Okay, my plugs in my '98 Contour are, no doubt, in need of changing (109K mi and presumably original). This model is distributorless, but not coils over plugs.
Okay, so I’ve grown accustomed to replacing the wires, too. But these wires are actually expensive, and as far as my electrical theory goes, engine spark is “pass/fail”: either you can produce enough volts to jump the gap, or you can’t: the strength of the spark is determined by the gap size.
Considering that an OBD-II car will produce a code in the event of a “miss,” is “breakdown maintenance” acceptable practice here?
You will get different answers,if the wires pass a visual inspection,no cracks,not oil soaked,held correctly in their holders I don’t mess with them.
What may happen is when you go to pull the wire off the plug you damage the wire,bummer, try all the “make it easy” techniques mainly twisting and grabbing on the boot not the wire. I have never had much luck with special pliers to get the wire off undamaged but they do exist.
If a wire does fail it’s not a can’t drive another mile situation,you do want to minimize driving with a misfire (cat damage)
The OBDII engine management system will NOT set a DTC (Diagnostic Trouble Code) if there is a misfire, until the misfire reaches enough to cause emissions to increase to 150% of maximum permitted values.
Since most people don’t have the equipment to check for how much misfire is occurring, they choose to change the spark plug wires at a “best guess” period. 100,000 miles isn’t too soon.
I would replace the wires. If I am reading your question correctly this car has 109,000 miles on the plugs and wires. It's time. Likely you will not need to replace them again for a long time, but if you don't replace them now, you will likely just end up replacing them in the not too distant future anyway.How old are the wires? You don’t need to replace them routinely.
I’ve had wires last 200k miles or more. One thing I always did when I changed my plugs was to wipe the wires down with a damp cloth. This removes any dirt or oil that my be on them. This drastically extends the life of wires.
OBDII detects cylinder misfire by measuring the crankshaft rotational speed. When a misfire occurs in a cylinder, the crankshaft rotates slower than what the computer anticipates. From this the computer is able determine which cylinder is misfiring, and set a code indicating the cylinder that is misfiring.
Tester
You do not need to replace the wires, but you should get some of that silicone grease sold in the little plastic packs where you get your plugs and use that inside the boot of the spark plug wire.
If you do replace the wires, you have to be careful to get a set of wires with the correct resistance. Too much or too little will decrease the intensity of the spark. The wires you have are impedance matched to your coil and you want to keep it that way.