1) Purpose and failure modes (theory)
- What the timing belt does: it mechanically links crankshaft rotation to camshaft (and on some diesel variants the injection pump), prescribing valve opening/closing and (if driven) injection timing. Accurate phase keeps piston motion synchronous with valves and injection.
- How it fails: age, heat, oil contamination, mis-tension, worn idlers/tensioner, or seized water pump cause tooth wear, stretching, cracking or skipped teeth. Consequences: timing drift (rough running, loss of power, hard start), sudden tooth skip (misfire, run-on), or catastrophic valve/piston contact on interference engines. Replacing the timing belt restores fixed mechanical phase and prevents these failures.

2) Preparations (theory)
- Tools, replacement parts: correct OEM timing belt, tensioner(s), idler(s), water pump and seals, crank/cam seal(s) if leaking, service manual for timing marks and torques. The theory: replacing ancillary wear items prevents a new belt being ruined soon after install.
- Safety: disconnect battery, work with engine cool and supported. Theory: prevents unintended crank rotation, electrical hazards, and injury.

3) Set engine to known reference (TDC on #1 compression) (theory + action)
- Action: remove spark plugs/glow plugs or remove fuel pump power for safe turning; rotate crank by socket on crank pulley to bring TDC mark to timing cover “0” or TDC index. Verify cam mark aligns to its reference. For diesel, ensure injection pump timing is not disturbed (lock or mark pump if driven).
- Theory: starting from a known phase prevents a mismatched reassembly that would leave valves/pistons out of sync — the core goal is to preserve or re-establish exact cam/crank phase.

4) Mark existing alignment and belt orientation (theory + action)
- Action: with covers still on, mark crank, cam (and injection pump if belt-driven) sprocket positions with paint/chalk; mark belt rotation direction.
- Theory: if belt is reused briefly for reference, marks ensure you restore original phase. Direction matters because belts stretch asymmetrically; replacing with a new belt you’ll still want correct routing and rotational orientation for idler geometry.

5) Remove accessory belts, pulleys, and timing cover (theory)
- Action: remove drive belts, harmonic balancer/crank pulley if needed, and timing cover(s) to expose belt and sprockets.
- Theory: access is required to see/align marks and to change belt and ancillary parts.

6) Hold components and relieve tension (theory + action)
- Action: lock camshaft(s)/pump if service manual specifies, loosen tensioner to release belt tension, note slack side.
- Theory: controlled tension release prevents sudden rotation of cam/crank which could cause valve damage and preserves alignment so you can remove the belt safely.

7) Remove belt; inspect sprockets and components (theory + action)
- Action: remove belt; inspect teeth, sprockets, cam/crank keyways for wear, and check tensioner/idler bearings for roughness; inspect water pump for play/leakage and seals for oil leakage.
- Theory: worn sprockets or rough bearings will shorten the life of a new belt or cause it to fail prematurely. Water pump failure is a common cause of timing belt replacement because a leaking/seized pump can quickly damage the belt.

8) Replace tensioner, idler(s), water pump and seals (theory + action)
- Action: install new tensioner and idlers, replace water pump and front crank/cam seals if needed; torque bolts to spec.
- Theory: the tensioner maintains correct belt preload; a worn tensioner leads to slack, tooth skip, and accelerated wear. New idlers ensure smooth tracking. Water pump replacement prevents coolant leaks that contaminate and degrade the belt. New seals prevent oil contamination which causes belt swelling and deterioration.

9) Install new belt with correct routing and tensioner pre-load (theory + action)
- Action: align crank and cam marks to TDC, route new belt keeping teeth fully engaged and taut on the non-tensioned side first, apply specified pre-tension per manual (spring or hydraulic tensioner set procedure).
- Theory: the non-tensioned (slack) side controls phase while the tensioned side takes up slack; installing correctly ensures the belt teeth seat properly on sprockets so the cam timing equals the crank position. Proper pre-load prevents initial jump or tooth lash. For spring/hydraulic tensioners, follow manufacturer preload procedure (some require releasing a locking pin after belt in place).

10) Verify timing and rotate engine by hand (theory + action)
- Action: with the belt installed and tension set, rotate the crankshaft two full revolutions clockwise by socket and recheck alignment marks and belt tension. Re-inspect cam timing marks at TDC.
- Theory: rotating through two revolutions checks for skipped teeth and confirms there is no valve-to-piston contact, and that dynamic tensioning is stable. If marks don’t realign, loosen and re-check installation — do not start engine.

11) Final torques, reassembly, and leak checks (theory)
- Action: torque all bolts to spec, reinstall timing covers, harmonic balancer, accessory belts, reconnect battery, refill coolant if water pump changed.
- Theory: correct torque prevents fastener loosening that can change tension or alignment; reassembly restores normal accessory drive and cooling. Replacing coolant avoids introducing air pockets that can cause pump cavitation.

12) Start and observe (theory + action)
- Action: start engine and listen for abnormal noises; check for coolant or oil leaks, verify steady idle and no misfires. After a short run, recheck tensioner and belt condition as some tensioner types settle.
- Theory: a properly timed engine will idle smoothly and produce expected power; noises or rough running indicate timing issues, tensioner failure, or missed marks.

How the repair fixes the fault (concise)
- Symptom: worn/failed timing belt causes phase drift, tooth skipping, or sudden breakage.
- Fix: replacing the belt restores the precise mechanical phase relationship between crank and cam (and injection pump if driven), eliminating timing drift and preventing tooth skip/break. Replacing tensioner/idlers restores stable preload and tracking so the new belt keeps correct timing. Replacing water pump/seals removes contamination and failure sources that would otherwise soon damage the new belt. Verification by rotating and testing assures there is no interference or misalignment before engine operation.

Key theoretical checks and reasons not to skip them
- Always set to true TDC on compression stroke: avoids ambiguous cam alignment (cam may be near mark at different piston stroke).
- Rotate by hand twice and recheck marks: catches off-by-one-tooth errors and reveals binding before valves/pistons are under combustion loads.
- Replace tensioner/idlers/water pump together: they are the limiting reliability items; mixing old with new reduces overall life.
- Follow exact tensioning method for that tensioner type (spring vs hydraulic vs manual adjust): under- or over-tension both shorten belt life and can alter timing.

Safety note (brief)
- If uncertain about marks or torque specs, consult the Toyota service manual. On any engine where valves can contact pistons (possible depending on engine variant), incorrect timing can cause severe engine damage.
rteeqp73

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