How To Test Spark Plug Wires

Reader Question

Hey Austin!  I have  a quick question, I wanted to know what is the best way to test for bad spark plug wires? I have a voltage meter, but not really sure what is the setting to use to check them? What are some common symptoms of bad spark plug wires anyway?

Chuck J.

 

Hello there Chuck!

Let me first start with some basic symptoms of bad spark plug wires.

1. Engine missfire, since the spark plug is not getting the spark signal coming through the plug wire.

2. Rough idle and poor fuel mileage due to engine missfire.

In both cases, if the spark plug wire is bad that cylinder is not going to work and a missfire will occur.

Testing for a bad spark plug wire can be a little tricky for a novice though. Using a handheld voltage meter is not as easy as you might think. You need to measure the resistance of the plug wire in relation to the other wires and see if one or more are reading too high.

BUT, trust me on this….it’s much easier and faster to replace the spark plug wire with a new one or another known good wire than it is to try and test them.

In my shop we have a large box of “good used” plug wires we use to test customer vehicles.  With the price of new wires so cheap these days, just replace them as part of your regular maintenance program. Most wires will begin to fail at the 60,000 mile range.

Heat is the killer of the wire, both inside and outside of the wire itself. If the wires are touching or are very close to touching the hot engine or exhaust manifold they will begin to break down. Make sure all plug wires are shielded from the exhaust manifold and not laying on the engine itself.

We use a spray bottle with some soap and water to help determine if a plug wire is damaged.  It’s not a perfect system, but it does work and you can try it yourself.

Here is a very short video I made years ago…with a cell phone with no audio (sorry) but you get the idea. Do NOT use anything flammable!!

 

About the Author:

Austin Davis, consumer car repair advocate. "Hi there! I love to help people solve their car repair problems and I hope my site was helpful to you today. Thank you for stopping by."
  • Austin Davis

    Awesome Tobey!

  • Tobey

    This was a great video, and was very helpful to me…..I was able to fix my car myself, my wife is impressed :)