The higher compression ratio engines, two-stroke engines and flexible fuel vehicles currently under development tend to face the problems of spark plug fouling owing to the necessity of using cold type spark plugs.This paper analyzes the sparking of fouled spark plugs and investigates the characteristics required of an ignition system in order to avoid fouling problems. The results clearly establish that to maintain a strong spark even when the plug is fouled, a high voltage should be instantaneously applied to the spark plug. A series-gap on the high-tension side was confirmed to be an effective means of achieving this and a new plug cap provided with a series-gap has resolved fouling problems such as failure to start.Lately, fuel economy and long-term energy conservation have become critically important. For automobiles, higher compression ratio engines, two-stroke engines and flexible fuel vehicles (FFVs) are being developed. Such engines require cold type spark plugs: higher compression ratios raise the combustion temperature, especially at high engine speeds; two-stroke engines increase plug temperature because they fire twice as often; FFVs, which can run on any blend of methanol and gasoline, tend to suffer from pre-ignition when operated on methanol if a normal type spark plug is used . Unfortunately, cold type spark plugs are liable to foul under certain conditions, such as repeated short trips in cold weather. Even the long-life, maintenance-free platinum electrode plugs  are prone to this problem if they are cold type, and must be replaced. Hence, in order to realize these new engines, it will be necessary to resolve the problem of cold type plug fouling, since previous research [3, 4 and 5] has not established effective countermeasures for the cold type plug. This paper investigates the actual behavior of fouled plugs and proposes a countermeasure based upon the ignition system.