ASE Practice Test (L3: Test Electronic Diesel Engine Diagnosis) 2

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?

Accepted Answer

The ECM may calculate excessive boost, reducing fueling to protect the engine, resulting in reduced power output

Answer

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?

Accepted 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

Answer

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?

Accepted Answer

The injector resistance is BELOW specification, suggesting a shorted injector winding — the injector should be replaced

Answer

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?

Accepted 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

Answer

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?

Accepted 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

Answer

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.