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NYC Auto Mechanic (Diesel) Exam | DCAS
Exam No. 2105 | Questions with Correct
Answers & Detailed Rationales
Question 1: In a High-Pressure Common Rail (HPCR) diesel fuel system, what
component is responsible for maintaining constant fuel pressure in the rail
regardless of engine speed or load?*
A. Mechanical lift pump
B. Fuel pressure control valve / regulator
C. Variable geometry turbocharger
D. Unit fuel injector nozzle
CORRECT ANSWER: B. Fuel pressure control valve / regulator
Rationale: The fuel pressure control valve (or pressure regulator), commanded by the
ECM, modulates the high-pressure pump output or bypasses excess fuel back to the
tank to maintain precise target injection pressure in the common rail across all
engine operating conditions.
Question 2: Which exhaust aftertreatment component in a modern heavy-duty
diesel vehicle uses Diesel Exhaust Fluid (DEF) to reduce oxides of nitrogen (NOx)
into harmless nitrogen gas and water vapor?*
A. Diesel Oxidation Catalyst (DOC)
B. Diesel Particulate Filter (DPF)
C. Selective Catalytic Reduction (SCR) system
D. Exhaust Gas Recirculation (EGR) valve
CORRECT ANSWER: C. Selective Catalytic Reduction (SCR) system
Rationale: The SCR system injects DEF (a urea-water solution) into the exhaust stream
ahead of the SCR catalyst. The heat hydrolyzes DEF into ammonia, which reacts with
Page 1 of 110
,2
NOx over the catalyst to form diatomic nitrogen ($N_2$) and water ($H_2O$).
Question 3: In an S-cam heavy-duty air brake system, what component transfers
rotational movement from the slack adjuster directly to the brake shoes?*
A. Pushrod
B. Brake chamber diaphragm
C. S-camshaft
D. Return spring
CORRECT ANSWER: C. S-camshaft
Rationale: When air enters the brake chamber, it pushes the diaphragm and pushrod,
rotating the slack adjuster. The slack adjuster turns the S-camshaft, whose S-shaped
head forces the brake shoes outward against the brake drum.
Standard Commercial Vehicle Air Brake System Layout. Source: ResearchGate
Question 4: What is the primary function of the safety valve installed on the
supply (wet) tank of a commercial truck air brake system?*
A. Regulate system working pressure between 100 and 120 psi
B. Protect the air tank from over-pressurization if the governor fails
C. Automatically drain accumulated water and oil condensate
D. Direct air pressure equally to the primary and secondary service circuits
CORRECT ANSWER: B. Protect the air tank from over-pressurization if the
governor fails
Rationale: The safety valve is factory preset (typically around 150 psi) to blow off
excessive air pressure if the air compressor governor malfunctions and fails to cut out
the compressor.
Question 1
Vehicles are exempt from the emissions inspection if they are
Page 2 of 110
,3
Correct Answer
diesel-powered, except diesel-powered vehicles that are 1997 or newer and that
have a gross vehicle weight rating (GVRW) less than 8,501 pounds
Question 2
How is the critical area of the windshield defined?
Correct Answer
The area cleaned by the windshield wipers.
Question 3
How is a light truck defined?
Correct Answer
6,000 lbs or less registered vehicle weight.
Question 5: A heavy-duty diesel engine experiences white exhaust smoke during
cold starts that continues even after reaching operating temperature. Which
condition is the MOST likely cause?*
A. Clogged diesel particulate filter
B. Incomplete combustion caused by a misfiring or defective fuel injector
C. Worn oil control rings leaking engine oil into the combustion chamber
D. Excessive exhaust gas recirculation (EGR) flow at high load
CORRECT ANSWER: B. Incomplete combustion caused by a misfiring or
defective fuel injector
Rationale: White smoke indicates unburned or partially burned atomized diesel fuel
vaporized in the combustion chamber. Low compression, retarded injection timing,
or a failing/leaking fuel injector cause this condition.
Question 6: According to standard NYC fleet maintenance and safety procedures,
what tool MUST be used when checking for high-pressure hydraulic leaks on
municipal equipment?*
A. Bare fingers to feel fluid movement
B. Cardboard or piece of wood
C. High-voltage insulation tester
D. Soap and water spray solution
CORRECT ANSWER: B. Cardboard or piece of wood
Rationale: High-pressure hydraulic fluid pinhole leaks can easily penetrate human
skin (fluid injection injury), causing severe tissue necrosis. A piece of cardboard or
wood must always be used to scan across a pressurized hydraulic hose or line.
