Common signs of failing spark plug wires include a decrease in power, acceleration, and fuel efficiency. In addition, the engine light coming on or visible damange to the cables can be signs of a failing wire.

As the spark plugs wear, the spark plug gaps widen, also weakening MPG. This is because the wider the spark plug gap, the more voltage is required to arch between the electrodes. The smoother the combustion process, the lower the demand for engine power and the lower the demand for fuel.

about every 30,000 to 90,000 miles

Due to the constant use that the spark plug wires on a car gets, they will wear out over time and can cause a variety of different issues. The ignition cables on a car are made to last about 60,000 miles before they have to be replaced.

Weak spark may cause starting and running difficulties as there may not be enough heat generated in the spark to accomplish ignition of the fuel/air mix in the combustion chamber. Common reasons for weak spark are faulty flywheel magnets and faulty stator.

On an automobile ignition system the spark should be bright blue. That’s because the compression ratio is higher than that on an air cooled small engine. A low compression engine running at a steady state with a constant fuel supply won’t quench even a spark plug with a yellow spark.

do get a weak spark may be indicative of a poor ground. . Coils don’t have a ground wire. They have a positive terminal that gets voltage from the ignition switch and a negative terminal that goes to the points.

The only safe way to test for spark is to use a spark plug tester tool. If a coil problem is suspected, measure the coil’s primary and secondary resistance with an ohmmeter. If either is out of specifications, the coil needs to be replaced. A coil can be easily bench tested with a digital 10 megaohm impedance ohmmeter.