NYC AUTO MECHANIC (DIESEL) (DCAS)
CERTIFICATION PRACTICE EXAM QUESTIONS
AND VERIFIED ANSWERS WITH RATIONALES
JUST RELEASED.pdf
1. A diesel engine with a common rail fuel system exhibits rough idle and black smoke. Fuel
pressure at idle is 30 MPa (spec: 35 MPa). The rail pressure control valve is commanded open at
50% duty cycle. Which is the most likely cause?
A. Clogged fuel filter
B. Faulty rail pressure sensor
C. Leaking injector return line
D. Stuck open pressure control valve
Answer: D
Rationale: The pressure control valve regulates rail pressure by bleeding fuel back to the tank. A stuck
open valve would cause low pressure despite high duty cycle. A clogged filter would cause low pressure
but the pump would compensate; a faulty sensor would cause incorrect readings but not necessarily low
pressure with high command. Leaking return lines would affect volume but not pressure as directly.
2. During a diesel particulate filter (DPF) regeneration, the exhaust temperature reaches 650°C.
The engine control unit (ECU) commands post-injection. Which of the following best describes the
primary purpose of post-injection?
A. Increase engine torque to overcome exhaust backpressure
B. Raise exhaust gas temperature by combusting fuel in the cylinder late
C. Inject fuel directly into the exhaust stream upstream of the DPF
D. Reduce NOx emissions by cooling the combustion chamber
Answer: B
Rationale: Post-injection occurs after the main injection event, allowing fuel to burn late in the cycle or in
the exhaust system, raising exhaust temperature to oxidize soot in the DPF. Option A is incorrect
because torque is not the goal; C describes a different strategy (fuel injection into exhaust) not typical
for post-injection; D is opposite-post-injection increases temperature, which can increase NOx.
3. A technician measures voltage drop across the starter motor positive cable during cranking: 0.8
V. Battery voltage at the battery posts is 12.6 V. What is the actual voltage at the starter motor
terminal during cranking?
A. 11.8 V
B. 12.6 V
C. 13.4 V
Page 1
,D. 0.8 V
Answer: A
Rationale: Voltage drop is the difference between source and load. With 12.6 V at the battery and a 0.8 V
drop in the cable, the starter terminal voltage is 12.6 - 0.8 = 11.8 V. Option B ignores the drop; C would
imply a voltage rise; D is the drop itself.
4. Which of the following best describes the function of the exhaust gas recirculation (EGR) cooler
bypass valve in a modern diesel engine?
A. Redirects exhaust gas away from the EGR cooler during cold starts to prevent condensation
B. Allows coolant to bypass the EGR cooler when engine temperature is too high
C. Increases EGR flow rate during high load conditions to reduce NOx
D. Bypasses the EGR system entirely when the DPF is regenerating
Answer: A
Rationale: The EGR cooler bypass valve routes exhaust gas around the cooler during cold starts to avoid
condensation of acidic gases that can damage the cooler and engine. Option B is incorrect because
coolant bypass is not typical; C is opposite-bypass reduces cooling, not flow; D is not standard-DPF
regeneration uses post-injection, not EGR bypass.
5. A heavy-duty diesel engine with a turbocharger and wastegate has low boost pressure at full
load. The wastegate actuator is connected to a pressure hose from the compressor outlet.
Technician A says a leaking wastegate diaphragm can cause low boost. Technician B says a stuck
closed wastegate can cause low boost. Who is correct?
A. Technician A only
B. Technician B only
C. Both A and B
D. Neither A nor B
Answer: A
Rationale: A leaking wastegate diaphragm would allow boost pressure to escape, preventing the actuator
from opening the wastegate fully, but if it leaks, the wastegate may open prematurely or not close
properly, causing low boost. A stuck closed wastegate would prevent exhaust gas from bypassing the
turbine, leading to overboost, not low boost. Therefore only Technician A is correct.
6. A diesel engine exhibits a knocking noise that is most pronounced when cold and under light
load. Which of the following is the most likely cause?
A. Retarded injection timing
B. Advanced injection timing
C. Low compression in one cylinder
D. Worn main bearings
Answer: B
Rationale: Advanced injection timing causes fuel to ignite too early, leading to a knocking sound (diesel
knock), especially when cold due to longer ignition delay. Retarded timing causes late combustion, less
knock. Low compression would cause misfire, not distinct knock. Worn main bearings produce a
thumping sound, not a metallic knock.
