Engine and Fuel Systems Flashcards

A diesel engine on a Class 8 truck exhibits intermittent low power complaints only under full load at highway speeds. Fuel pressure at the high-pressure pump inlet reads 45 psi at idle but drops to 18 psi during the load event. The OEM specification is a minimum 35 psi at all operating conditions. Which component failure is MOST likely causing this pressure drop?

Answer: A worn transfer pump (lift pump) unable to maintain adequate feed pressure under high-demand conditions

The transfer (lift) pump is responsible for supplying fuel to the high-pressure pump at adequate pressure under all operating conditions. At idle, demand is low and a worn transfer pump can still maintain pressure; however, under full load, the high-pressure pump requires maximum fuel flow, exposing the transfer pump's inability to keep up. A clogged high-pressure filter would cause a similar symptom but would typically also affect idle pressure. A pressure regulator fault would appear as a consistent, not load-dependent, issue. Air ingestion at the pickup would usually show as stalling or rough running rather than a clean pressure drop under load.

During an injector return flow (leak-off) test on a common rail diesel engine, injectors 1, 2, and 4 return approximately 15 mL over 30 seconds. Injector 3 returns 58 mL over the same period. What is the correct interpretation and next step?

Answer: Injector #3 has excessive internal leakage and must be replaced; excessive return flow reduces rail pressure and causes rough running on that cylinder

Injector leak-off (return flow) tests measure internal leakage past the injector's control valve and needle seat. A result of 58 mL versus 15 mL for the others is approximately 4× higher — far outside normal tolerance for most OEM specifications. Excessive return flow means fuel is bypassing the injection event and returning to tank, robbing rail pressure and resulting in a lean condition and misfire on that cylinder. This definitively condemns injector #3. While a cylinder contribution test can support the diagnosis, the leak-off result already isolates the faulty unit. A cracked body would cause external leakage, not elevated return flow.

A natural gas (CNG) powered transit bus engine is running rough and setting a P0301 misfire code on cylinder 1. A relative compression test shows all cylinders within 5% of each other. Spark plugs were replaced 10,000 km ago. The technician swaps the ignition coil from cylinder 1 to cylinder 3 and clears codes. After a road test, the misfire code follows to cylinder 3. What should the technician inspect NEXT?

Answer: Inspect the gas injector on cylinder 1 for a stuck-open or leaking condition causing an over-rich mixture

When a fault follows the swapped component (the coil moved to cylinder 3, and the misfire code moved to cylinder 3), that definitively confirms the coil is the root cause — the coil is faulty. However, the question asks what to inspect NEXT after this diagnostic step confirms the coil is bad. On CNG engines, a coil that is repeatedly damaged or failing on a specific cylinder is often a symptom of an underlying cause: a leaking or stuck-open gas injector creating an excessively rich charge that contaminates the plug and overloads the ignition system. Simply replacing the coil without investigating the CNG injector on cylinder 1 risks repeat failure. The cylinder cut-out test is redundant once the swap test has isolated the cylinder. The ignition module is not implicated — the fault followed the coil, not the module.

A technician is diagnosing a turbocharged diesel engine that exhibits black smoke at idle but clears under load. Boost pressure at full load is within specification. EGR is disabled for diagnostic purposes. Fuel trim data shows no commanded over-fueling. Which condition BEST explains black smoke at idle that clears under load?

Answer: A stuck-open EGR valve (despite being commanded closed) recirculating exhaust gases and displacing intake air at idle, reducing excess air ratio

Black smoke indicates incomplete combustion due to insufficient air relative to fuel — a low air-excess ratio (lambda <1). At idle, boost pressure is minimal so any air displacement is proportionally severe. An EGR valve stuck open (mechanically, not electronically) continues to recirculate exhaust gases even when commanded closed, displacing fresh intake air. At idle, the engine relies on naturally aspirated airflow; the displaced air causes a rich condition and black smoke. Under load, increased boost pressure overwhelms the displaced air volume, restoring adequate air-excess ratio and clearing the smoke. Worn injectors would typically cause smoke under load (higher injection pressure). A clogged DPF would cause backpressure affecting both idle and load. A wastegate stuck open reduces boost under load, which contradicts the specification-passing boost data.

During a high-pressure common rail (HPCR) fuel system diagnosis on a heavy truck, the technician observes that commanded rail pressure is 1,400 bar but actual rail pressure oscillates between 1,200 and 1,600 bar. Injector return flow values are normal for all cylinders. The pressure limiting valve (PLV) is new. What is the MOST likely cause of the rail pressure oscillation?

Answer: A high-pressure pump with a worn or damaged inlet metering valve (IMV) causing erratic volumetric output

Rail pressure oscillation (actual pressure swinging above and below commanded pressure) with normal injector return values points to the high-pressure pump's metering system. The inlet metering valve (IMV), also called the fuel volume control valve, regulates how much fuel is admitted to the pump's high-pressure section. A worn or damaged IMV cannot hold a precise duty-cycle-controlled opening, causing the pump to alternately over- and under-deliver fuel to the rail. This produces the characteristic oscillating pressure pattern. A faulty rail pressure sensor would produce erratic readings but the actual pressure would be stable (sensor fault ≠ system fault). A cracked rail would cause a consistent pressure drop, not oscillation. Excessive ECU-commanded injection duration would show as overconsumption and low rail pressure, not oscillation around the setpoint.

A 310T technician is performing a cylinder balance test on a 6-cylinder diesel engine using a scan tool. Cylinders 1 through 5 show fuel quantity corrections between +2 mg/stroke and -3 mg/stroke. Cylinder 6 shows a correction of -18 mg/stroke (the ECU is reducing fuel delivery significantly on #6 to achieve balance). What does this data indicate, and what should be tested FIRST?

Answer: Cylinder 6 injector is over-delivering fuel; perform an injector return flow test to check for a stuck-open needle

A cylinder balance test trims fuel delivery to equalize contribution across all cylinders. A large negative correction (-18 mg/stroke) on cylinder 6 means the ECU must drastically reduce fuel to that injector to match the power output of other cylinders — indicating cylinder 6 is naturally producing more power than commanded, which means it is receiving more fuel than commanded. This is the hallmark of an injector with a mechanically stuck-open or leaking needle that delivers excess fuel independent of electronic command. The return flow test will reveal excessive internal leakage confirming over-delivery. High compression would be a long-term/permanent condition unlikely to cause this magnitude of imbalance. An ECU masking a mechanical fault doesn't explain why reducing fuel creates balance; a burnt valve would reduce, not increase, cylinder output. A restricted return line would affect rail pressure globally, not one cylinder.