Automotive Service Excellence (ASE) A8 Engine PerformanceComputerized Engine Controls Diagnosis and RepairHard
A vehicle owner reports that their vehicle occasionally stalls when coming to a stop, especially after extended highway driving. There are no DTCs stored. The technician observes that the engine RPM drops significantly below normal idle speed just before stalling. Which of the following is the most likely cause?
- AA faulty engine coolant temperature (ECT) sensor.
- BAn intermittent vacuum leak.
- CA sticking exhaust gas recirculation (EGR) valve.
- DA clogged positive crankcase ventilation (PCV) valve.
Show answer & explanationAnswer & explanation
Correct answer: C. A sticking exhaust gas recirculation (EGR) valve.
A sticking EGR valve, particularly one that is stuck open intermittently, will allow exhaust gases to enter the intake manifold at idle or low RPMs when it should be closed. This dilutes the incoming air/fuel mixture, causing a lean condition, rough idle, and often stalling, especially when decelerating or coming to a stop.
Why the other options are wrong
- A. A faulty ECT sensor would primarily affect fuel enrichment or cooling fan operation, not typically cause stalling at stops without DTCs or a significant drop in RPM.
- B. An intermittent vacuum leak would generally cause a high or erratic idle, as uncontrolled air enters the engine, not a significant drop below normal idle leading to stalling.
- D. A clogged PCV valve would typically lead to increased crankcase pressure, possibly oil leaks, or rough idle due to unmetered air, but is less likely to cause specific stalling at stops with a significant RPM drop than a sticking EGR.
EGR Valve Sticking Open
An EGR (Exhaust Gas Recirculation) valve that sticks open allows exhaust gases to flow into the intake manifold at inappropriate times, such as idle or low RPMs, leading to engine performance issues.
- EGR should be closed at idle and wide-open throttle.
- Stuck open EGR dilutes air/fuel mixture, causing lean condition.
- Symptoms: rough idle, stalling, hesitation, poor fuel economy.
- Often no DTCs if the valve is only partially stuck open or intermittent.
Memory trick: EGR stuck open, engine chokes, at a stop it croaks.