when do spark plugs need replacing?
between 30,000 and 160,000 kilometres, which is why the advice is confusing: the answer depends entirely on what the plugs are made of.
the three types
copper. the cheapest and the shortest lived, roughly 30,000 to 50,000 kilometres. mostly older vehicles.
platinum. around 100,000 kilometres.
iridium. 100,000 to 160,000 kilometres, and what most modern engines come with from the factory.
fitting copper plugs to an engine designed for iridium saves money once and costs it three times over, because you are back in there every 40,000 kilometres.
the signs they are due
- rough idle, or a misfire you feel as a stumble
- hard starting, particularly on a cold morning
- fuel economy drifting down with no other explanation
- a flashing check engine light, which means an active misfire and means stop driving it: unburned fuel is going into the catalytic converter, and that is an expensive part to destroy
- hesitation under acceleration
why the labour matters more than the plugs
on an older four cylinder, plugs are a straightforward job. on plenty of modern engines they are not: some V6 layouts need the intake manifold removed to reach the rear bank, and the labour is most of the bill.
that is worth knowing when you compare quotes, because a price that seems high for "just spark plugs" may be entirely reasonable for the engine you have.
do not stretch them too far
plugs that stay in far too long can seize in an aluminium cylinder head, and extracting a seized plug is a much bigger job than replacing one. on some engines it means removing the head.
the interval exists for that reason as well as for how the engine runs.