Automotive Service Excellence (ASE) A8 Engine PerformanceGeneral Engine DiagnosisHard
A technician is performing an engine condition diagnosis. The vehicle starts and runs, but the engine experiences a severe, consistent vibration that feels like a 'jackhammer' and is directly proportional to engine RPM. A cylinder power balance test shows that cylinder #5 is completely dead, with no RPM drop when its spark is disabled. What is the most likely cause?
- AA severely bent connecting rod on cylinder #5.
- BA clogged catalytic converter.
- CA faulty fuel pump.
- DA worn crankshaft main bearing.
Show answer & explanationAnswer & explanation
Correct answer: A. A severely bent connecting rod on cylinder #5.
A severe, jackhammer-like vibration directly proportional to engine RPM, combined with a completely dead cylinder #5, strongly suggests a major mechanical failure within that cylinder. A severely bent connecting rod would cause the piston to not move correctly or at all, leading to zero compression, no power contribution, and a significant engine imbalance and vibration.
Why the other options are wrong
- B. A clogged catalytic converter would cause a loss of power and potentially a rough idle, but not typically a localized 'jackhammer' vibration from a specific cylinder.
- C. A faulty fuel pump would affect all cylinders or cause general fuel starvation, not a specific dead cylinder with a localized 'jackhammer' vibration.
- D. A worn crankshaft main bearing would cause a deep 'thump' or 'knock' from the bottom end, not a 'jackhammer' vibration tied to a specific dead cylinder.
Engine Vibration - Severe Mechanical
Diagnosing severe, localized engine vibrations that are indicative of major internal mechanical damage within a specific cylinder.
- Severe, RPM-dependent vibration often points to internal component failure.
- Combined with a dead cylinder, suspect piston/rod issues.
- Requires internal inspection for confirmation.
Memory trick: Engine's deep groan, internal troubles are shown.