Engine and Fuel Systems Flashcards

A diesel engine on a Class 8 truck exhibits black smoke under load but clears at idle. Fuel pressure and injector return flow tests are within spec. A compression test shows all cylinders within 10% of each other. Which condition is MOST likely causing this symptom?

Answer: A restricted charge air cooler reducing boost density at high load

Black smoke under load that clears at idle indicates a rich air-fuel ratio specifically when demand is high. A restricted charge air cooler (CAC/intercooler) reduces the density and volume of intake air reaching the cylinders under boost conditions, creating a fuel-rich mixture and black smoke. Compression is acceptable, injectors are fine, and the symptom being load-dependent points to the air-supply side, not fuel-side over-delivery.

During a diesel high-pressure common rail (HPCR) system diagnosis, a technician finds that fuel trim data shows a large positive correction on cylinders 3 and 4 only. Cylinder contribution tests confirm low power from these cylinders. Injector return-leak tests show cylinders 3 and 4 have significantly LOWER return flow than the others. What is the MOST accurate interpretation?

Answer: Injectors 3 and 4 have worn nozzle tips causing restricted fuel delivery

In HPCR systems, low return flow from an injector indicates it is not leaking internally — the control valve is holding. However, combined with low cylinder contribution and a positive fuel trim correction on those cylinders, this points to restricted nozzle orifices (worn or partially clogged tips) that limit the actual fuel mass injected despite the ECM commanding more fuel. High return flow would indicate a leaking control valve. The pump affects rail pressure globally, not cylinder-selectively.

A technician is performing a cylinder cutout test on a 6-cylinder diesel. Cylinders 1, 2, 4, 5, and 6 each produce a 12–15 RPM drop when cut out. Cylinder 3 produces only a 4 RPM drop. Compression on cylinder 3 is 520 psi (within the manufacturer's minimum). Injector return flow is normal. What should the technician suspect FIRST?

Answer: A defective injector with a partially blocked nozzle on cylinder 3

A low RPM drop during cylinder cutout means that cylinder is contributing little power — cutting it out doesn't change engine speed much because it wasn't doing much work. Compression meets minimum spec, so major mechanical failure is less likely. Normal injector return flow rules out a stuck-open control valve. A partially blocked nozzle would restrict atomization and fuel quantity delivered, reducing that cylinder's power contribution without flagging compression or return flow tests. This is a classic nozzle restriction scenario.

A natural gas (CNG) coach engine misfires intermittently at operating temperature but runs smoothly when cold. No fault codes are stored. Fuel system pressure at the regulator outlet is stable. Which diagnostic step would MOST directly isolate a temperature-dependent fuel delivery fault?

Answer: Perform a cylinder balance test cold and compare contribution values to a hot retest

Because the fault is temperature-dependent and intermittent with no stored codes, the goal is to capture data during the actual fault condition. Performing a cylinder balance (contribution) test when cold and then repeating it once the engine is at operating temperature and misfiring directly compares each cylinder's contribution in both states, pinpointing which cylinder(s) degrade with heat. This could indicate a thermally expanding component (valve, injector seat) or a sensor drifting with heat. Static tests at ambient temperature will not reproduce a hot-only fault.

A heavy-duty diesel uses a Variable Geometry Turbocharger (VGT). The engine produces low power and excessive black smoke. Boost pressure reaches only 18 psi under full load where 28 psi is expected. The VGT actuator moves freely across its full range when commanded by a scan tool. Exhaust backpressure measured upstream of the turbocharger is within specification. What is the MOST likely root cause?

Answer: Sticking VGT vanes that are mechanically seized in the high-flow (low-boost) position despite actuator movement

The VGT actuator rod moves freely when commanded in isolation, but this does not confirm the vanes inside the turbine housing are actually responding. Carbon and soot buildup commonly seizes the variable vanes in the open (high-flow, low-boost) position. The actuator mechanism can move but if the linkage is disconnected or the vanes are seized, boost won't build. Exhaust backpressure being normal rules out a downstream blockage. A cracked housing would typically produce audible leaks and codes. A MAP sensor fault would trigger a code. Compressor wear would show consistent low boost across all conditions, not a dramatic deficit.

A technician finds a diesel engine with a fuel system that uses a mechanical unit injector (MUI) design. The engine has low power and a misfire on cylinder 5. Adjustment of the injector timing on cylinder 5 to the manufacturer's specification does not resolve the complaint. A cylinder contribution test shows cylinder 5 still low. The injector is replaced with a new unit and retimed — the problem persists. What should the technician investigate NEXT?

Answer: The camshaft lobe for cylinder 5 for abnormal wear or damage

In a mechanical unit injector system, the injector is actuated by the camshaft through a rocker arm — there is no electronic solenoid controlling injection quantity; the cam lobe profile and lift determine injection timing and duration. Replacing the injector and correctly retiming it eliminates the injector and timing adjustment as causes. Since the fault persists with a new, properly timed injector, the mechanical drive mechanism must be at fault. A worn or damaged camshaft lobe on cylinder 5 would reduce injector push-rod travel, decreasing injection quantity and producing the observed low contribution. MUI systems have no ECM trim files for individual injectors.