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A burnt distributor cap can cause an engine to run or idle rough and vibrate excessively. It may also cause poor fuel economy and higher emissions. While there are other causes of these problems, checking the distributor cap to see if it is burnt or damaged is high on the list of things to look at. Burnt areas on distributor caps are usually caused by arcing because of carbon deposits.
Exterior Burns
Closely inspect the outside of the distributor cap. Look for burn marks around the towers and contacts. Burn marks here indicate a faulty spark plug wire or improper contact. If the wire and the cap both show burnt areas, replace both. If replaced, make sure the plug wire is completely seated onto the contact to avoid future burn problems. If the burn area is very light, resulting from just a dusting of carbon deposits, it is possible to clean the affected areas. If in doubt at all, replace all affected parts.
Interior Burns
On the inside of the distributor cap, look for carbon deposits or melted areas. These marks indicate a faulty arc from the points or rotor to the plug wire contacts. In some cases where there is just a light carbon deposit, sandpaper can be used to clean off the carbon. Be aware, though, that too much filing will create too large of a gap on the points and result in further arcing problems.
The Problem with Carbon
One of the reasons a new distributor cap works so well is that there is no carbon in the cap. Once it is put into use, the carbon will start to build. This usually happens very slowly and will continue to build up over time. The carbon conducts electricity, which can cause a misfire to the wrong plug or arcing between several plugs at once. By keeping the cap clean and looking for damage during engine maintenance, the cap will last longer. If there are any burn marks or carbon deposits on the outside of the cap, always check the inside for damage as well. It is best to replace any worn or burnt parts as they will continue to get worse and cause more engine problems.
