Help with focus tune-up

I am going to do a tune-up on my 2000 Ford Focus. It is making a horrible missing noise. It has 89000 miles at present. I believe I might have a bad
spark plug wire. I would be interested to hear advice from other owner's on their own tune-ups
Tia,
Larry
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If you have an ohmeter you can ohm the sparkplug wires and determine if one is higher than the others. You will need to use the recommended plug and set the gap with a gap gauge. Gary

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Thanks for the advice. In the past we have observed an spark in one of the plug wires and the car sometimes will start run very smooth after about 30 miles.

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