How do you know when your vehicle needs new spark plugs and wires?

Answer 1

Plugs need replacing about every 60,000 or so. Don't use those darn multi-prong Bosch plugs. Platinum tip types are fine, I like Autolite (owned by Honeywell). You can remove and inspect yours. If they look black, have a bridged gap or look otherwise no-so-new replace them. They have to have the proper gap. A gapping tool is cheap and you can get one at any autoparts store.

The plug wires could need replacement for a number of reasons. I have seen some that lasted 100,000 miles with no problems. A dead giveaway is when you look under the hood at night with all lights off with engine running. Look all along (but don't touch them) and wherever they lay across one another or come close to metal you may see a blue electrical leakage discharge. That energy is supposed to be insulated by the outside of the wires and make it's way to the plugs, not jump to ground at a bad (worn, abraded, pinched) place. You will likely hear a soft popping sound along with the blue discharge. If you see that, replace them all. If yours has a mechanical distributor, remove and inspect it for wear, rust colored deposits (oxidization), cracks in the cap, loose "tower" posts and the like. Replace the rotor button too if you replace the cap. Move only one wire at a time from the old cap to the new. Each wire's position on the cap is very very important. If you have access to a digital camera take lots of pics for reference.