Question 7: When testing a 12V heavy-duty dual-battery starter circuit under
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cranking load, what is the MAXIMUM allowable total voltage drop across the
positive battery cable?*
A. 0.1 Volts
B. 0.5 Volts
C. 2.0 Volts
D. 4.0 Volts
CORRECT ANSWER: B. 0.5 Volts
Rationale: Industry standard (SAE/TMC) dictates that total voltage drop on high-
current 12V starter motor circuits (positive side) should not exceed 0.5 Volts during
active cranking. Excessive drop points to loose connections, corroded cables, or
failing contacts.
Question 8: What type of diesel engine liner is in direct contact with engine
coolant on its outer circumference?*
A. Dry liner
B. Wet liner
C. Parent bored cylinder
D. Composite sleeve
CORRECT ANSWER: B. Wet liner
Rationale: Wet cylinder liners come into direct contact with the engine coolant in the
cylinder block jacket, requiring upper and lower O-ring seals to prevent coolant from
entering the crankcase oil supply.
Question 9: What is the purpose of the tractor protection valve in a heavy-duty
tractor-trailer air brake system?*
A. Prevent trailer spring brakes from applying during normal braking
B. Retain air pressure in the tractor system if the trailer breaks away or loses pressure
C. Equalize air pressure between front and rear tractor service axles
D. Automatically drain water from the tractor primary air tank
CORRECT ANSWER: B. Retain air pressure in the tractor system if the trailer
breaks away or loses pressure
Rationale: The tractor protection valve closes automatically if trailer supply pressure
drops below a critical threshold (typically 20–45 psi), preserving tractor air pressure
so the tractor driver retains braking control.
Question 10: When diagnosing a J1939 CAN Bus network with two 120-ohm
terminating resistors connected in parallel, what total resistance reading should
be measured across CAN-HI and CAN-LO with the battery disconnected?*
A. 120 Ohms
B. 240 Ohms
C. 60 Ohms
D. 0 Ohms
Page 4 of 110
NYC Auto Mechanic (Diesel) Exam | DCAS
Exam No. 2105 | Questions with Correct
Answers & Detailed Rationales
Question 1: In a High-Pressure Common Rail (HPCR) diesel fuel system, what
component is responsible for maintaining constant fuel pressure in the rail
regardless of engine speed or load?*
A. Mechanical lift pump
B. Fuel pressure control valve / regulator
C. Variable geometry turbocharger
D. Unit fuel injector nozzle
CORRECT ANSWER: B. Fuel pressure control valve / regulator
Rationale: The fuel pressure control valve (or pressure regulator), commanded by the
ECM, modulates the high-pressure pump output or bypasses excess fuel back to the
tank to maintain precise target injection pressure in the common rail across all
engine operating conditions.
Question 2: Which exhaust aftertreatment component in a modern heavy-duty
diesel vehicle uses Diesel Exhaust Fluid (DEF) to reduce oxides of nitrogen (NOx)
into harmless nitrogen gas and water vapor?*
A. Diesel Oxidation Catalyst (DOC)
B. Diesel Particulate Filter (DPF)
C. Selective Catalytic Reduction (SCR) system
D. Exhaust Gas Recirculation (EGR) valve
CORRECT ANSWER: C. Selective Catalytic Reduction (SCR) system
Rationale: The SCR system injects DEF (a urea-water solution) into the exhaust stream
ahead of the SCR catalyst. The heat hydrolyzes DEF into ammonia, which reacts with
Page 1 of 110
,2
NOx over the catalyst to form diatomic nitrogen ($N_2$) and water ($H_2O$).
Question 3: In an S-cam heavy-duty air brake system, what component transfers
rotational movement from the slack adjuster directly to the brake shoes?*
A. Pushrod
B. Brake chamber diaphragm
C. S-camshaft
D. Return spring
CORRECT ANSWER: C. S-camshaft
Rationale: When air enters the brake chamber, it pushes the diaphragm and pushrod,
rotating the slack adjuster. The slack adjuster turns the S-camshaft, whose S-shaped
head forces the brake shoes outward against the brake drum.
