Volkswagen Beetle Glow Plugs Available Here. Glow plugs are a component in diesel engines that work to ensure that when starting your engine, there is some additional energy. This source of energy also helps when cold-starting an engine and it helps to make combustion more efficient to help reduce emissions and smoke development. When glow plugs are not functioning properly, you might notice a variety of vehicle symptoms, such as hard starting, reduction in acceleration and power, the engine misfiring and black smoke coming out of the vehicle’s exhaust. Since you want to ensure that your vehicle emissions are low and it is operating at peak efficiency, if your vehicle has bad glow plugs, it is important to replace them quickly.