DTC DTC Air Intake and Turbocharger Systems 1 — Questions and Answers
Question 1: What is the primary purpose of the charge air cooler (intercooler) in a turbocharged diesel engine?
- To cool the engine coolant before it enters the radiator
- To reduce the temperature of compressed air before it enters the intake manifold (Correct answer)
- To lower the exhaust gas temperature before the turbocharger
- To cool the turbocharger center housing
Correct answer: To reduce the temperature of compressed air before it enters the intake manifold
The charge air cooler reduces the temperature of compressed air exiting the turbocharger, which increases air density and oxygen content entering the cylinders for better combustion efficiency.
Question 2: A diesel engine exhibits black smoke at full load accompanied by lower than normal boost pressure. Which component is MOST likely at fault?
- Clogged fuel injectors
- Worn or damaged turbocharger (Correct answer)
- Faulty EGR valve stuck open
- Restricted exhaust back-pressure sensor
Correct answer: Worn or damaged turbocharger
A worn or damaged turbocharger will fail to build adequate boost pressure, resulting in insufficient air for complete combustion, which produces black smoke under load.
Question 3: When inspecting a turbocharger for shaft play, what type of movement indicates a worn center bearing?
- Rotational play only
- Excessive axial (end-to-end) play only
- Excessive radial (side-to-side) play (Correct answer)
- No detectable movement at all
Correct answer: Excessive radial (side-to-side) play
Excessive radial (side-to-side) shaft play indicates worn center bearings in the turbocharger, which can allow the compressor or turbine wheel to contact the housing.
Question 4: What does a Variable Geometry Turbocharger (VGT) use to control boost pressure at different engine speeds?
- A wastegate valve that bypasses exhaust gases around the turbine
- Adjustable vanes in the turbine housing that redirect exhaust gas flow (Correct answer)
- A bypass valve on the compressor outlet
- A mechanically driven supercharger that supplements the turbo
Correct answer: Adjustable vanes in the turbine housing that redirect exhaust gas flow
A VGT uses adjustable vanes inside the turbine housing to vary the angle and velocity of exhaust gas striking the turbine wheel, allowing precise boost control across a wide RPM range.
Question 5: A technician notices oil in the intake air piping between the turbocharger and the charge air cooler. What is the MOST likely cause?
- Overfilled engine oil level causing crankcase pressure
- Worn turbocharger compressor seal allowing oil to be ingested (Correct answer)
- Clogged crankcase ventilation (breather) tube
- Defective positive crankcase ventilation (PCV) valve
Correct answer: Worn turbocharger compressor seal allowing oil to be ingested
A worn turbocharger compressor seal allows engine oil from the center bearing housing to migrate past the seal and into the compressor outlet, contaminating the intake air piping.
Question 6: What is the purpose of the wastegate on a conventional turbocharger?
- To allow exhaust gases to bypass the diesel particulate filter during regeneration
- To limit maximum boost pressure by routing excess exhaust gas around the turbine wheel (Correct answer)
- To control the EGR rate by metering exhaust back into the intake
- To vent crankcase pressure to the atmosphere
Correct answer: To limit maximum boost pressure by routing excess exhaust gas around the turbine wheel
The wastegate is a valve that bypasses exhaust gases around the turbine wheel once a preset boost pressure is reached, preventing overboosting and protecting the engine.
Question 7: Which air filter restriction measurement tool is most commonly used to determine when a diesel engine air filter requires replacement?
- A vacuum gauge connected to the crankcase breather
- A manometer or restriction indicator on the air cleaner housing (Correct answer)
- A pressure gauge on the turbocharger compressor outlet
- An infrared thermometer measuring filter temperature
Correct answer: A manometer or restriction indicator on the air cleaner housing
A restriction indicator (or manometer) mounted on the air cleaner housing measures intake vacuum in inches of water column, signaling when restriction exceeds the manufacturer's specified limit.
DTC DTC Air Intake and Turbocharger Systems 1
What is the primary purpose of the charge air cooler (intercooler) in a turbocharged diesel engine?