DTC DTC Air Intake and Turbocharger Systems 3

Question 1

What is the purpose of the compressor bypass valve (also called a blow-off valve or recirculation valve) used on some turbocharged diesel engines?

Accepted Answer

To recirculate compressed air back to the turbocharger inlet during rapid deceleration, preventing compressor surge

Answer

To relieve excess fuel pressure when the throttle is suddenly closed

Answer

To vent crankcase pressure into the intake manifold under boost

Answer

To bypass the charge air cooler when intake temperatures are too low

Question 2

A diesel engine displays hesitation and low power at low RPM but performs normally at high RPM. Which condition is MOST consistent with these symptoms?

Accepted Answer

Excessive turbo lag due to a large turbocharger sized for high-RPM performance

Answer

Turbocharger over-boosting due to a failed wastegate

Answer

Cracked intake manifold allowing boost to escape only at high pressure

Answer

Blocked exhaust back-pressure sensor causing retarded injection timing

Question 3

When checking turbocharger axial (end-play) shaft movement, what is the typical maximum allowable limit before the turbocharger is considered unserviceable?

Accepted Answer

0.001–0.003 inch (0.025–0.076 mm)

Answer

0.025–0.050 inch (0.635–1.270 mm)

Answer

0.010–0.030 inch (0.254–0.762 mm)

Answer

0.100–0.150 inch (2.54–3.81 mm)

Question 4

What is the primary reason diesel engine air intake systems use a two-stage air filtration design (pre-cleaner plus primary filter)?

Accepted Answer

To remove large debris and moisture in the pre-cleaner stage before the primary filter to extend primary filter service life

Answer

To allow air recirculation between filter stages for temperature control

Answer

To split airflow between two parallel primary filters for increased engine breathing

Answer

To meet EPA regulations requiring dual filtration on commercial diesel engines

Question 5

On a diesel engine with an electronically controlled VGT turbocharger, which component or system is responsible for commanding vane position?

Accepted Answer

The engine control module (ECM) via an actuator solenoid or electric motor based on sensor inputs

Answer

The mechanical wastegate actuator responding to boost pressure directly

Answer

The driver directly through the accelerator pedal position sensor only

Answer

The turbocharger's internal centrifugal governor independent of the ECM

Question 6

During a charge air cooler pressure test, what is typically used to pressurize the system and what failure condition would indicate a defective cooler?

Accepted Answer

Pressurize with shop air to 40 psi; rapid pressure loss indicates a cracked core or leaking end tank

Answer

Pressurize with nitrogen to 30 psi; a drop in pressure over 5 minutes indicates a leak

Answer

Pressurize with engine coolant to system pressure; a rise in pressure indicates a crack

Answer

Pressurize with fuel to 10 psi; discoloration indicates porosity

Question 7

What effect does a restricted diesel engine air filter have on turbocharger operation and exhaust emissions?

Accepted Answer

It causes the turbocharger to spin slower due to reduced airflow, leading to lower boost, richer mixture, and increased particulate matter (black smoke)

Answer

It increases boost pressure and reduces NOx emissions

Answer

It has no effect on turbocharger speed but increases EGR flow automatically

Answer

It increases turbocharger speed due to increased pressure differential across the turbine