Page 1 of 171
NYC Auto Mechanic (Diesel) EXAM LATEST
EDITION WITH 300 QUESTIONS AND CORRECT
RATIONALIZED SOLUTIONS JUST RELEASED
THIS YEAR (2026-2027)
NYC Auto Mechanic (Diesel) Exam — 300 Randomized Scenario-Based MCQs
Summarized Exam Coverage
The NYC Auto Mechanic (Diesel) exam (DCAS Exam No. 2105) is a computer-based multiple-
choice civil service examination administered by the Department of Citywide Administrative
Services (DCAS). The exam evaluates technical knowledge, diagnostic reasoning, information
ordering, and mathematical reasoning, focusing on the maintenance, troubleshooting, and
repair of diesel engines and heavy-duty vehicles. The required passing score is 70%. Content
areas include internal combustion engine components and operation, diesel fuel injection
systems, electrical and electronic systems, vehicle systems repair (steering, suspension, brakes,
transmissions, drivetrain), preventive maintenance and inspection, diagnostic tools and
techniques, and safety and ethics (including OSHA standards and NYC operating rules). The
exam covers ten most-tested areas: Diesel Engine Fundamentals & Components, Fuel Injection
Systems, Electrical & Electronic Systems, Emissions & Aftertreatment Systems, Air Brake
Systems, Engine Diagnosis & Troubleshooting, Turbochargers & Air Systems, Preventive
Maintenance & Fluids, Steering/Suspension/Drivetrain, and Safety & Diagnostic Procedures.
SECTION 1: DIESEL ENGINE FUNDAMENTALS (Questions 1–50)
1. A diesel engine is diagnosed with excessive black smoke, reduced power, and a noticeable
surge at high RPM. The fuel injection timing is found to be within specifications. What is the
MOST likely cause?
1
SUCCESS!
,Page 2 of 171
A) Air intake restriction
B) Excessive valve lash
C) Broken piston ring
D) Faulty turbocharger wastegate
Answer: A
Inadequate air supply relative to fuel injected creates a rich mixture, leading to incomplete
combustion, black smoke, and power loss. A surge can occur as the ECM attempts to
compensate for the lack of air.
2. During a cylinder contribution test on a heavy-duty diesel engine, one cylinder shows a
significant drop in RPM contribution. A relative compression test indicates normal cranking
compression. What is the MOST probable fault?
A) Broken piston ring
B) Leaking injector
C) Bent connecting rod
D) Worn camshaft lobe
2
SUCCESS!
,Page 3 of 171
Answer: B
A leaking injector can cause a rich condition in that cylinder, reducing its power contribution
during the test despite normal mechanical compression. The excess fuel washes the cylinder and
disrupts combustion.
3. A diesel engine relies on which principle for ignition of the air-fuel mixture?
A) Spark plug firing
B) Glow plug heating
C) Heat of compression
D) External flame ignition
Answer: C
Diesel engines use compression ignition, where air is compressed to high pressures (300–500
psi), generating temperatures of 800–1000°F. Diesel fuel is then injected into this hot,
compressed air, causing spontaneous ignition without a spark plug.
4. What are the four strokes of a four-stroke diesel engine in correct order?
3
SUCCESS!
, Page 4 of 171
A) Intake, compression, power, exhaust
B) Intake, power, compression, exhaust
C) Compression, intake, power, exhaust
D) Intake, exhaust, compression, power
Answer: A
A four-stroke diesel engine follows the sequence: Intake (piston moves down, drawing in air),
Compression (piston moves up, compressing air and raising temperature), Power (fuel is injected
and ignites, forcing piston down), and Exhaust (piston moves up, pushing out combustion gases).
5. During the compression stroke of a diesel engine, what occurs?
A) Air and fuel mixture is compressed and ignited by a spark plug.
B) Only air is compressed, raising its temperature above the autoignition point of diesel fuel.
C) Exhaust gases are forced out through the open exhaust valve.
D) The intake valve opens to allow fresh air into the cylinder.
Answer: B
4
SUCCESS!