Page 2
,7. A technician is diagnosing a no-start condition on a diesel engine. The glow plug system is
functioning. The engine cranks but does not start. Fuel is present at the injectors. Which test
should be performed next?
A. Measure cylinder compression
B. Check for air in the fuel system
C. Test the injection pump timing
D. Verify the camshaft position sensor signal
Answer: A
Rationale: Since fuel is at the injectors and glow plugs work, the next likely cause is low compression,
preventing ignition. Air in fuel would cause erratic running but not necessarily no-start with fuel at
injectors. Injection pump timing could be off, but compression test is quicker and more common.
Camshaft sensor fault would prevent injection timing but fuel presence suggests injection occurs.
8. Which of the following is the most likely result of a restricted diesel exhaust system?
A. Increased fuel economy
B. Decreased exhaust gas temperature
C. Increased turbocharger boost pressure
D. Reduced engine power and black smoke
Answer: D
Rationale: A restricted exhaust increases backpressure, reducing the engine's ability to expel exhaust
gases, leading to lower volumetric efficiency and power loss. It also causes incomplete combustion,
resulting in black smoke. Boost pressure would decrease, not increase. Fuel economy worsens. Exhaust
temperature may increase due to restriction, not decrease.
9. A diesel engine uses a high-pressure common rail injection system. The injector return flow rate
is measured and found to be 15 mL/min for all injectors (spec: 10 mL/min max). What is the most
likely consequence?
A. Hard starting and white smoke
B. Engine overheating
C. Low rail pressure and poor performance
D. Increased fuel economy
Answer: C
Rationale: Excessive return flow from injectors indicates internal leakage, which reduces the effective
fuel delivered to the cylinders and lowers rail pressure because the high-pressure pump cannot
compensate. This leads to poor performance, not hard starting (though possible) or overheating. Fuel
economy would decrease, not increase.
10. A technician observes that the engine coolant temperature rises rapidly after a cold start, but
the heater output is weak. The thermostat is new. What is the most likely cause?
A. Air pocket in the cooling system
B. Faulty water pump impeller
C. Stuck open thermostat
Page 3
, D. Restricted heater core
Answer: D
Rationale: A restricted heater core reduces coolant flow through the heater, resulting in weak heat
output. The engine may still warm quickly if the thermostat is functioning (closed initially) and coolant
circulation through the radiator is minimal. An air pocket would cause erratic temperature and possible
overheating, not rapid warm-up with weak heat. A faulty water pump would cause overheating, not
rapid warm-up. A stuck open thermostat would cause slow warm-up.
11. A diesel engine exhibits excessive black smoke under load, reduced power, and increased fuel
consumption. A compression test reveals cylinder pressures within spec, and the injection timing is
correct. Which of the following is the most likely cause?
A. Injector nozzle orifice erosion causing reduced injection pressure
B. Turbocharger wastegate stuck open
C. Fuel injection pump delivery valve spring fatigue
D. Air intake restriction due to clogged air filter
Answer: A
Rationale: Excessive black smoke under load with normal compression and timing indicates incomplete
combustion due to poor atomization. Eroded injector nozzles lower injection pressure, producing larger
fuel droplets that don't burn fully. A stuck-open wastegate would cause low boost but not black smoke; a
clogged air filter would also cause black smoke but is less likely if compression and timing are correct
and the symptom is specific to load.
12. A technician is diagnosing a diesel engine that cranks but fails to start. The fuel tank is full, and
the fuel filter was recently replaced. Which test should be performed first to isolate the problem?
A. Check for air in the fuel system by cracking the injector lines
B. Measure fuel pressure at the injection pump inlet
C. Perform a cylinder contribution test
D. Inspect the glow plug relay operation
Answer: A
Rationale: After a fuel filter change, air ingress is a common cause of no-start. Cracking injector lines to
check for fuel and air bubbles quickly identifies air in the system. Fuel pressure measurement is
secondary; cylinder contribution and glow plug checks are premature without confirming fuel delivery.
13. During a road test, a heavy-duty diesel truck experiences a sudden loss of engine power
accompanied by a loud hissing sound from the engine bay. The technician suspects a turbocharger
issue. Which diagnostic step is most appropriate?
