ASE Test Electronic Diesel Engine Diagnosis 3 — Questions and Answers
Question 1: An electronic diesel sets a DTC for low boost pressure under load. Which component should be inspected first?
- Turbocharger, boost sensor, and charge-air piping for leaks (Correct answer)
- The starter motor
- The fuel cap seal
- The cabin air filter
Correct answer: Turbocharger, boost sensor, and charge-air piping for leaks
Low boost codes are commonly caused by turbo wear, a faulty boost sensor, or leaks in the charge-air (intercooler) piping.
Question 2: What is the purpose of the manifold absolute pressure (MAP) sensor on a turbo diesel?
- To monitor boost/intake pressure for fueling and EGR control (Correct answer)
- To measure oil pressure
- To detect knock
- To sense steering angle
Correct answer: To monitor boost/intake pressure for fueling and EGR control
The MAP sensor reports intake manifold (boost) pressure so the ECM can manage fueling and emissions controls.
Question 3: A diesel exhibits white smoke at idle when cold but clears as it warms. What is the most likely benign cause?
- Incomplete combustion / unburned fuel during cold operation (Correct answer)
- A blown head gasket every time
- A failed alternator
- A slipping clutch
Correct answer: Incomplete combustion / unburned fuel during cold operation
Cold-start white smoke that clears with warm-up is typically unburned fuel from incomplete combustion before the engine reaches operating temperature.
Question 4: When verifying a MAF sensor concern on a modern diesel, what symptom commonly results from a contaminated MAF?
- Reduced power, derate, and incorrect EGR/fueling (Correct answer)
- Loud exhaust pop
- Steering wander
- Flickering headlights
Correct answer: Reduced power, derate, and incorrect EGR/fueling
A contaminated mass airflow sensor reports incorrect airflow, causing power loss, derate, and faulty EGR/fuel calculations.
Question 5: Which scan tool data PID best helps diagnose an EGR valve that is sticking open?
- Commanded vs. actual EGR position (Correct answer)
- Battery voltage
- Vehicle speed
- Fuel level percentage
Correct answer: Commanded vs. actual EGR position
Comparing commanded EGR position to actual feedback reveals a valve that fails to follow commands, such as sticking open.
Question 6: A diesel goes into reduced-power (limp) mode and the lamp is on. What is the proper diagnostic approach?
- Retrieve and analyze DTCs, then test the indicated system (Correct answer)
- Replace the ECM immediately
- Clear codes and return the vehicle
- Replace all injectors
Correct answer: Retrieve and analyze DTCs, then test the indicated system
Limp mode is triggered by a stored fault, so reading and analyzing DTCs before testing the specific system is the correct path.
Question 7: What does a slow or no glow-plug current draw on one cylinder typically indicate during testing?
- An open or high-resistance glow plug on that cylinder (Correct answer)
- A low coolant level
- A failed fuel pressure regulator
- A worn timing belt
Correct answer: An open or high-resistance glow plug on that cylinder
Little or no current draw on a single glow plug circuit indicates an open or high-resistance plug for that cylinder.
ASE Test Electronic Diesel Engine Diagnosis 3
An electronic diesel sets a DTC for low boost pressure under load.
Which component should be inspected first?