GM Fuel Systems and Emissions 1 — Questions and Answers
Question 1: What is the primary purpose of the GM Direct Injection (GDI) fuel system compared to port fuel injection?
- Inject fuel directly into the intake manifold at higher pressure
- Inject fuel directly into the combustion chamber at high pressure for improved efficiency (Correct answer)
- Use a carburetor to mix fuel and air before the intake
- Deliver fuel at lower pressure to reduce pump wear
Correct answer: Inject fuel directly into the combustion chamber at high pressure for improved efficiency
GDI injects fuel directly into the combustion chamber at pressures up to 2,900 psi, enabling better fuel atomization, improved thermal efficiency, and reduced emissions.
Question 2: Which GM system monitors fuel trim values to diagnose fuel delivery issues?
- Stabilitrak
- Powertrain Control Module (PCM) (Correct answer)
- Body Control Module (BCM)
- Transmission Control Module (TCM)
Correct answer: Powertrain Control Module (PCM)
The PCM continuously monitors short-term and long-term fuel trim values to adjust the air/fuel mixture and detect deviations that indicate fuel system faults.
Question 3: What does a long-term fuel trim (LTFT) value of +25% most likely indicate on a GM vehicle?
- The engine is running too rich and injectors are over-delivering fuel
- The PCM is adding fuel to compensate for a lean condition (Correct answer)
- Normal adaptive fuel control within acceptable range
- Excessive fuel pressure from a faulty regulator
Correct answer: The PCM is adding fuel to compensate for a lean condition
A LTFT of +25% means the PCM is consistently adding 25% more fuel than the base calibration, indicating a lean condition likely caused by a vacuum leak, low fuel pressure, or faulty injector.
Question 4: On GM vehicles equipped with E85 flex-fuel capability, what sensor determines the ethanol content of the fuel?
- Mass Air Flow (MAF) sensor
- Fuel composition sensor (ethanol content sensor) (Correct answer)
- Oxygen sensor downstream of the catalytic converter
- Fuel pressure sensor in the high-pressure pump
Correct answer: Fuel composition sensor (ethanol content sensor)
The fuel composition sensor measures the dielectric properties of the fuel blend to determine ethanol percentage, allowing the PCM to adjust spark timing and fuel delivery accordingly.
Question 5: What is the function of the Evaporative Emission Control (EVAP) system's purge valve on a GM vehicle?
- Vent fuel tank pressure directly to atmosphere during refueling
- Control the flow of fuel vapors from the charcoal canister into the intake manifold (Correct answer)
- Regulate fuel pressure between the pump and fuel rail
- Seal the fuel tank during EVAP leak detection tests
Correct answer: Control the flow of fuel vapors from the charcoal canister into the intake manifold
The EVAP purge valve opens under PCM command to allow stored fuel vapors from the charcoal canister to be drawn into the intake manifold and burned during engine operation.
Question 6: Which GM diagnostic tool feature allows a technician to perform an EVAP leak test using a nitrogen-based smoke machine connection?
- Fuel Trim Reset function in GDS2
- Enhanced EVAP System Test in GDS2 with the EVAP service port (Correct answer)
- OBD-II Mode $06 live data view
- Fuel injector balance test via Tech2
Correct answer: Enhanced EVAP System Test in GDS2 with the EVAP service port
GDS2 provides an Enhanced EVAP System Test that commands the PCM to seal the system so a smoke machine can be connected to the EVAP service port to locate leaks.
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?
- 14-17 psi
- 35-45 psi (Correct answer)
- 58-72 psi
- 90-110 psi
Correct answer: 35-45 psi
Most GM port-injection systems specify 35-45 psi base fuel pressure with the vacuum line removed from the regulator; values outside this range indicate a pump, regulator, or restriction issue.
GM Fuel Systems and Emissions 1
What is the primary purpose of the GM Direct Injection (GDI) fuel system compared to port fuel injection?