Automotive Service Excellence (ASE) A8 Engine PerformanceFuel, Air Induction, and Exhaust Systems Diagnosis and RepairEasy
A technician is performing an exhaust gas analysis on a vehicle. The readings show consistently high levels of hydrocarbons (HC) and carbon monoxide (CO), with low oxygen (O2) levels. The catalytic converter is at operating temperature. What does this indicate about the engine's air-fuel mixture?
- AThe engine is running rich.
- BThe engine is running lean.
- CThe engine is operating at stoichometric conditions.
- DThe catalytic converter is efficient.
Show answer & explanationAnswer & explanation
Correct answer: A. The engine is running rich.
High HC (unburnt fuel) and CO (incomplete combustion) combined with low O2 indicate that there is too much fuel and not enough oxygen for complete combustion, which is characteristic of a rich air-fuel mixture.
Why the other options are wrong
- B. A lean mixture would typically show low HC, low CO, and high O2.
- C. Stoichiometric operation aims for optimal combustion, resulting in low HC, CO, and O2 (ideally around 0.5-1% O2 post-cat).
- D. An efficient catalytic converter would reduce HC and CO levels, even with some engine richness, by converting them to H2O and CO2.
Rich Mixture Exhaust Gas Analysis
Interpreting exhaust gas analyzer readings to identify an engine running with an excess of fuel relative to air.
- High Hydrocarbons (HC) indicate unburnt fuel.
- High Carbon Monoxide (CO) indicates incomplete combustion due to insufficient oxygen.
- Low Oxygen (O2) confirms that available oxygen is consumed by excess fuel.
Memory trick: HC-CO high, O2 low, rich mixture, watch it go!