A vehicle owner reports that the engine occasionally stalls at idle after driving for about 15-20 minutes. There are no Diagnostic Trouble Codes (DTCs) stored. The technician observes that the fuel pump runs continuously when the key is ON, even before cranking, and does not shut off after a few seconds as it should. What is the MOST likely cause of the stalling at idle?
- AA defective engine coolant temperature (ECT) sensor reporting cold
- BA leaking fuel injector
- CA faulty Crankshaft Position (CKP) sensor
- DA faulty fuel pump relay that is stuck closed
Show answer & explanationAnswer & explanation
Correct answer: C. A faulty Crankshaft Position (CKP) sensor
The fuel pump running continuously with the key ON, even before cranking and not shutting off, is a critical clue. This behavior is often a 'limp home' or default strategy implemented by the PCM when it does NOT receive a valid engine speed signal (from the CKP sensor) but assumes a problem with the fuel pump control. The PCM typically runs the fuel pump for 2-3 seconds and then shuts it off if no engine RPM signal is detected. If it runs continuously, it's often because the PCM isn't seeing the CKP signal, but is forced to keep the pump on. The intermittent stalling at idle after 15-20 minutes points to an intermittent CKP sensor failure, which can be heat-related. Without a reliable CKP signal, the PCM loses track of engine position and speed, leading to stalling.
Why the other options are wrong
- A. A defective ECT sensor reporting cold would cause a rich running condition, poor fuel economy, and possibly rough idle, but it wouldn't explain the continuous fuel pump operation or the specific stalling behavior linked to the CKP sensor.
- B. A leaking fuel injector would cause a rich condition, hard starting (especially hot), and possibly misfires, but not typically a fuel pump running continuously before crank or stalling specifically linked to the pump behavior.
- D. A faulty fuel pump relay stuck closed would cause the fuel pump to run continuously, but it wouldn't directly cause a stalling issue unless it also prevented proper fuel pressure regulation. More importantly, the continuous pump operation is often a symptom of a deeper PCM issue related to engine speed signal, not just a stuck relay.
Fuel Pump Priming Logic
The PCM typically activates the fuel pump for a few seconds when the ignition is turned ON (key ON, engine OFF) to prime the system, then deactivates it if no engine RPM signal is detected.
- Pump should run briefly (2-3 seconds) then stop if engine isn't cranking/running.
- Continuous pump operation without cranking can indicate a fault in the engine speed sensor (CKP).
- This is a diagnostic clue for PCM's perception of engine operation.
Memory trick: If the 'PUMP' won't 'STOP', and the engine 'DROPS', the 'CKP' sensor is at the top.