GM Fuel Systems and Emissions

Question 1

What is the primary purpose of the GM Direct Injection (GDI) fuel system compared to port fuel injection?

Accepted Answer

Inject fuel directly into the combustion chamber at high pressure for improved efficiency

Answer

Inject fuel directly into the intake manifold at higher pressure

Answer

Use a carburetor to mix fuel and air before the intake

Answer

Deliver fuel at lower pressure to reduce pump wear

Question 2

Which GM system monitors fuel trim values to diagnose fuel delivery issues?

Accepted Answer

Powertrain Control Module (PCM)

Answer

Stabilitrak

Answer

Body Control Module (BCM)

Answer

Transmission Control Module (TCM)

Question 3

What does a long-term fuel trim (LTFT) value of +25% most likely indicate on a GM vehicle?

Accepted Answer

The PCM is adding fuel to compensate for a lean condition

Answer

The engine is running too rich and injectors are over-delivering fuel

Answer

Normal adaptive fuel control within acceptable range

Answer

Excessive fuel pressure from a faulty regulator

Question 4

On GM vehicles equipped with E85 flex-fuel capability, what sensor determines the ethanol content of the fuel?

Accepted Answer

Fuel composition sensor (ethanol content sensor)

Answer

Mass Air Flow (MAF) sensor

Answer

Oxygen sensor downstream of the catalytic converter

Answer

Fuel pressure sensor in the high-pressure pump

Question 5

What is the function of the Evaporative Emission Control (EVAP) system's purge valve on a GM vehicle?

Accepted Answer

Control the flow of fuel vapors from the charcoal canister into the intake manifold

Answer

Vent fuel tank pressure directly to atmosphere during refueling

Answer

Regulate fuel pressure between the pump and fuel rail

Answer

Seal the fuel tank during EVAP leak detection tests

Question 6

Which GM diagnostic tool feature allows a technician to perform an EVAP leak test using a nitrogen-based smoke machine connection?

Accepted Answer

Enhanced EVAP System Test in GDS2 with the EVAP service port

Answer

Fuel Trim Reset function in GDS2

Answer

OBD-II Mode $06 live data view

Answer

Fuel injector balance test via Tech2

Question 7

What GM specification typically defines the minimum acceptable fuel pressure at idle for a port-injected engine with the vacuum line disconnected from the fuel pressure regulator?

Accepted Answer

35-45 psi

Answer

14-17 psi

Answer

58-72 psi

Answer

90-110 psi