GM Fuel Systems and Emissions 3

Question 1

What pressure does GM specify for the high-pressure fuel pump outlet on a typical GDI engine during full-load operation?

Accepted Answer

1,000-2,900 psi

Answer

GM GDI high-pressure fuel systems operate at 1,000-2,900 psi (approximately 70-200 bar) at the fuel rail during full load to achieve fine atomization directly in the cylinder.

Question 2

Which GM scan tool function allows a technician to perform a fuel injector balance test to identify a faulty injector?

Accepted Answer

Injector Balance Test under Special Functions in GDS2

Answer

GDS2's Injector Balance Test fires each injector individually and monitors RPM drop or fuel trim response to identify injectors that are restricted, leaking, or not performing uniformly.

Question 3

What is the function of the charcoal canister in a GM EVAP system?

Accepted Answer

Adsorb and store fuel vapor from the fuel tank until the engine can purge it

Answer

The charcoal (activated carbon) canister adsorbs hydrocarbon fuel vapors venting from the fuel tank and stores them until engine vacuum and the purge valve draw them into the intake to be burned.

Question 4

When performing an EVAP system small leak test on a GM vehicle, what leak rate defines a 'small leak' per OBD-II standards?

Accepted Answer

0.020-inch diameter equivalent orifice leak

Answer

OBD-II defines a small EVAP leak as one equivalent to a 0.020-inch (0.5 mm) diameter orifice, which the PCM can detect via fuel tank pressure change rate during the leak monitor.

Question 5

On a GM flex-fuel vehicle, how does the PCM compensate for a higher ethanol blend (E85) compared to E10 gasoline?

Accepted Answer

Advances ignition timing and increases fuel injector pulse width to account for ethanol's lower energy density

Answer

Ethanol has a lower energy density than gasoline but a higher octane rating, so the PCM increases injector pulse width (more fuel) and advances timing to take advantage of ethanol's knock resistance.

(GM) General Motors Certified Practice Test

(GM) General Motors Certified Practice Test

GM Fuel Systems and Emissions 3