Troubleshooting, Fault Diagnosis, and Repair
Core concepts
Diagnosis starts with symptoms: loss of power, abnormal noise, vibration, overheating, excessive gland leakage, or poor steam economy. Candidates should link symptoms to causes: worn piston rings (blow-by, reduced power), scored liner (noise, oil consumption), valve timing errors (poor economy, rough running), water hammer (condensate), bearing wear (vibration, heat), and gland packing failure (leakage).
Use the indicator diagram as a diagnostic tool: a rounded admission line suggests restricted steam passages; a high compression line suggests excessive cushioning; a low expansion line suggests leakage. Repair methods include re-ringing, honing or reboring liners, re-packing glands, re-setting valve gear, and replacing bearings. Always verify root cause before repair; repeated failures often indicate alignment or lubrication issues.
Traps in questions
- Replacing rings without checking liner wear and ovality.
- Adjusting valve gear without confirming the true dead-centre position.
- Treating symptoms (noise) without finding the cause (misalignment).
- Ignoring indicator diagrams when diagnosing economy loss.
Worked example / rule of thumb
Rule of thumb: if power falls and exhaust is steamy or blow-by is heard, suspect ring/liner wear first. Compare indicator diagrams before and after repair to confirm improvement. Check alignment after bearing replacement; misalignment is a frequent cause of repeat failures.