possibility.
* Computer Problems (ECM/PCM): The engine control module (ECM) or powertrain control module (PCM) could be malfunctioning, leading to erratic engine behavior and stalling.
* Throttle Position Sensor (TPS) Failure: An inaccurate reading from the TPS can cause the engine to shut down.
Troubleshooting Steps (Don't attempt complex repairs yourself unless experienced):
1. Check the obvious: Is the fuel gauge showing empty?
2. Listen for the fuel pump: When you turn the key to the "on" position (without starting), you should hear a whirring sound from the fuel pump (usually near the fuel tank). If not, it's a strong suspect.
3. Check the battery: Is the battery terminal connections secure and clean? Have the battery tested at an auto parts store.
4. Have the car scanned for diagnostic trouble codes (DTCs): A mechanic or auto parts store can plug a scan tool into the car's OBD-II port to retrieve any diagnostic trouble codes stored by the car's computer. These codes will point towards specific potential problems.
Important Note: It's dangerous to try to diagnose and repair this type of problem yourself, especially on a freeway. A sudden stall on the freeway can cause a serious accident. Have your 1998 Toyota Avalon towed to a qualified mechanic for diagnosis and repair. The DTCs are crucial for efficiently pinpointing the issue.
