310T Engine and Fuel Systems 5 — Questions and Answers
Question 1: What is the primary purpose of a diesel engine's unit injector (UI) system compared to a common rail system?
- Unit injectors use lower injection pressures for better atomization
- Each unit injector generates its own high pressure, eliminating a separate high-pressure pump (Correct answer)
- Unit injectors allow multiple injection events per cycle more easily
- Unit injectors are controlled mechanically without ECM input
Correct answer: Each unit injector generates its own high pressure, eliminating a separate high-pressure pump
In a unit injector system, the injector itself generates high pressure via a rocker-arm-driven plunger, unlike common rail which uses a central pump.
Question 2: A technician finds gray, milky residue on the oil filler cap of a diesel engine. What is the most probable cause?
- Condensation from short trip driving
- Coolant intrusion into the engine oil (Correct answer)
- Excessive fuel dilution of the oil
- Normal blow-by from worn rings
Correct answer: Coolant intrusion into the engine oil
A milky or gray emulsion on the oil cap indicates coolant mixing with engine oil, typically from a head gasket failure or cracked cylinder head.
Question 3: On a turbocharged diesel, what is the purpose of the charge air cooler (CAC or intercooler)?
- Cool the turbocharger bearing housing to prevent oil coking
- Cool the compressed intake air to increase air density and power (Correct answer)
- Reduce exhaust gas temperature before the DPF
- Pre-heat intake air for cold weather starting
Correct answer: Cool the compressed intake air to increase air density and power
The charge air cooler reduces intake air temperature after turbo compression, increasing air density so more oxygen is available for combustion.
Question 4: Which measurement tool is used to check diesel injection timing by detecting pressure rise in the fuel system?
- Vacuum gauge
- Dial indicator at the injection pump
- Diesel timing light with clamp-on transducer (Correct answer)
- Fuel pressure gauge with chart recorder
Correct answer: Diesel timing light with clamp-on transducer
A diesel timing light uses a clamp-on transducer on the injection line to detect the pressure pulse when injection occurs, indicating timing relative to crankshaft position.
Question 5: A coach engine develops a 'fuel knock' that is distinct from normal diesel combustion knock. What condition causes fuel knock?
- Over-advanced injection timing causing rapid pressure rise (Correct answer)
- Injector dribble depositing raw fuel that ignites late
- High cetane fuel burning too slowly
- Low engine oil pressure reducing injector lubrication
Correct answer: Over-advanced injection timing causing rapid pressure rise
Over-advanced injection timing causes fuel to ignite too early, creating an abnormally rapid pressure rise rate that produces a characteristic sharp knock.
Question 6: When replacing a diesel fuel injector, why must the injector hold-down torque be tightened to specification?
- To ensure the injector electrical connector seats properly
- To prevent combustion gas blow-by past the injector seat and seal (Correct answer)
- To align the injector spray pattern with the piston bowl
- To compress the fuel return fitting for a leak-free connection
Correct answer: To prevent combustion gas blow-by past the injector seat and seal
Proper torque ensures the injector sealing washer or copper crush washer seats correctly, preventing high-pressure combustion gases from escaping past the injector.
Question 7: A diesel engine's DPF (diesel particulate filter) requires frequent active regeneration cycles. What underlying condition most likely causes this?
- High engine load operation that burns soot quickly
- Excessive oil consumption or fuel contamination causing excess soot production (Correct answer)
- Cold ambient temperatures reducing exhaust temperature
- Using low-sulfur diesel fuel
Correct answer: Excessive oil consumption or fuel contamination causing excess soot production
Excessive soot from burning engine oil or fuel dilution overloads the DPF faster than normal, triggering more frequent regeneration events.
310T Engine and Fuel Systems 5
What is the primary purpose of a diesel engine's unit injector (UI) system compared to a common rail system?