A. Perform a boost pressure test with a scan tool
B. Inspect the charge air cooler for cracks
C. Check the turbocharger shaft play and compressor wheel condition
D. Measure exhaust back pressure at the turbo inlet
Answer: C
Rationale: A sudden hissing sound and power loss often indicate a turbocharger failure, such as a broken
shaft or damaged compressor wheel. Direct inspection of shaft play and wheel condition is the most
Page 4
CERTIFICATION PRACTICE EXAM QUESTIONS
AND VERIFIED ANSWERS WITH RATIONALES
JUST RELEASED.pdf
1. A diesel engine with a common rail fuel system exhibits rough idle and black smoke. Fuel
pressure at idle is 30 MPa (spec: 35 MPa). The rail pressure control valve is commanded open at
50% duty cycle. Which is the most likely cause?
A. Clogged fuel filter
B. Faulty rail pressure sensor
C. Leaking injector return line
D. Stuck open pressure control valve
Answer: D
Rationale: The pressure control valve regulates rail pressure by bleeding fuel back to the tank. A stuck
open valve would cause low pressure despite high duty cycle. A clogged filter would cause low pressure
but the pump would compensate; a faulty sensor would cause incorrect readings but not necessarily low
pressure with high command. Leaking return lines would affect volume but not pressure as directly.
2. During a diesel particulate filter (DPF) regeneration, the exhaust temperature reaches 650°C.
The engine control unit (ECU) commands post-injection. Which of the following best describes the
primary purpose of post-injection?
A. Increase engine torque to overcome exhaust backpressure
B. Raise exhaust gas temperature by combusting fuel in the cylinder late
C. Inject fuel directly into the exhaust stream upstream of the DPF
D. Reduce NOx emissions by cooling the combustion chamber
Answer: B
Rationale: Post-injection occurs after the main injection event, allowing fuel to burn late in the cycle or in
the exhaust system, raising exhaust temperature to oxidize soot in the DPF. Option A is incorrect
because torque is not the goal; C describes a different strategy (fuel injection into exhaust) not typical
for post-injection; D is opposite-post-injection increases temperature, which can increase NOx.
3. A technician measures voltage drop across the starter motor positive cable during cranking: 0.8
V. Battery voltage at the battery posts is 12.6 V. What is the actual voltage at the starter motor
terminal during cranking?
A. 11.8 V
B. 12.6 V
C. 13.4 V
Page 1
,D. 0.8 V
Answer: A
Rationale: Voltage drop is the difference between source and load. With 12.6 V at the battery and a 0.8 V
drop in the cable, the starter terminal voltage is 12.6 - 0.8 = 11.8 V. Option B ignores the drop; C would
imply a voltage rise; D is the drop itself.
4. Which of the following best describes the function of the exhaust gas recirculation (EGR) cooler
bypass valve in a modern diesel engine?
A. Redirects exhaust gas away from the EGR cooler during cold starts to prevent condensation
B. Allows coolant to bypass the EGR cooler when engine temperature is too high
C. Increases EGR flow rate during high load conditions to reduce NOx
D. Bypasses the EGR system entirely when the DPF is regenerating
Answer: A
Rationale: The EGR cooler bypass valve routes exhaust gas around the cooler during cold starts to avoid
condensation of acidic gases that can damage the cooler and engine. Option B is incorrect because
coolant bypass is not typical; C is opposite-bypass reduces cooling, not flow; D is not standard-DPF
regeneration uses post-injection, not EGR bypass.
5. A heavy-duty diesel engine with a turbocharger and wastegate has low boost pressure at full
load. The wastegate actuator is connected to a pressure hose from the compressor outlet.
Technician A says a leaking wastegate diaphragm can cause low boost. Technician B says a stuck
closed wastegate can cause low boost. Who is correct?
A. Technician A only
B. Technician B only
C. Both A and B
D. Neither A nor B
Answer: A
Rationale: A leaking wastegate diaphragm would allow boost pressure to escape, preventing the actuator
from opening the wastegate fully, but if it leaks, the wastegate may open prematurely or not close
properly, causing low boost. A stuck closed wastegate would prevent exhaust gas from bypassing the
turbine, leading to overboost, not low boost. Therefore only Technician A is correct.
6. A diesel engine exhibits a knocking noise that is most pronounced when cold and under light
load. Which of the following is the most likely cause?
A. Retarded injection timing
B. Advanced injection timing
C. Low compression in one cylinder
D. Worn main bearings
Answer: B
Rationale: Advanced injection timing causes fuel to ignite too early, leading to a knocking sound (diesel
knock), especially when cold due to longer ignition delay. Retarded timing causes late combustion, less
knock. Low compression would cause misfire, not distinct knock. Worn main bearings produce a
thumping sound, not a metallic knock.
Page 2
,7. A technician is diagnosing a no-start condition on a diesel engine. The glow plug system is
functioning. The engine cranks but does not start. Fuel is present at the injectors. Which test
should be performed next?
