Part of Lesson Plan: Ignition System Service (updated) by Curt Erales

Activity Overview / Details

Some of today’s engines use spark plugs designed to last for up to 100,000 miles, but most spark plugs need to be changed about every 30,000 miles. During a tune-up, a complete set of spark plugs is usually installed. When a spark plug is dirty or has worn electrodes, a misfire can result. Symptoms of misfire are usually only noticeable at idle.

Some spark plugs have carbon resistors. Others have a semiconductor suppressor that is not energized until 1000 volts. This kind of spark plug can be checked with an ohmmeter.

When replacing spark plugs, it is important that the correct replacement is used. The code number on the spark plug tells about the hear range, thread size, type of seat, whether the tip is extended, or whether or not it has a resistor.

The spark plug gap is usually set from the factory. It is important to double-check the gap before installation.  It is not unusual to find closed gaps or gaps that have changed because of rough handling.

A wire gauge can be used to check the gap. It has an arm that is used to reposition the bendable electrode on the plug.

When installing spark plugs, it is recommended to apply a little bit of anti-seize compound on the front two threads of the plug before installation. Anti-seize is especially needed on aluminum heads to prevent electrolytic action between the steel spark plug body and aluminum head. 

Spark plug gaps in older cars are specified small gaps around 0.035 inches. The gaps on newer cars can vary up to 0.80 inches. It is important to look up the specification of the gap.

Removing spark plug cables must be a careful operation if cables are to be reused. The rubber boots usually become formed to the ridges on the spark plug. They must be twisted to loosen them. Handle them only by the plug boots so they do not suffer internal breaks. Changing cables one at a time will avoid accidental mixing them up in the firing order.