* Vacuum Leak: This is the most common cause. When you press the brakes, the brake booster uses engine vacuum to assist in stopping. A leak in the vacuum lines (running from the intake manifold to the brake booster), a failing brake booster, or a leak in the intake manifold itself can cause a sudden drop in engine vacuum, leading to a stall, especially if the engine is already idling low.
* Low Idle Speed: If your idle speed is already too low, the additional load of the brake booster engaging could be enough to stall the engine. Check your idle speed setting.
* Clogged Fuel Filter: A severely clogged fuel filter restricts fuel flow. The extra demand placed on the engine by the brake booster could exacerbate this, resulting in a stall.
* Failing Ignition System: While less likely to be directly related to brake application, a weak ignition coil, faulty distributor (if equipped), or failing ignition wires could cause a misfire or complete engine shutdown under increased load.
* Alternator Problems: A failing alternator might not provide enough power to the engine when the extra load of the brake booster is added. This is less likely to cause an immediate stall but could lead to progressively worsening performance until a stall occurs.
* Throttle Position Sensor (TPS): A faulty TPS can give the engine's computer incorrect information about throttle position. This could lead to erratic idling and stalling, especially under load.
* Mass Airflow Sensor (MAF): Similar to the TPS, a bad MAF sensor provides inaccurate readings of air intake, which can affect fuel mixture and lead to stalling.
Troubleshooting Steps:
1. Check Vacuum Lines: Visually inspect all vacuum lines running to and from the brake booster and intake manifold for cracks, leaks, or disconnections. Listen for hissing sounds near these lines while the engine is running.
2. Check Idle Speed: Verify your engine's idle speed is within the manufacturer's specifications. Consult your owner's manual.
3. Check Brake Booster: A failing brake booster often manifests as reduced braking power and sometimes stalling upon brake application. A mechanic can test this.
4. Listen for Unusual Noises: Pay attention to any unusual noises coming from the engine compartment when braking, which could indicate a problem with the alternator, ignition system, or other components.
5. Check Fuel Filter: Inspect the fuel filter for visible signs of clogging or restriction. If you're unsure, replace it – it's a relatively inexpensive part.
Important Note: If your vehicle stalls while braking, it poses a significant safety hazard. Do not drive the vehicle until the problem is diagnosed and repaired by a qualified mechanic. A faulty brake booster is a particularly serious issue that needs immediate attention.
