Page 1 of 131
NYC Auto Mechanic (Diesel) — DCAS Exam No. 2105
QUESTIONS AND VERIFIED ANSWERS WITH RATIONALES
JUST RELEASED
NYC Auto Mechanic (Diesel) — DCAS Exam No. 2105
MOST-TESTED EXAM COVERAGE
1. Diesel engine operation, construction, and component diagnosis
2. Fuel injection, common-rail, air-intake, turbocharging, and starting systems
3. Electrical, electronic, charging, starting, sensors, and computerized controls
4. Emission-control systems, EGR, DPF, SCR, DEF, and exhaust diagnostics
5. Cooling, lubrication, heating, and engine temperature management
6. Braking systems, ABS, air brakes, hydraulic brakes, and brake troubleshooting
7. Steering, suspension, wheel-end, driveline, transmission, and differential systems
8. Preventive maintenance, inspections, service procedures, and fleet maintenance
9. Diagnostic tools, electrical testing, scan tools, measurements, and troubleshooting
10. Shop safety, hazardous materials, lifting equipment, reports, ethics, supervision, and vehicle
operation
, Page 2 of 131
Diesel Engine Fundamentals
1. A four-stroke diesel engine completes its normal operating cycle through which sequence of piston
strokes?
A. Intake, compression, power, exhaust
B. Compression, intake, exhaust, power
C. Intake, exhaust, power, compression
D. Power, compression, intake, exhaust
Answer: A
Rationale: The four strokes occur in the sequence intake, compression, power, and exhaust.
2. During the compression stroke of a conventional diesel engine, what is primarily compressed inside
the cylinder?
A. Fuel only
B. Air only
C. Coolant and air
D. Fuel and exhaust gases
, Page 3 of 131
Answer: B
Rationale: Diesel engines compress air to raise its temperature before fuel injection.
3. What primary function does the crankshaft perform within a reciprocating diesel engine during
normal operation?
A. Controls injector pressure
B. Circulates engine coolant
C. Converts piston motion into rotary motion
D. Regulates turbocharger boost
Answer: C
Rationale: The crankshaft converts the pistons' reciprocating movement into rotational movement.
4. Which component controls the opening and closing of the intake and exhaust valves at specified
engine timing?
A. Flywheel
B. Oil pump
, Page 4 of 131
C. Connecting rod
D. Camshaft
Answer: D
Rationale: Camshaft lobes operate the valve train according to predetermined timing.
5. What is the primary purpose of piston rings installed around a diesel engine piston?
A. Seal combustion gases, control oil, and transfer heat
B. Increase fuel pressure
C. Control injector timing
D. Operate the exhaust valves
Answer: A
Rationale: Piston rings provide combustion sealing, oil control, and heat transfer.
6. A cylinder head gasket must maintain a seal between which major engine components?
A. Crankshaft and connecting rod
B. Cylinder head and engine block
NYC Auto Mechanic (Diesel) — DCAS Exam No. 2105
QUESTIONS AND VERIFIED ANSWERS WITH RATIONALES
JUST RELEASED
NYC Auto Mechanic (Diesel) — DCAS Exam No. 2105
MOST-TESTED EXAM COVERAGE
1. Diesel engine operation, construction, and component diagnosis
2. Fuel injection, common-rail, air-intake, turbocharging, and starting systems
3. Electrical, electronic, charging, starting, sensors, and computerized controls
4. Emission-control systems, EGR, DPF, SCR, DEF, and exhaust diagnostics
5. Cooling, lubrication, heating, and engine temperature management
6. Braking systems, ABS, air brakes, hydraulic brakes, and brake troubleshooting
7. Steering, suspension, wheel-end, driveline, transmission, and differential systems
8. Preventive maintenance, inspections, service procedures, and fleet maintenance
9. Diagnostic tools, electrical testing, scan tools, measurements, and troubleshooting
10. Shop safety, hazardous materials, lifting equipment, reports, ethics, supervision, and vehicle
operation
, Page 2 of 131
Diesel Engine Fundamentals
1. A four-stroke diesel engine completes its normal operating cycle through which sequence of piston
strokes?
A. Intake, compression, power, exhaust
B. Compression, intake, exhaust, power
C. Intake, exhaust, power, compression
D. Power, compression, intake, exhaust
Answer: A
Rationale: The four strokes occur in the sequence intake, compression, power, and exhaust.
2. During the compression stroke of a conventional diesel engine, what is primarily compressed inside
the cylinder?
A. Fuel only
B. Air only
C. Coolant and air
D. Fuel and exhaust gases
, Page 3 of 131
Answer: B
Rationale: Diesel engines compress air to raise its temperature before fuel injection.
3. What primary function does the crankshaft perform within a reciprocating diesel engine during
normal operation?
A. Controls injector pressure
B. Circulates engine coolant
C. Converts piston motion into rotary motion
D. Regulates turbocharger boost
Answer: C
Rationale: The crankshaft converts the pistons' reciprocating movement into rotational movement.
4. Which component controls the opening and closing of the intake and exhaust valves at specified
engine timing?
A. Flywheel
B. Oil pump
, Page 4 of 131
C. Connecting rod
D. Camshaft
Answer: D
Rationale: Camshaft lobes operate the valve train according to predetermined timing.
5. What is the primary purpose of piston rings installed around a diesel engine piston?
A. Seal combustion gases, control oil, and transfer heat
B. Increase fuel pressure
C. Control injector timing
D. Operate the exhaust valves
Answer: A
Rationale: Piston rings provide combustion sealing, oil control, and heat transfer.
6. A cylinder head gasket must maintain a seal between which major engine components?
A. Crankshaft and connecting rod
B. Cylinder head and engine block
