ASE Practice Test (L3: Test Electronic Diesel Engine Diagnosis) 2 — Questions and Answers
Question 1: A diesel engine's ECM has a DTC for a MAP (manifold absolute pressure) sensor that reads higher than expected. What would be the effect on engine operation?
- The engine would run lean and produce white smoke
- The ECM may calculate excessive boost, reducing fueling to protect the engine, resulting in reduced power output (Correct answer)
- The engine would immediately shut down
- No effect — MAP sensor data is not used for fueling calculations
Correct answer: The ECM may calculate excessive boost, reducing fueling to protect the engine, resulting in reduced power output
The MAP sensor provides intake manifold pressure data that the diesel ECM uses to calculate air mass (along with temperature and engine speed) for proper fueling. If the MAP reads abnormally high (suggesting excessive boost), the ECM may interpret this as a turbocharger overboost condition and reduce fuel delivery to protect the engine. The result is lower power, which is the limp-home strategy. Verifying actual boost with a calibrated gauge and comparing to MAP sensor data confirms sensor accuracy.
Question 2: During diesel engine diagnosis, a technician uses a scan tool to view "relative compression" data. What does this data reveal?
- The absolute PSI in each cylinder measured during cranking
- Cylinders with lower contribution to cranking speed — lower compression cylinders slow down engine cranking speed during their compression stroke, revealing weak cylinders without a compression gauge (Correct answer)
- The ignition timing of each cylinder
- The fuel delivery volume to each injector
Correct answer: Cylinders with lower contribution to cranking speed — lower compression cylinders slow down engine cranking speed during their compression stroke, revealing weak cylinders without a compression gauge
Relative (or running) compression testing uses the scan tool and crank sensor data to measure how much each cylinder resists engine rotation during cranking. A cylinder with low compression resists less, so the engine cranks faster through that cylinder's compression stroke. The scan tool plots cranking RPM fluctuations. Cylinders that cause less cranking resistance show as weak cylinders. This non-intrusive test quickly identifies compression problems without removing glow plugs or injectors.
Question 3: A technician finds that a diesel injector contributes code P0201 (Injector Circuit Open – Cylinder 1). Using a digital multimeter, the technician measures the injector solenoid resistance at 0.3 ohms. Specification is 0.4–0.8 ohms. What does this indicate?
- The injector is within specification and likely the ECM is faulty
- The injector resistance is BELOW specification, suggesting a shorted injector winding — the injector should be replaced (Correct answer)
- The resistance is within normal variation — no issue
- The wiring harness between ECM and injector is open
Correct answer: The injector resistance is BELOW specification, suggesting a shorted injector winding — the injector should be replaced
Injector solenoid resistance specifications define the allowable range. A reading below the minimum spec (0.3 ohms vs. 0.4 ohm minimum) indicates internal winding resistance is too low — the coil is partially shorted. While an open circuit (infinite resistance) would cause a P0201 open code, a shorted winding increases current draw, which can also trigger codes and damage the ECM driver circuit. The injector must be replaced. The harness should also be checked for chafing that could cause a partial short.
Question 4: What does the "smoke switch" or "engine derate" strategy in a diesel truck ECM do when an EGR fault is detected?
- Immediately shuts the engine off to protect emission components
- Limits maximum fuel delivery and torque output to keep emissions within limits during the fault, while allowing the vehicle to continue operating in a reduced-performance mode (Correct answer)
- Adds extra fuel to burn off the EGR valve deposits
- Disables all emission-related sensors only
Correct answer: Limits maximum fuel delivery and torque output to keep emissions within limits during the fault, while allowing the vehicle to continue operating in a reduced-performance mode
Modern diesel ECMs manage emission system faults through derate strategies rather than immediate shutdown (except in extreme cases). When an EGR fault is detected, the ECM limits maximum fuel delivery (torque derate) to reduce combustion temperatures and NOx output, keeping the vehicle legally drivable to a repair facility while protecting emission components. More severe faults may trigger a multi-stage derate that progressively limits vehicle speed. This is also called "limp home" mode.
Question 5: Which diagnostic procedure is used to identify a misfiring diesel cylinder when a scan tool does not show individual cylinder contribution data?
- Remove all glow plugs and check each for carbon fouling
- Perform a "cut-out" test — disable individual injectors one at a time via the scan tool and note which cylinder's disabling causes no change in RPM or smoothness (Correct answer)
- Measure exhaust gas temperature at each cylinder's manifold runner with an infrared thermometer
- Remove and flow-test all injectors on a bench
Correct answer: Perform a "cut-out" test — disable individual injectors one at a time via the scan tool and note which cylinder's disabling causes no change in RPM or smoothness
The cylinder cut-out test (injector disable test) commands the ECM to disable one injector at a time while the engine idles. When a normally contributing injector is cut, the engine should stumble or RPM should drop. If disabling an injector causes NO change in engine operation, that cylinder was already not contributing — identifying the dead or misfiring cylinder. This test can be performed on most modern diesel trucks via a factory or professional scan tool without disassembly.
Question 6: A diesel technician is performing an aftertreatment DEF (diesel exhaust fluid) system diagnosis. The vehicle has a code for "DEF quality low." The tank was recently filled. What should be checked?
- The DEF injector nozzle for clogging only
- The DEF quality/concentration sensor (NOx or refractive index sensor), and verify the DEF quality by testing its urea concentration — DEF should be 32.5% urea solution (Correct answer)
- The diesel fuel quality at the tank
- The DPF differential pressure sensor
Correct answer: The DEF quality/concentration sensor (NOx or refractive index sensor), and verify the DEF quality by testing its urea concentration — DEF should be 32.5% urea solution
DEF (AdBlue) must be precisely 32.5% urea solution for SCR (selective catalytic reduction) to work correctly. Diluted or contaminated DEF (e.g., water mixed in, or DEF substituted with another fluid) will not provide adequate NOx reduction. The DEF quality sensor detects incorrect urea concentration. A "DEF quality low" code with a recently filled tank suggests the wrong fluid or contaminated DEF was added. Testing DEF concentration with a refractometer confirms the actual urea percentage before replacing the sensor.
ASE Practice Test (L3: Test Electronic Diesel Engine Diagnosis) 2
A diesel engine's ECM has a DTC for a MAP (manifold absolute pressure) sensor that reads higher than expected.
What would be the effect on engine operation?