How to Inspect Your Spark Plug Wires

Plug wires can cause serious problems if they go bad. David Baker/Flickr

Spark plug wires are pretty durable. They aren't a moving part so they don't wear out too often, but their regular exposure to heat and cold can cause the rubber insulation to break down. A careful inspection of your plug wires can avoid any problems. If you're having problems that you think could be related to your plug wires, the good news is they are usually pretty easy to troubleshoot. However, it's a good idea to inspect them periodically because a problematic spark plug wire can lead to a range of other problems if left alone. Most of them aren't dangerous unless you have an underlying condition like a fuel leak. If you have anything leaking, be sure to pay close attention and determine what type of fluid it is. And if your leak smells like gas, have it looked at immediately. 

Safety Note

Be sure to inspect your plug wires when the engine is not running. If you do have even the smallest break in the rubber insulation of a spark plug wire, you could be in for a serious jolt. When the ignition coil sends current through the wire, it's not just a trickle. There is enough voltage there to give you a painful jolt. Somebody with a medical condition could even be seriously injured by the amount of juice flowing through a plug wire. For this reason, you should always be sure the engine is not running and the ignition is in the OFF position before you jump in and start grabbing the wires. 

Wire Breakdown

The only thing that can really go wrong with a plug wire is a break in the insulation. The insulation (the rubber on the outside of the wire) keeps the electricity where it needs to be so it sparks on the inside of your engine, not someplace else before it gets there. If the insulation is cracked, the spark will jump off the wire, or arc, onto something metal under the hood. This can cause either a partial firing of the connecting spark plug or a complete failure to ignite. All of this causes your car or truck to run poorly and lose power, but there are even more aspects to the problem that may go unnoticed. 

An arcing plug wire can cause a weak spark or no spark at all in the cylinder with the bad wire. This makes your car run rough and can affect your ​gas mileage. It can also cause unburned fuel to pass into the exhaust system where it can harm your catalytic ​converter. There have even been stories that involve both a fuel leak and an arcing plug wire, resulting in a fire! It can happen.

A good time to check your wires would be while you change your spark plugs. So do a quick inspection and save yourself some headache.

Doing the Inspection

With your engine off, start at the distributor end of the plug wire and work your way toward the plug end. You're looking for anything that is not smooth, pliable rubber. Bend the wires slightly to be sure no cracks appear. Check the boots at the distributor end of the wires to be sure they are not torn or cracked. Finally, check the wires at the spark plug end one at a time by pulling it off the plug and inspecting the end for any tears or cracks. Also look to be sure there is no burning or darkening of the end.

If you find any damage, it's time to buy a new set. They can be as little as $20 or as much as $100+ for a set depending on your application. It's worth the cost, though. A bad plug wire can be a little monster, it can even trigger your Check Engine Light. If you think your plug wires may be causing a problem but can't find a fault, you can check your OBD Engine Codes to see if there is more info to be gleaned from your car's computer.