A. Measure cylinder compression
B. Check for air in the fuel system
C. Test the injection pump timing
D. Verify the camshaft position sensor signal
Answer: A
Rationale: Since fuel is at the injectors and glow plugs work, the next likely cause is low compression,
preventing ignition. Air in fuel would cause erratic running but not necessarily no-start with fuel at
injectors. Injection pump timing could be off, but compression test is quicker and more common.
Camshaft sensor fault would prevent injection timing but fuel presence suggests injection occurs.
8. Which of the following is the most likely result of a restricted diesel exhaust system?
A. Increased fuel economy
B. Decreased exhaust gas temperature
C. Increased turbocharger boost pressure
D. Reduced engine power and black smoke
Answer: D
Rationale: A restricted exhaust increases backpressure, reducing the engine's ability to expel exhaust
gases, leading to lower volumetric efficiency and power loss. It also causes incomplete combustion,
resulting in black smoke. Boost pressure would decrease, not increase. Fuel economy worsens. Exhaust
temperature may increase due to restriction, not decrease.
9. A diesel engine uses a high-pressure common rail injection system. The injector return flow rate
is measured and found to be 15 mL/min for all injectors (spec: 10 mL/min max). What is the most
likely consequence?
A. Hard starting and white smoke
B. Engine overheating
C. Low rail pressure and poor performance
D. Increased fuel economy
Answer: C
Rationale: Excessive return flow from injectors indicates internal leakage, which reduces the effective
fuel delivered to the cylinders and lowers rail pressure because the high-pressure pump cannot
compensate. This leads to poor performance, not hard starting (though possible) or overheating. Fuel
economy would decrease, not increase.
10. A technician observes that the engine coolant temperature rises rapidly after a cold start, but
the heater output is weak. The thermostat is new. What is the most likely cause?
A. Air pocket in the cooling system
B. Faulty water pump impeller
C. Stuck open thermostat
Page 3
, D. Restricted heater core
Answer: D
Rationale: A restricted heater core reduces coolant flow through the heater, resulting in weak heat
output. The engine may still warm quickly if the thermostat is functioning (closed initially) and coolant
circulation through the radiator is minimal. An air pocket would cause erratic temperature and possible
overheating, not rapid warm-up with weak heat. A faulty water pump would cause overheating, not
rapid warm-up. A stuck open thermostat would cause slow warm-up.
11. A diesel engine exhibits excessive black smoke under load, reduced power, and increased fuel
consumption. A compression test reveals cylinder pressures within spec, and the injection timing is
correct. Which of the following is the most likely cause?
A. Injector nozzle orifice erosion causing reduced injection pressure
B. Turbocharger wastegate stuck open
C. Fuel injection pump delivery valve spring fatigue
D. Air intake restriction due to clogged air filter
Answer: A
Rationale: Excessive black smoke under load with normal compression and timing indicates incomplete
combustion due to poor atomization. Eroded injector nozzles lower injection pressure, producing larger
fuel droplets that don't burn fully. A stuck-open wastegate would cause low boost but not black smoke; a
clogged air filter would also cause black smoke but is less likely if compression and timing are correct
and the symptom is specific to load.
12. A technician is diagnosing a diesel engine that cranks but fails to start. The fuel tank is full, and
the fuel filter was recently replaced. Which test should be performed first to isolate the problem?
A. Check for air in the fuel system by cracking the injector lines
B. Measure fuel pressure at the injection pump inlet
C. Perform a cylinder contribution test
D. Inspect the glow plug relay operation
Answer: A
Rationale: After a fuel filter change, air ingress is a common cause of no-start. Cracking injector lines to
check for fuel and air bubbles quickly identifies air in the system. Fuel pressure measurement is
secondary; cylinder contribution and glow plug checks are premature without confirming fuel delivery.
13. During a road test, a heavy-duty diesel truck experiences a sudden loss of engine power
accompanied by a loud hissing sound from the engine bay. The technician suspects a turbocharger
issue. Which diagnostic step is most appropriate?
A. Perform a boost pressure test with a scan tool
B. Inspect the charge air cooler for cracks
C. Check the turbocharger shaft play and compressor wheel condition
D. Measure exhaust back pressure at the turbo inlet
Answer: C
Rationale: A sudden hissing sound and power loss often indicate a turbocharger failure, such as a broken
shaft or damaged compressor wheel. Direct inspection of shaft play and wheel condition is the most
Page 4