NYC Auto Mechanic (Diesel) EXAM LATEST
EDITION WITH 300 QUESTIONS AND CORRECT
RATIONALIZED SOLUTIONS JUST RELEASED
THIS YEAR (2026-2027)
NYC Auto Mechanic (Diesel) Exam — 300 Randomized Scenario-Based MCQs
Summarized Exam Coverage
The NYC Auto Mechanic (Diesel) exam (DCAS Exam No. 2105) is a computer-based multiple-
choice civil service examination administered by the Department of Citywide Administrative
Services (DCAS). The exam evaluates technical knowledge, diagnostic reasoning, information
ordering, and mathematical reasoning, focusing on the maintenance, troubleshooting, and
repair of diesel engines and heavy-duty vehicles. The required passing score is 70%. Content
areas include internal combustion engine components and operation, diesel fuel injection
systems, electrical and electronic systems, vehicle systems repair (steering, suspension, brakes,
transmissions, drivetrain), preventive maintenance and inspection, diagnostic tools and
techniques, and safety and ethics (including OSHA standards and NYC operating rules). The
exam covers ten most-tested areas: Diesel Engine Fundamentals & Components, Fuel Injection
Systems, Electrical & Electronic Systems, Emissions & Aftertreatment Systems, Air Brake
Systems, Engine Diagnosis & Troubleshooting, Turbochargers & Air Systems, Preventive
Maintenance & Fluids, Steering/Suspension/Drivetrain, and Safety & Diagnostic Procedures.
SECTION 1: DIESEL ENGINE FUNDAMENTALS (Questions 1–50)
1. A diesel engine is diagnosed with excessive black smoke, reduced power, and a noticeable
surge at high RPM. The fuel injection timing is found to be within specifications. What is the
MOST likely cause?
1
SUCCESS!
,Page 2 of 171
A) Air intake restriction
B) Excessive valve lash
C) Broken piston ring
D) Faulty turbocharger wastegate
Answer: A
Inadequate air supply relative to fuel injected creates a rich mixture, leading to incomplete
combustion, black smoke, and power loss. A surge can occur as the ECM attempts to
compensate for the lack of air.
2. During a cylinder contribution test on a heavy-duty diesel engine, one cylinder shows a
significant drop in RPM contribution. A relative compression test indicates normal cranking
compression. What is the MOST probable fault?
A) Broken piston ring
B) Leaking injector
C) Bent connecting rod
D) Worn camshaft lobe
2
SUCCESS!
,Page 3 of 171
Answer: B
A leaking injector can cause a rich condition in that cylinder, reducing its power contribution
during the test despite normal mechanical compression. The excess fuel washes the cylinder and
disrupts combustion.
3. A diesel engine relies on which principle for ignition of the air-fuel mixture?
A) Spark plug firing
B) Glow plug heating
C) Heat of compression
D) External flame ignition
Answer: C
Diesel engines use compression ignition, where air is compressed to high pressures (300–500
psi), generating temperatures of 800–1000°F. Diesel fuel is then injected into this hot,
compressed air, causing spontaneous ignition without a spark plug.
4. What are the four strokes of a four-stroke diesel engine in correct order?
3
SUCCESS!
, Page 4 of 171
A) Intake, compression, power, exhaust
B) Intake, power, compression, exhaust
C) Compression, intake, power, exhaust
D) Intake, exhaust, compression, power
Answer: A
A four-stroke diesel engine follows the sequence: Intake (piston moves down, drawing in air),
Compression (piston moves up, compressing air and raising temperature), Power (fuel is injected
and ignites, forcing piston down), and Exhaust (piston moves up, pushing out combustion gases).
5. During the compression stroke of a diesel engine, what occurs?
A) Air and fuel mixture is compressed and ignited by a spark plug.
B) Only air is compressed, raising its temperature above the autoignition point of diesel fuel.
C) Exhaust gases are forced out through the open exhaust valve.
D) The intake valve opens to allow fresh air into the cylinder.
Answer: B
4
SUCCESS!
