NYC Auto Mechanic Diesel Exam Prep 2026 | DCAS Exam No. 2105 | 150+
Practice Questions with Correct Answers & Detailed Rationales | New York
City Auto Mechanic Diesel Exam Study Guide | Latest Updated Test Preparation
| Diesel Engine Systems, Automotive Maintenance, Diagnostics & Repair |
Comprehensive Practice Test & Exam Review | Complete Question Bank for
Exam Success
EXAM BLUEPRINT
Topic Area Number of Questions
Diesel Engine Systems & Components 25
Fuel Systems & Injection 20
Air Intake & Exhaust Systems 15
Electrical & Electronic Systems 20
Cooling & Lubrication Systems 15
Braking Systems (Air & Hydraulic) 15
Steering & Suspension Systems 10
Drivetrain & Transmission 10
Emission Control Systems 10
, Topic Area Number of Questions
Preventive Maintenance & Diagnostics 10
Question #1
A heavy-duty diesel engine exhibits excessive blow-by gases at the crankcase breather tube. Which of
the following is the MOST likely primary cause?
A. Clogged air filter element
B. Worn piston rings
C. Overfilled engine oil
D. Failed turbocharger seals
Correct Answer: B. Worn piston rings
Explanation: Excessive blow-by is primarily caused by worn piston rings failing to seal combustion gases
from entering the crankcase. While clogged air filters can increase cylinder pressure slightly, worn rings
are the definitive cause. Oil overfill can cause frothing but not direct blow-by.
Question #2
During a compression test on a 6-cylinder diesel engine, cylinder #3 reads 275 psi while all others read
380-400 psi. What is the MOST appropriate next step?
A. Replace the fuel injector for cylinder #3
B. Perform a cylinder leak-down test on cylinder #3
C. Adjust the valve clearance on cylinder #3
D. Replace the cylinder head gasket
Correct Answer: B. Perform a cylinder leak-down test on cylinder #3
Explanation: A low compression reading (approximately 30% below normal) warrants further
investigation through a leak-down test. This test isolates whether leakage occurs through
intake/exhaust valves, piston rings, or head gasket, providing diagnostic direction before any component
replacement.
Question #3
What is the purpose of the crankshaft vibration damper (harmonic balancer) on a diesel engine?
A. To increase engine horsepower output
B. To reduce torsional vibrations that could damage the crankshaft
C. To balance the weight of the connecting rods
D. To dampen noise from the valvetrain
Practice Questions with Correct Answers & Detailed Rationales | New York
City Auto Mechanic Diesel Exam Study Guide | Latest Updated Test Preparation
| Diesel Engine Systems, Automotive Maintenance, Diagnostics & Repair |
Comprehensive Practice Test & Exam Review | Complete Question Bank for
Exam Success
EXAM BLUEPRINT
Topic Area Number of Questions
Diesel Engine Systems & Components 25
Fuel Systems & Injection 20
Air Intake & Exhaust Systems 15
Electrical & Electronic Systems 20
Cooling & Lubrication Systems 15
Braking Systems (Air & Hydraulic) 15
Steering & Suspension Systems 10
Drivetrain & Transmission 10
Emission Control Systems 10
, Topic Area Number of Questions
Preventive Maintenance & Diagnostics 10
Question #1
A heavy-duty diesel engine exhibits excessive blow-by gases at the crankcase breather tube. Which of
the following is the MOST likely primary cause?
A. Clogged air filter element
B. Worn piston rings
C. Overfilled engine oil
D. Failed turbocharger seals
Correct Answer: B. Worn piston rings
Explanation: Excessive blow-by is primarily caused by worn piston rings failing to seal combustion gases
from entering the crankcase. While clogged air filters can increase cylinder pressure slightly, worn rings
are the definitive cause. Oil overfill can cause frothing but not direct blow-by.
Question #2
During a compression test on a 6-cylinder diesel engine, cylinder #3 reads 275 psi while all others read
380-400 psi. What is the MOST appropriate next step?
A. Replace the fuel injector for cylinder #3
B. Perform a cylinder leak-down test on cylinder #3
C. Adjust the valve clearance on cylinder #3
D. Replace the cylinder head gasket
Correct Answer: B. Perform a cylinder leak-down test on cylinder #3
Explanation: A low compression reading (approximately 30% below normal) warrants further
investigation through a leak-down test. This test isolates whether leakage occurs through
intake/exhaust valves, piston rings, or head gasket, providing diagnostic direction before any component
replacement.
Question #3
What is the purpose of the crankshaft vibration damper (harmonic balancer) on a diesel engine?
A. To increase engine horsepower output
B. To reduce torsional vibrations that could damage the crankshaft
C. To balance the weight of the connecting rods
D. To dampen noise from the valvetrain
