Automotive Service Excellence (ASE) A8 Engine PerformanceEmissions Control Systems Diagnosis and RepairMedium
A vehicle experiences intermittent rough idle and occasional stalling, particularly when coming to a stop. There are no DTCs stored. The technician observes that the engine RPMs drop significantly when the brake pedal is depressed, and then recover. A visual inspection reveals a small, soft vacuum hose connected to the brake booster. What is the most likely cause of these symptoms?
- AA restricted PCV valve
- BA clogged EGR valve
- CA leaking brake booster diaphragm
- DA faulty Idle Air Control (IAC) valve
Show answer & explanationAnswer & explanation
Correct answer: C. A leaking brake booster diaphragm
The key symptom here is the rough idle and stalling when depressing the brake pedal, accompanied by a drop in RPMs. This strongly suggests a vacuum leak that is exacerbated when the brake booster is activated. A leaking brake booster diaphragm would create a significant vacuum leak when the brake pedal is depressed, drawing unmetered air into the engine and causing a lean condition, leading to rough idle or stalling.
Why the other options are wrong
- A. A restricted PCV valve would cause increased crankcase pressure, potentially leading to oil leaks and rough idle, but not typically in response to brake pedal activation.
- B. A clogged EGR valve would typically cause symptoms like pinging/detonation under load, or rough idle if stuck open, but not directly linked to brake pedal actuation.
- D. A faulty IAC valve can cause rough idle or stalling, but typically wouldn't be directly triggered by depressing the brake pedal.
Brake Booster Vacuum Leak
A leak in the brake booster diaphragm or vacuum hose that allows unmetered air into the intake manifold, typically causing engine symptoms when the brake pedal is pressed.
- Symptoms include rough idle, stalling, or RPM drop when braking.
- Caused by a faulty booster diaphragm or damaged vacuum hose.
- Can lead to a lean condition and poor braking performance.
Memory trick: Brake pedal drops RPM? Booster's sucking air!