Why would a 1989 Pontiac Bonneville continue to cut off at stops if the check engine light is on but you have replaced starter battery coilpack spark plugs and wires?

Even with a new battery, coil pack, spark plugs, and wires, a 1989 Pontiac Bonneville cutting off at stops with a check engine light on points to several potential problems, beyond simple ignition. The check engine light is crucial here – you *must* get the codes read. This will narrow down the possibilities significantly.

Here are some likely culprits, even after replacing those parts:

* Crankshaft Position Sensor (CKP Sensor): This sensor tells the computer the engine's rotational speed. A faulty CKP sensor can cause the engine to stall, especially at idle. This is a very common cause of stalling in older vehicles.

* Camshaft Position Sensor (CMP Sensor): Similar to the CKP sensor, but for the camshaft. A faulty CMP sensor will disrupt timing and cause stalling.

* Throttle Position Sensor (TPS): This sensor tells the computer the throttle's position. A faulty TPS can cause erratic idling and stalling.

* Idle Air Control (IAC) Valve: This valve regulates airflow at idle. A dirty or faulty IAC valve can cause stalling, especially at stops.

* Fuel Pump or Fuel Delivery Issues: While less likely given the other replacements, a weak fuel pump or problems with the fuel filter or injectors could cause the engine to cut out when fuel demand is low (like at a stop). The fuel pump might be failing under low pressure conditions.

* Computer/ECU Problems: The engine control unit (ECU or computer) itself could be faulty. This is less common but possible.

* Wiring Harness Issues: A short or corroded wire somewhere in the harness could be causing intermittent problems that only manifest at certain points, like when the engine is idling.

* Alternator: Though you replaced the battery, a failing alternator won't directly cause a stall but could lead to power fluctuations that contribute to the problem, especially if the battery is low.

Why the initial replacements didn't fix it: Replacing the battery, coil, plugs, and wires addresses the *ignition* system. However, the problem could lie elsewhere in the fuel system, the engine management system (including sensors and the ECU), or the vehicle's electrical system.

What you MUST do:

1. Get the diagnostic trouble codes (DTCs) read: This is the single most important step. An auto parts store often offers free code reading services. The codes will tell you exactly what the computer has detected as a problem.

2. Inspect for vacuum leaks: A vacuum leak can disrupt the engine's idle and cause stalling.

3. Check fuel pressure: A fuel pressure gauge can test if the fuel system is delivering adequate fuel.

Ignoring the check engine light is a recipe for further damage. Get the codes read, and you'll be much closer to a solution.