Automotive Service Excellence (ASE) A8 Engine PerformanceEmissions Control Systems Diagnosis and RepairHard
A technician is diagnosing a vehicle with a fluctuating idle, especially after the engine warms up, and a 'Check Engine' light with DTC P0505 (Idle Control System Malfunction). The PCV valve was recently replaced. Inspection reveals that the PCV hose connecting to the intake manifold is collapsed and brittle. What is the MOST likely effect of this condition on engine operation?
- AInconsistent airflow through the PCV system, leading to an unstable idle.
- BIncreased flow of blow-by gases to the intake, causing a rich condition.
- CReduced vacuum in the intake manifold, leading to a lean condition.
- DExcessive crankcase pressure, causing oil leaks.
Show answer & explanationAnswer & explanation
Correct answer: A. Inconsistent airflow through the PCV system, leading to an unstable idle.
A collapsed or brittle PCV hose would restrict or intermittently block the flow of crankcase vapors into the intake manifold. This causes inconsistent vacuum regulation and unmetered air/vapor flow through the PCV system, directly leading to an unstable or fluctuating idle as the idle control system tries to compensate.
Why the other options are wrong
- B. A restricted PCV hose would reduce, not increase, the flow of blow-by gases to the intake, making a rich condition less likely from this specific fault.
- C. A restricted PCV hose would reduce the amount of air/vapor drawn from the crankcase, but its primary effect on intake manifold vacuum would be localized to the PCV port, leading to inconsistent airflow rather than a general lean condition.
- D. While a completely blocked PCV system can cause excessive crankcase pressure, a collapsed hose would primarily affect flow, leading to idle instability before major oil leaks.
PCV Hose Restriction
A restricted PCV hose (collapsed/brittle) limits the flow of crankcase vapors, leading to unstable idle, poor crankcase ventilation, and potential oil contamination.
- Affects idle stability due to inconsistent airflow.
- Can lead to P0505 (Idle Control System Malfunction).
- Prevents proper removal of blow-by gases.
Memory trick: Collapsed PCV hose? Engine can't breathe, idle bounces like a crazy bee!