CPT CPT Fuel Systems & Engine Performance 2 — Questions and Answers
Question 1: What does injector duty cycle (IDC) percentage represent, and at what approximate IDC should a CPT technician consider upgrading injectors?
- Percentage of time injectors are open per cycle; upgrade when IDC exceeds 80% to maintain safe headroom (Correct answer)
- Total fuel volume per minute; upgrade at 50% IDC
- Fuel pressure differential; upgrade at 100% IDC
- Pulse width in milliseconds; upgrade at 60% IDC
Correct answer: Percentage of time injectors are open per cycle; upgrade when IDC exceeds 80% to maintain safe headroom
IDC is the percentage of engine cycle time the injector is open; above 80% leaves insufficient headroom for enrichment, requiring larger injectors.
Question 2: A turbocharged engine with a return-less fuel system shows fuel pressure drop at high boost levels. What modification would a CPT technician recommend first?
- Install a boost-referenced fuel pressure regulator to maintain pressure differential under boost (Correct answer)
- Increase injector pulse width through ECU tuning only
- Add a second fuel filter
- Lower the boost pressure target
Correct answer: Install a boost-referenced fuel pressure regulator to maintain pressure differential under boost
A boost-referenced FPR increases fuel rail pressure proportionally with boost pressure, maintaining the correct pressure differential across injectors at high boost.
Question 3: When testing a high-pressure GDI fuel pump, a technician measures fuel pressure at idle that fluctuates rapidly between 500 and 2,500 psi. What is the most likely cause?
- A worn high-pressure pump cam lobe or damaged pump internals causing inconsistent pumping strokes (Correct answer)
- A faulty low-pressure fuel pump
- An ECU calibration error in the fuel demand map
- A leaking high-pressure fuel injector
Correct answer: A worn high-pressure pump cam lobe or damaged pump internals causing inconsistent pumping strokes
GDI high-pressure pump pressure should be stable; rapid fluctuation indicates the pump cannot maintain consistent displacement due to internal or cam lobe wear.
Question 4: Which fuel system component prevents vapor lock in a high-performance fuel system operating in hot conditions?
- An in-tank fuel pump with a return line keeping cool fuel circulating through the rail (Correct answer)
- A larger fuel filter
- Braided stainless fuel lines
- An inline fuel pressure gauge
Correct answer: An in-tank fuel pump with a return line keeping cool fuel circulating through the rail
A return-type fuel system with an in-tank pump circulates cool fuel continuously, preventing heat soak and vapor lock in the fuel rail.
Question 5: A performance technician notices that fuel pressure rises above spec when the EVAP purge valve opens at light throttle. What does this indicate?
- The purge valve is introducing fuel vapors that are enriching the mixture, reflected as a fuel pressure increase from reduced injector demand (Correct answer)
- A stuck-closed fuel pressure regulator
- A faulty MAF sensor reading high
- Normal EVAP system operation
Correct answer: The purge valve is introducing fuel vapors that are enriching the mixture, reflected as a fuel pressure increase from reduced injector demand
When EVAP vapors enter the intake, the ECU reduces injector pulse width to compensate, causing fuel pressure to rise as less fuel flows through the rail.
Question 6: What is the purpose of a fuel accumulator in some return-type fuel systems?
- To dampen fuel pressure pulsations from the pump and maintain rail pressure briefly after shutdown (Correct answer)
- To increase total fuel volume capacity
- To filter water from the fuel
- To regulate maximum fuel pressure during boost conditions
Correct answer: To dampen fuel pressure pulsations from the pump and maintain rail pressure briefly after shutdown
A fuel accumulator uses a spring-loaded diaphragm to absorb pump pressure pulses and maintain residual rail pressure briefly after the pump stops.
CPT CPT Fuel Systems & Engine Performance 2
What does injector duty cycle (IDC) percentage represent, and at what approximate IDC should a CPT technician consider upgrading injectors?