ASE A1 Engine RepairGeneral Engine DiagnosisMedium
A technician is diagnosing an engine that repeatedly stalls shortly after starting, especially when cold, but eventually runs normally once warm. A scan tool shows no active Diagnostic Trouble Codes (DTCs). The technician suspects an issue with a sensor that provides crucial information for cold engine operation. Which sensor is MOST likely at fault?
- AKnock Sensor (KS)
- BEngine Coolant Temperature (ECT) sensor
- CThrottle Position Sensor (TPS)
- DVehicle Speed Sensor (VSS)
Show answer & explanationAnswer & explanation
Correct answer: B. Engine Coolant Temperature (ECT) sensor
The ECT sensor provides critical cold engine temperature data to the PCM for proper fuel enrichment and idle control. If it sends an incorrect 'warm' signal when the engine is actually cold, the engine will run too lean and stall until it genuinely warms up.
Why the other options are wrong
- A. A knock sensor detects engine knock and signals the PCM to retard timing, which would affect power but not typically cause repeated cold-start stalling that resolves when warm.
- C. A faulty TPS would affect throttle response and idle stability at various temperatures, but is less specific to cold-start stalling that resolves when warm.
- D. A VSS primarily affects speedometer readings, cruise control, and transmission shift points, not cold-start engine stalling.
Engine Coolant Temperature (ECT) Sensor
The ECT sensor measures the temperature of the engine coolant and sends this data to the Powertrain Control Module (PCM) to adjust fuel mixture, ignition timing, and cooling fan operation.
- Crucial for cold starts, providing fuel enrichment data.
- A faulty sensor can cause rich or lean conditions, affecting idle and starting.
- Often a two-wire thermistor, resistance changes with temperature.
Memory trick: Cold start stall? Check the temp, or it won't jump and romp.