CPT CPT Fuel Systems & Engine Performance 2

Question 1

What does injector duty cycle (IDC) percentage represent, and at what approximate IDC should a CPT technician consider upgrading injectors?

Accepted Answer

Percentage of time injectors are open per cycle; upgrade when IDC exceeds 80% to maintain safe headroom

Answer

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?

Accepted Answer

Install a boost-referenced fuel pressure regulator to maintain pressure differential under boost

Answer

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?

Accepted Answer

A worn high-pressure pump cam lobe or damaged pump internals causing inconsistent pumping strokes

Answer

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?

Accepted Answer

An in-tank fuel pump with a return line keeping cool fuel circulating through the rail

Answer

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?

Accepted Answer

The purge valve is introducing fuel vapors that are enriching the mixture, reflected as a fuel pressure increase from reduced injector demand

Answer

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.

CPT - Certified Performance Technician Practice Test

CPT - Certified Performance Technician Practice Test

CPT CPT Fuel Systems & Engine Performance 2