How to replace the spark plug wires (ignition harness)

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  • OVERALL DIFFICULTY INDEX: 2/10
  • NO TOOLS NEEDED FOR THIS JOB
  • IF YOU HAVE NO EXPERIENCE YOU SPEND 45 MINUTES TO CARRY OUT THIS JOB
  • RISK OF BURNS, SEE BELOW
BEFORE YOU START
  • Be careful not to touch any hot part of the engine.
  • Always work on cold engines
  • Make sure the ignition key is on the engine stop position.
  • Cylinder No1 is the one to the front of the vehicle on longitudinal engines and the one nearby the gearbox on transverse engine mounts.
  • Watch well the wires length and to which cylinder each goes for an easier refit of the new ones.
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Symptoms of worn spark plug wires (ignition harness)

The symptoms of worn or damaged spark plug wires are as follows:
  • Misfires after engine warms up or while driving
  • Sudden misfires that occur randomly from time to time
  • Rough idle 
  • Engine hesitation upon accelerating
  • In come cases, engine won't start at all
You can inspect your spark plug cords one by one and check for any trace of spark on their boot or measure their resistance to see if any wire returns a much different value and proceed to the replacement of the defective wire, or simply change the full ignition harness, which is strongly recommended if your spark plug wires have more than 30,000 miles.

Spark plug ignition cords replacement

Start from the distributor side.
Pull back the distributor soft cover if it has one.
​Prepare stickers with as many numbers as your cylinders.​
Pull out any wire and follow it to see to which spark plug it is connected.  
​Stick the corresponding cylinder number on the distributor cover.
​Repeat the same for all cylinders. One can read with no surprise the classic firing order on this 4 cylinder engine 1-3-4-2.
Pull out the ignition wire from the ignition coil, fit the new spark plug wires and reassemble. 

See also our other tutorials in relation with ignition problems...

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