Among the many various gasoline and diesel fuels, seafoam is a safe and effective additive used in all kinds of fuel mixtures. When added to fuel, seafoam can make even 4-stroke engines smoke, especially if you add more than one ounce per gallon. Fortunately, an occurrence of this nature is common and should not be alarming. Smoke is not consistent with SeaFoam use, so you should stop using it and check if you have a head gasket failure or associated coolant leak.