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Why replace a part that is working well?
The quality of timing belts means they are not cheap, and because of their cost and importance it is usual for the bearings and idlers that assist them to be changed as well.
Car manufacturers recommend that timing belts should be changed after a certain mileage even if they are still working perfectly well. Why? KH
Several parts of your car require “preventive” maintenance (or replacement) while they are still working well and have no overt signs or consequences of defect.
Common examples are engine oil, the filters for oil, fuel and air, and greasing. Their lifespans are relatively short and they are inexpensive and quick to change, so you don’t wait for them to start causing problems. Most motorists will check and change them as routine service items. Two or three times every year.
Other things that “wear out”, but still work, include spark plugs, tyres, brake linings, fanbelts, shock-absorbers, bushes, ball-joints, bearings and clutch plates etc.
But they age progressively, usually not suddenly, and they are either easy to see or have distinct warning symptoms for some time before they completely fail.
Timing belts are different in several respects. They are designed to last a lot longer and are not routinely checked; they are out of mind and out of sight; they can fail suddenly without clear warning; and if they do they can cause instant and expensive engine damage.
Their main job is to operate and co-ordinate the opening and closing of the engine’s inlet and exhaust valves, in split-second harmony with the up-and-down movements of the pistons.
That’s happening several thousand (!) times per minute, to usually four pistons driven by the crankshaft, and eight valves operated by the camshaft. They are synchronised by the timing belt which directly connects a cog on the driven crankshaft to a cog on the camshaft.
If the belt breaks, the camshaft has no power source and stops, so the valves are left in a random arrangement of open and closed positions that no longer tally with what the pistons, fuel injection and ignition sparks are doing or require.
Induction strokes get no fuel, compression strokes aren’t enclosed and force huge pressure the wrong way up inlet channels, and exhaust strokes have nowhere to escape.
The problem only lasts a few seconds because piston conrods get bent, piston crowns smash against valves, valve shafts are buckled and might be rammed into the valve gear… all with such force that if a conrod snaps it can be blasted through the cast iron wall of the engine block like a cannon ball.
The final degree of damage will vary from case to case, depending on what the car was doing when the belt snapped, and on whether the engine design is “interference” or “non-interference” (see adjacent explanation of that). But some severe and expensive damage is almost inevitable.
Knowing the consequences, manufacturers do what they can – through materials, design, specification and quality control - to ensure timing belts do not break.
They establish a life-span during which the risk of a spontaneous failure is virtually nil. And that is the “belt change” limit they set (commonly 100,000 kms). They know that in normal use the belt might last twice or even three times that long, but that the risk of failure increases – at first slightly but then exponentially - beyond the super-safe benchmark.
If your dashboard has a timing belt warning light, what triggers it? Probably not a condition assessment. More likely just a reminder of the mileage it has done. The belt probably – but not certainly – has a lot more life in it.
Symptoms of impending failure “might” include a ticking sound in the engine, hard starting and perhaps misfire, but sudden breakage without warning is also possible.
The quality of timing belts means they are not cheap, and because of their cost and importance it is usual for the bearings and idlers that assist them to be changed as well.
The total cost can be tens of thousands of shillings, so the balance between cost and risk can depend on whether the car is worth millions or peanuts.
The age of the timing belt is something you should consider when buying a used car, and if that is unknown at least have the belt visually checked when you first have it serviced.