CPT CPT Fuel Systems & Engine Performance 1 — Questions and Answers
Question 1: What is the primary purpose of a fuel pressure regulator in a return-type fuel system?
- To increase fuel flow volume to the injectors
- To maintain a consistent fuel pressure differential across the injectors relative to manifold pressure (Correct answer)
- To filter contaminants from the fuel supply
- To control the injector pulse width
Correct answer: To maintain a consistent fuel pressure differential across the injectors relative to manifold pressure
The fuel pressure regulator maintains a fixed pressure drop across injectors so fuel delivery remains consistent regardless of manifold vacuum changes.
Question 2: A technician measures fuel pressure that drops more than 5 psi within 5 minutes after the engine is shut off. What is the most likely cause?
- Leaking fuel injector(s) or a faulty check valve in the fuel pump (Correct answer)
- A blocked fuel filter
- A clogged fuel return line
- Normal fuel system bleed-down
Correct answer: Leaking fuel injector(s) or a faulty check valve in the fuel pump
Rapid post-shutdown fuel pressure drop indicates fuel is leaking back past either leaky injectors or a failed check valve in the fuel pump.
Question 3: When upgrading fuel injectors on a performance engine, which parameter MUST be updated in the ECU calibration to maintain correct fuel delivery?
- Injector dead time (latency) and flow rate scaling (Correct answer)
- Ignition coil dwell time
- Throttle body bore size compensation
- EGR valve duty cycle
Correct answer: Injector dead time (latency) and flow rate scaling
Injector dead time (the delay before the injector opens) and the injector's flow rate constant must be updated so the ECU calculates correct pulse widths.
Question 4: On a port-injected engine, what symptom would result from a fuel injector with a stuck-open condition on one cylinder?
- A rich misfire on that cylinder, fouled spark plug, and possible hydraulic lock if severe (Correct answer)
- A lean misfire with a clean spark plug
- Increased fuel economy on that cylinder
- No noticeable change in performance
Correct answer: A rich misfire on that cylinder, fouled spark plug, and possible hydraulic lock if severe
A stuck-open injector continuously delivers fuel, causing a rich misfire, fuel-fouled spark plug, and risk of hydraulic lock if liquid fuel fills the cylinder.
Question 5: What is the purpose of ethanol content sensor data in a flex-fuel vehicle's performance calibration?
- To allow the ECU to adjust fuel delivery and ignition timing based on ethanol percentage for optimal performance (Correct answer)
- To control the transmission shift points
- To monitor fuel temperature for vapor lock prevention
- To measure fuel filter restriction
Correct answer: To allow the ECU to adjust fuel delivery and ignition timing based on ethanol percentage for optimal performance
Ethanol has different stoichiometric ratios and higher octane than gasoline, requiring the ECU to enrich fueling and advance timing as ethanol content increases.
Question 6: A direct injection (GDI) engine exhibits carbon buildup on intake valves despite proper fueling. Why does this occur and how is it addressed?
- Fuel injected directly into the cylinder bypasses the intake valves, so PCV oil mist deposits accumulate; addressed with catch cans or walnut blasting (Correct answer)
- Excessive rich mixture washes the valves with raw fuel
- Intake manifold runners are too long, trapping carbon
- Incorrect spark plug heat range causing incomplete combustion
Correct answer: Fuel injected directly into the cylinder bypasses the intake valves, so PCV oil mist deposits accumulate; addressed with catch cans or walnut blasting
GDI fuel never washes the intake valves, so PCV blow-by deposits accumulate over time; solutions include oil catch cans or periodic walnut shell blasting.
CPT CPT Fuel Systems & Engine Performance 1
What is the primary purpose of a fuel pressure regulator in a return-type fuel system?