310T Engine and Fuel Systems 3 — Questions and Answers
Question 1: A diesel engine exhibits excessive white smoke at startup that clears after warm-up. What is the most likely cause?
- Worn piston rings
- Low compression in one cylinder
- Incomplete fuel combustion due to cold engine (Correct answer)
- Stuck open injector
Correct answer: Incomplete fuel combustion due to cold engine
White smoke at cold startup is typically unburned fuel vapor from incomplete combustion that dissipates as the engine reaches operating temperature.
Question 2: On a common rail diesel fuel system, what component maintains system pressure between injection events?
- Fuel lift pump
- High-pressure accumulator rail (Correct answer)
- Fuel pressure regulator valve
- Injection control pressure sensor
Correct answer: High-pressure accumulator rail
The high-pressure accumulator rail stores pressurized fuel and dampens pressure fluctuations to maintain consistent injection pressure.
Question 3: Which condition would cause a diesel engine to run rough and produce black smoke under load?
- Restricted fuel filter (Correct answer)
- Air in the fuel system
- Clogged EGR valve
- Leaking fuel injector return line
Correct answer: Restricted fuel filter
A restricted fuel filter starves injectors of fuel at high demand, causing incomplete combustion and black smoke under load.
Question 4: When performing a diesel engine compression test, a reading of 380 psi on one cylinder versus 480 psi on all others indicates:
- Normal variation within specification
- Worn or damaged rings or valves in that cylinder (Correct answer)
- Incorrect test procedure
- Faulty compression gauge
Correct answer: Worn or damaged rings or valves in that cylinder
A cylinder reading more than 10% below the average indicates internal sealing problems such as worn rings, damaged valves, or a blown head gasket.
Question 5: A technician notices the crankcase pressure is excessive and oil is pushed out through seals. What test should be performed first?
- Cylinder leakdown test (Correct answer)
- Fuel pressure test
- Coolant pressure test
- Turbocharger boost pressure test
Correct answer: Cylinder leakdown test
A cylinder leakdown test identifies which cylinders are passing combustion gases into the crankcase, causing excessive blow-by pressure.
Question 6: What is the purpose of the fuel return line on a diesel injection system?
- To supply additional fuel during high load
- To return excess fuel and cool the injectors back to the tank (Correct answer)
- To recirculate fuel through the water separator
- To bypass the fuel filter under high demand
Correct answer: To return excess fuel and cool the injectors back to the tank
The fuel return line carries excess fuel and absorbed heat from the injectors back to the fuel tank, cooling the injection components.
Question 7: A truck's engine idles smoothly but stumbles when throttle is applied quickly. On a diesel, this most likely indicates:
- Faulty throttle position sensor
- Turbocharger lag or restricted intake (Correct answer)
- Worn camshaft lobes
- Low engine oil pressure
Correct answer: Turbocharger lag or restricted intake
Stumbling under rapid throttle application in a turbocharged diesel often results from turbo lag or a restricted air intake causing a momentary lean condition.
310T Engine and Fuel Systems 3
A diesel engine exhibits excessive white smoke at startup that clears after warm-up.
What is the most likely cause?