TM 10-3930-653-14&P 4-2-2. TROUBLE DIAGNOSES AND CORRECTIONS Condition Probable cause Corrective action I.  Noisy engine Knocking of crankshaft Loose main bearing Replace. and bearing. Seized bearing. Replace. Bent crankshaft. Repair or replace. Uneven wear of journal. Correct. Excessive crankshaft end play. Replace center thrust bearing. Piston and connecting Loose bearing Replace. rod knocking Seized bearing Replace. Loose piston pin. Replace pin or bushing. Loose piston in cylinder Recondition cylinder Broken piston ring. Replace Improper connecting rod alignment Realign. Camshaft knocking Loose bearing. Replace. Excessive axial play. Replace bearing thrust plate Rough gear teeth. Repair Broken cam gear Replace Timing chain noise Improper chain tension Adjust Worn and/or damaged chain. Replace Worn sprocket. Replace Worn and/or broken tension adjusting Replace mechanism. Excessive camshaft and bearing clearance Replace. Camshaft and valve Improper valve clearance Adjust mechanism knocking Worn adjusting screw. Replace Worn rocker face Replace. Loose valve stem in guide. Replace guide. Weakened valve spring Replace. Seized valve Repair or replace. Water pump knocking Improper shaft end play. Replace. Broken impeller Replace. II.  Other mechanical troubles Sticking valve Improper valve clearance. Adjust Insufficient clearance between valve stem Clean stem or ream guide and guide. Weakened or broken valve spring Replace Seized or damaged valve stem. Replace or clean Poor fuel quality. Use better fuel. 4 - 21