Given that your engine creates dozens of explosions every single second, there’s a lot of combustion going on under your bonnet. With any burning of fuel, there will be deposits and debris that forms. Sometimes it’s obvious, like the ash that forms when you burn a log in a fireplace. Other times, like in your car’s engine, these deposits are microscopic, but the principle is the same. Over time, these deposits build up and affect your ability to keep burning the fuel properly.