Standard Commercial Vehicle Air Brake System Layout. Source: ResearchGate
Question 4: What is the primary function of the safety valve installed on the
supply (wet) tank of a commercial truck air brake system?*
A. Regulate system working pressure between 100 and 120 psi
B. Protect the air tank from over-pressurization if the governor fails
C. Automatically drain accumulated water and oil condensate
D. Direct air pressure equally to the primary and secondary service circuits
CORRECT ANSWER: B. Protect the air tank from over-pressurization if the
governor fails
Rationale: The safety valve is factory preset (typically around 150 psi) to blow off
excessive air pressure if the air compressor governor malfunctions and fails to cut out
the compressor.
Question 1
Vehicles are exempt from the emissions inspection if they are
Page 2 of 110
,3
Correct Answer
diesel-powered, except diesel-powered vehicles that are 1997 or newer and that
have a gross vehicle weight rating (GVRW) less than 8,501 pounds
Question 2
How is the critical area of the windshield defined?
Correct Answer
The area cleaned by the windshield wipers.
Question 3
How is a light truck defined?
Correct Answer
6,000 lbs or less registered vehicle weight.
Question 5: A heavy-duty diesel engine experiences white exhaust smoke during
cold starts that continues even after reaching operating temperature. Which
condition is the MOST likely cause?*
A. Clogged diesel particulate filter
B. Incomplete combustion caused by a misfiring or defective fuel injector
C. Worn oil control rings leaking engine oil into the combustion chamber
D. Excessive exhaust gas recirculation (EGR) flow at high load
CORRECT ANSWER: B. Incomplete combustion caused by a misfiring or
defective fuel injector
Rationale: White smoke indicates unburned or partially burned atomized diesel fuel
vaporized in the combustion chamber. Low compression, retarded injection timing,
or a failing/leaking fuel injector cause this condition.
Question 6: According to standard NYC fleet maintenance and safety procedures,
what tool MUST be used when checking for high-pressure hydraulic leaks on
municipal equipment?*
A. Bare fingers to feel fluid movement
B. Cardboard or piece of wood
C. High-voltage insulation tester
D. Soap and water spray solution
CORRECT ANSWER: B. Cardboard or piece of wood
Rationale: High-pressure hydraulic fluid pinhole leaks can easily penetrate human
skin (fluid injection injury), causing severe tissue necrosis. A piece of cardboard or
wood must always be used to scan across a pressurized hydraulic hose or line.
Question 7: When testing a 12V heavy-duty dual-battery starter circuit under
Page 3 of 110
, 4
cranking load, what is the MAXIMUM allowable total voltage drop across the
positive battery cable?*
A. 0.1 Volts
B. 0.5 Volts
C. 2.0 Volts
D. 4.0 Volts
CORRECT ANSWER: B. 0.5 Volts
Rationale: Industry standard (SAE/TMC) dictates that total voltage drop on high-
current 12V starter motor circuits (positive side) should not exceed 0.5 Volts during
active cranking. Excessive drop points to loose connections, corroded cables, or
failing contacts.
Question 8: What type of diesel engine liner is in direct contact with engine
coolant on its outer circumference?*
A. Dry liner
B. Wet liner
C. Parent bored cylinder
D. Composite sleeve
CORRECT ANSWER: B. Wet liner
Rationale: Wet cylinder liners come into direct contact with the engine coolant in the
cylinder block jacket, requiring upper and lower O-ring seals to prevent coolant from
entering the crankcase oil supply.
Question 9: What is the purpose of the tractor protection valve in a heavy-duty
tractor-trailer air brake system?*
A. Prevent trailer spring brakes from applying during normal braking
B. Retain air pressure in the tractor system if the trailer breaks away or loses pressure
C. Equalize air pressure between front and rear tractor service axles
D. Automatically drain water from the tractor primary air tank
CORRECT ANSWER: B. Retain air pressure in the tractor system if the trailer
breaks away or loses pressure
Rationale: The tractor protection valve closes automatically if trailer supply pressure
drops below a critical threshold (typically 20–45 psi), preserving tractor air pressure
so the tractor driver retains braking control.
Question 10: When diagnosing a J1939 CAN Bus network with two 120-ohm
terminating resistors connected in parallel, what total resistance reading should
be measured across CAN-HI and CAN-LO with the battery disconnected?*
A. 120 Ohms
B. 240 Ohms
C. 60 Ohms
D. 0 Ohms
Page 4 of 110
