After an engine has been driven it gets very hot. When it is stopped it has gasoline vapors and engine oil vapors that evaporate while the engine is cooling. As it evaporates it leaves behind deposits that build up in the form of a sticky sludge inside the intake system and air throttling plates. After lots of the sludge has built up inside the air intake system, the ability to accurately monitor and adjust fuel is diminished. After the air throttling plates get this sludge built up they tend to hang and not work freely. This sludge builds up so slowly the driver usually does not even notice the slight hesitation, slower starting and slow decline in fuel economy. For these reasons we recommend a service about every 30,000 miles to clean the system. We inject a chemical directly onto the throttle plates and into the intake system while the engine is running. This cleans the intake system all the way to the tops of the pistons. There is also a chemical that gets added into the gas tank. After this service it is always best to fill the fuel tank to get the best results of the tank additive.