DTC DTC Air Intake and Turbocharger Systems 2 — Questions and Answers
Question 1: What condition can cause a turbocharger to 'surge,' producing a repetitive whooshing or cyclical noise?
- Excessive boost pressure from a stuck-closed wastegate
- A partially blocked air filter restricting compressor inlet flow
- Compressor wheel operating in an unstable flow region due to low airflow demand (Correct answer)
- A cracked charge air cooler allowing boost to escape
Correct answer: Compressor wheel operating in an unstable flow region due to low airflow demand
Turbocharger surge occurs when the compressor wheel operates beyond its surge line — typically at low airflow conditions — causing a cyclic breakdown and recovery of airflow that produces a characteristic repetitive sound.
Question 2: When performing a boost pressure test on a turbocharged diesel, where should the pressure gauge be connected?
- At the turbocharger turbine inlet (exhaust side)
- At the intake manifold or the charge air cooler outlet pipe (Correct answer)
- At the crankcase breather outlet
- At the fuel return line
Correct answer: At the intake manifold or the charge air cooler outlet pipe
Boost pressure is the pressurized air delivered to the engine cylinders, so the gauge is connected at the intake manifold or the air piping after the charge air cooler to measure actual delivered boost.
Question 3: A diesel technician finds the charge air cooler core is clogged with oil residue internally. What is the CORRECT repair procedure?
- Replace the charge air cooler and diagnose the source of the oil contamination before reinstalling a new unit (Correct answer)
- Flush the charge air cooler with solvent, reinstall, and monitor for further contamination
- Leave the charge air cooler in place and repair the turbocharger seal only
- Replace only the turbocharger without cleaning the charge air cooler
Correct answer: Replace the charge air cooler and diagnose the source of the oil contamination before reinstalling a new unit
The contaminated charge air cooler must be replaced because residual oil deposits restrict airflow and can be ingested into the engine; the root cause (typically a turbo seal or crankcase ventilation issue) must also be corrected.
Question 4: What is the recommended procedure immediately after installing a new or rebuilt turbocharger before starting the engine?
- Pre-lube the turbocharger center bearing by cranking the engine without starting, then let the engine idle before applying load (Correct answer)
- Install the turbocharger, start the engine immediately, and idle at high RPM to seat the bearings
- Fill the turbocharger oil inlet port with clean engine oil, then start and immediately load the engine
- Remove the oil drain line, fill with gear oil, reinstall, then start normally
Correct answer: Pre-lube the turbocharger center bearing by cranking the engine without starting, then let the engine idle before applying load
Pre-lubricating the turbocharger by cranking without starting, and then idling before applying load, ensures the center bearing receives adequate oil pressure before the turbine spins at high speed, preventing dry-start damage.
Question 5: Which symptom would MOST likely indicate an air leak between the turbocharger compressor outlet and the intake manifold?
- Excessive black exhaust smoke at all engine speeds
- Blue exhaust smoke at startup only
- Low power with white exhaust smoke and low boost pressure
- Low power, lower-than-normal boost, and possible hissing noise under load (Correct answer)
Correct answer: Low power, lower-than-normal boost, and possible hissing noise under load
An air leak on the pressurized intake side causes boost pressure loss, resulting in reduced power output; the escaping compressed air often produces a hissing or whistling noise under load.
Question 6: In a diesel engine with a two-stage turbocharger system, what is the function of the low-pressure (LP) turbocharger?
- To compress exhaust gases before they enter the high-pressure turbine
- To provide the initial compression stage, feeding pre-compressed air into the high-pressure compressor (Correct answer)
- To regulate EGR flow between the two compressor stages
- To act as a wastegate for the high-pressure turbocharger
Correct answer: To provide the initial compression stage, feeding pre-compressed air into the high-pressure compressor
In a two-stage system, the low-pressure turbocharger provides an initial compression stage, partially compressing intake air before it is further compressed by the high-pressure turbocharger, achieving higher total boost ratios.
Question 7: What is the most likely cause of blue-white smoke from a diesel engine that occurs primarily at idle and clears up at higher engine speeds?
- Worn turbocharger seals allowing oil to be drawn into the intake under low-boost vacuum (Correct answer)
- Overfueling caused by a stuck-open injector
- Coolant leaking past a failed head gasket into the combustion chamber
- A clogged diesel particulate filter causing excess back pressure
Correct answer: Worn turbocharger seals allowing oil to be drawn into the intake under low-boost vacuum
At low idle, the turbocharger creates a slight vacuum on the compressor inlet; worn compressor seals allow engine oil to be drawn into the intake, burning and producing blue-white smoke that clears as boost builds at higher RPM.
DTC DTC Air Intake and Turbocharger Systems 2
What condition can cause a turbocharger to 'surge,' producing a repetitive whooshing or cyclical noise?