my neighbor asked me what he could do to determine whether or not his plug wires were shot. Profile: 1990 Toyota Corolla 4 cylinder 5 speed 180,000 miles last set of wires at about 100,000 miles 5 years ago. The car “skips” once in a while when he goes up at hill, doesn’t do it all the time. I told him to replace the wires/plugs/cap since it has been 80k miles since they were changed. He said he wants to diagnose it before he puts any $$ into an old Toyota with 180,000 miles. I listened to it run, smooth idle, seems to rev up ok. Oh, one more thing . . it skips more when it is raining.
At night, open the hood and start the engine. Using a spray bottle, spray a mist of water on the wires. You will see it if the plug wires are shorting to the engine block. It looks like miniature blue lightning bolts.
While MC answered your question although I might add that it is also possible to use a ohm meter, I might make another suggestion.
At five years and about 100,000 miles, I would not even bother to check them, I would just replace them. It would be a rare set of wires that would last that long and still be fully functional, and even then you would know that they were on their last legs.
The simplest test is to keep the current wires as long as the engine runs smoothly, but to replace them when the car starts to misfire climbing hills or in wet weather. Your friend’s car has already failed its wire test.
My time is worth more to me to waste it on diagnosing plug wires so old. But spraying water on them at night when the engine is started cold can be an educational experience and it is fun the first 25 times it is done. I’m up to 2 times now since 1969.
My thanks to you folks . . . replacing the wires was my diagnosis as well. I simply don’t want to spray water on anything electrical, and since the skipping occurs whilst going up hills, this might be problematic. I’ll help him to replace the wires and cap . . . we’ll see how it goes. Thanks friends!