ASE Fuel Systems & Emissions Control 2 — Questions and Answers
Question 1: What does a three-way catalytic converter simultaneously reduce?
- HC, CO, and NOx emissions (Correct answer)
- Only carbon monoxide (CO) via oxidation
- Diesel particulate matter and sulfur compounds
- CO2 and water vapor from combustion
Correct answer: HC, CO, and NOx emissions
A three-way catalyst uses oxidation (platinum/palladium) to convert HC and CO into CO2 and H2O, while a reduction catalyst (rhodium) breaks NOx down into harmless N2 and O2.
Question 2: A Long Term Fuel Trim (LTFT) reading of +18% most likely indicates:
- A lean condition — the ECM is adding fuel to compensate (Correct answer)
- A rich condition — the ECM is reducing fuel delivery
- Fuel pressure is too high at the injector rail
- The fuel injectors are oversized for the engine displacement
Correct answer: A lean condition — the ECM is adding fuel to compensate
Positive fuel trim means the ECM is commanding more fuel than the base calibration calls for, indicating a lean condition caused by a vacuum leak, weak fuel pump, dirty injectors, or a faulty MAF sensor.
Question 3: What is the purpose of the PCV (Positive Crankcase Ventilation) valve?
- Vent blow-by gases from the crankcase into the intake manifold for combustion (Correct answer)
- Regulate engine oil pressure by controlling crankcase venting
- Prevent exhaust backpressure from entering the crankcase
- Control the volume of oil vapor entering the air filter
Correct answer: Vent blow-by gases from the crankcase into the intake manifold for combustion
The PCV valve meters crankcase blow-by gases (containing unburned hydrocarbons and combustion byproducts) into the intake manifold where they are burned, preventing their release as pollutants.
Question 4: A Long Term Fuel Trim (LTFT) reading of -16% most likely indicates:
- A rich condition — the ECM is reducing fuel to compensate (Correct answer)
- A lean condition — the engine needs additional fuel delivery
- Fuel pressure is below specification at idle
- An EGR valve that is stuck in the open position
Correct answer: A rich condition — the ECM is reducing fuel to compensate
Negative fuel trim means the ECM is cutting fuel delivery below the base calibration, indicating a rich condition caused by leaking injectors, excessively high fuel pressure, or a MAF sensor reading too high.
Question 5: Which OBD-II readiness monitor specifically evaluates the EVAP system for leaks?
- EVAP monitor (Correct answer)
- Catalyst efficiency monitor
- Oxygen sensor response monitor
- Fuel system lean/rich monitor
Correct answer: EVAP monitor
The EVAP monitor performs sealed-system pressure and vacuum tests under specific drive-cycle conditions to detect leaks as small as 0.020 inches, setting a DTC if the system fails to hold.
Question 6: How does GDI (Gasoline Direct Injection) differ from multi-port fuel injection (MPFI)?
- GDI injects fuel directly into the combustion chamber at high pressure (Correct answer)
- GDI injects fuel upstream of the intake valve into the port
- GDI uses lower fuel pressure than conventional port injection
- GDI relies on intake vacuum to draw fuel through a carburetor jet
Correct answer: GDI injects fuel directly into the combustion chamber at high pressure
GDI injectors deliver fuel directly into the combustion chamber at pressures of 1,500–3,600 psi, enabling precise mixture control, improved fuel economy, and higher compression ratios compared to port injection.
Question 7: What is the function of the fuel pressure regulator in a return-style fuel system?
- Maintain constant pressure at the injectors by returning excess fuel to the tank (Correct answer)
- Boost fuel pressure above the pump's output pressure rating
- Filter contaminants from fuel before it reaches the injectors
- Control pulse width duration for each individual fuel injector
Correct answer: Maintain constant pressure at the injectors by returning excess fuel to the tank
The fuel pressure regulator maintains steady injector rail pressure (typically 35–65 psi) by opening a bypass valve that returns surplus fuel to the tank, ensuring accurate fuel metering at all engine loads.
ASE Fuel Systems & Emissions Control 2
What does a three-way catalytic converter simultaneously reduce?