Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisMedium
A client reports that their vehicle's engine runs smoothly at idle but develops a significant vibration and loss of power only when under load, such as climbing a hill or accelerating briskly. The vibration is directly felt from the engine and is RPM-dependent. A scan tool shows no misfire codes. What is the most likely cause?
- AA clogged fuel filter.
- BWeak valve springs.
- CWorn engine mounts.
- DA vacuum leak.
Show answer & explanationAnswer & explanation
Correct answer: A. A clogged fuel filter.
Engine problems that manifest only under load, such as a significant vibration and loss of power without misfire codes, often point to a fuel delivery issue. A clogged fuel filter restricts fuel flow, which becomes insufficient only when the engine demands more fuel under load, leading to a lean condition and potential misfires that are not always detected as codes immediately.
Why the other options are wrong
- B. Weak valve springs would typically cause valve float at higher RPMs, leading to a loss of power and potentially misfires, but often with more distinct engine noise or erratic vacuum fluctuations.
- C. Worn engine mounts would cause vibration at all times or under specific conditions (e.g., idle), but usually wouldn't cause a loss of power or a vibration that only manifests under engine load.
- D. A vacuum leak would typically cause a rough idle and potentially lean codes, but usually not a severe vibration and power loss only under heavy load.
Engine Issues Under Load
Diagnosing engine performance problems (vibration, power loss) that occur specifically when the engine is under a heavy load, often indicating a fuel delivery or air restriction issue.
- Load-dependent issues often relate to fuel or air supply.
- Clogged fuel filter limits fuel flow at high demand.
- Can cause lean misfires not immediately coded.
Memory trick: Under load, the weakness is showed.