Automotive Service Excellence (ASE) A8 Engine PerformanceIgnition System Diagnosis and RepairHard
A vehicle owner reports a significant loss of power and poor fuel economy, especially under heavy acceleration. The technician uses a scan tool and finds no misfire codes, but observes that the ignition timing advance is consistently lower than commanded values at higher RPMs and loads. All ignition components (coils, plugs, wires) test good. What could be the MOST likely cause for the reduced ignition timing advance?
- AA malfunctioning knock sensor detecting false knock.
- BExcessive carbon buildup in the combustion chambers.
- CA faulty engine coolant temperature (ECT) sensor reporting cold engine.
- DA defective crankshaft position (CKP) sensor providing an erratic signal.
Show answer & explanationAnswer & explanation
Correct answer: A. A malfunctioning knock sensor detecting false knock.
A knock sensor detects engine knock (pre-ignition or detonation). If it malfunctions and detects 'false knock' or 'phantom knock', the Engine Control Module (ECM) will retard ignition timing to prevent actual damage, even if no real knock is occurring. This constant timing retardation leads to reduced power and fuel economy, especially under load, without necessarily setting misfire codes.
Why the other options are wrong
- B. Excessive carbon buildup can cause pre-ignition and actual knock, leading to timing retardation, but the question implies the issue is with the *sensor* causing the retardation, not the engine condition itself.
- C. A faulty ECT sensor reporting a cold engine would typically make the ECM enrich the fuel mixture, not necessarily retard timing significantly under load.
- D. A defective CKP sensor would likely cause misfires, erratic RPM readings, or a no-start condition, rather than consistently lower-than-commanded timing advance without misfire codes.
False Knock Retard
Ignition timing retardation commanded by the ECM due to a faulty knock sensor incorrectly detecting engine knock, even when actual knock is not present.
- Leads to power loss and reduced fuel economy.
- Often no DTCs related to misfires are set.
- Requires diagnosis of knock sensor and related wiring/ECM.
Memory trick: Knock sensor's false alarm retards timing, stealing power.