How far do you take that allowance for use?
How would you compare total revolutions at idle in traffic, vs total revolutions at speed on a motorway.
Toyota engineers most things with a large safety margin built in to allow for worst case use.
In later iterations of these engines, timing belt service interval was increased to 150k km.
Of course someone had to ask, and to be fair regardless of who does it is a ridiculous question, this is completely subjective, how do you allow for the environment the car is used in? This is much in the same way a tosser of a mechanic tells a customer he has about 2,000 miles left on his brake pads! Is the car manual or automatic? What if the car only does motorway use as opposed to being in town? Does the owner live in a mountainous area? So for sure a ridiculous thing to say to a customer about his brakes much in the same way about asking the life of a timing belt, and just in case you are going to ask the question, the answer is no, you cannot reliably check the condition or possible life of the belt whilst in situ, and if you have gone to all the work to remove it, the answer is yes, you would replace it.
But to give you a mechanics experience/insight but please allow for cloudy memory, Fords for example years back were recommending something like a 80,000 mile belt interval change after engine testing the 1.8 diesel pre production engines (going in
Ford vehicles) and were destroying their belts at 90,000 miles! The same engines going into Mazda's was the same 90,000 miles. So Ford and Mazda start the production run with Ford belt changes recommended at something like 70,000 but Mazda intervals were something like 60,000 miles! The engines in Fords were breaking their belts much earlier than the service interval so they changed the width of the two belts (only one on the 1.6 engine albeit almost identical in design) to extend the service interval and they still broke earlier than the recommended service interval, it later turns out after more early breakages that the production engines going in Fords vehicles had nylon toothed idler gears, the original pre-production units had all steel gears however, Mazda (owned by Ford) had steel gears as well but still with lower service intervals to avoid the risk of failure, so things are not as clear as you would think.
And to answer the extended service intervals you mention is a lot easier and simpler to understand than you would think as well, for example, better lubrication of camshaft bearing surfaces with the higher quality of oils available today, redesigning the camshaft profiles, perhaps thinner bearing surfaces adding to the reduction in overall friction, even the slightest change in the camshaft belt idlers position can ease the angle the belt changes during running, also note that belt manufacturing tolerances and changes in materials have improved, timing belt teeth were originaly square edged to match the square segments in the timing wheels, now the teeth tend to be rounded, as does the segments in the wheels, this allows for a smoother transition into mesh again reducing contact wear, and now the camshaft driving wheels are slotted allowing for more accuracy with the timing and using automatic tensioners that allow for changes in belt tension as the engine block 'grows' as it heats up, all this would aid in extending a belts life, even better oil seal design to reduce seepage or 'sweating' can increase the belts life with less contamination. Look at the change of many vehicles that have fuel injectors that now work from an electrical pump means the work the mechanical pump had to do is no longer there, so again different engines loads with more or less changes the life expectancy of something as pliable as a timing belt.
As an aside, most vehicles these days have the water pump driven from the camshaft belt, it takes the load off the auxiliary belt (used to be called a fan belt for our younger readers) to allow for more powerful alternators to drive electric steering pumps and the extra gadgets vehicles are fitted with, you would be blown away when I explain to a customer that the tensioners, idlers
and the water pump has to be changed when changing the timing belts and of course they don't agree, I don't get into the arguments about not warrantying the belt if I don't fit the 'full kit', I simply tell them to go and find another mechanic.....politely of course.
Regards
Dave
