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I'm about to change spark plugs on my car, and given the cost of getting just the plugs or plugs + cables, I'm considering just keeping the current ones.

The car runs fine (well, it's consuming way too much fuel), so I'm interested in knowing what kind of wear they go through that would make them fail or degrade performance.

The car is a 2010 Fiat Uno 1L, possibly with original spark plugs and about 46k mi

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Spark plug cables can suffer from a few different things:

  • cumulative heat stress from being near the engine
  • cumulative electrical stress from the high voltages they carry
  • chafing against nearby things

Normally, I inspect them whenever I replace the plugs. Look for cracks or breaks in the outside insulator, damage to the boot, and if you have a multi-meter you can test their resistance.

  • So, if the resistance is within spec and they don't have any cracks, changing them would merely be a preventative measure? – Délisson Junio Jan 19 '17 at 11:01
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    More like a waste of money :) I only replace spark plug cables if there's something wrong with them. – tlhIngan Jan 19 '17 at 11:09
  • Could you point to where one would find the resistance rating? – rana Jan 19 '17 at 20:35
  • A repair manual for your specific make and model (Haynes, Chilton) – tlhIngan Jan 20 '17 at 1:00
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The most common type of spark plug cables is carbon core wires. In this kind the carbon eventually burns away and the wires can no longer transmit a good signal. If you don't have carbon wires, the main thing that could go wrong would just be a hole in the insulation or boot which could be caused by any number of things. Test your wires' resistance with a multimeter before you replace them. Another thing you can do to see if they are bad is wait until its dark and then spray the wire and boots with a water bottle. Any sparks arcing out of the wires or boots means they are bad.

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I've had the connector that mates to the top of the plug fail before. This caused an intermittent misfire and general poor running that weren't fixed by new plugs -- then I looked at the ends of the cables and saw why.

This was in a 90s car so things might have changed since then (it was the last petrol car I did much work on).

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