Throttle body and fuel injection service

Why?   In the old days we had carburetors and wet intake systems. The air and fuel was mixed above the throttle plate and delivered together in the intake. Now, we run only air through the intake system; the fuel is delivered separately through injectors, which are located so that they spray gasoline directly against the back of the intake valves. As Federally-mandated emissions controls have been added to our engines, we've introduced residual crankcase gases from the positive crankcase ventilation system (introduced nationally in 1963) and exhaust gases from the EGR system (which appeared in 1972) into the intake. This wasn't a problem as long as we ran fuel through the intake manifold because the gasoline washed away the carbon that forms from these gases. Then, fuel systems were redesigned to manage air and fuel separately for better power, fuel economy and emissions. But this created a new problem: intake system deposits. They form on the throttle plate, changing base idle settings (or minimum air, as we call it today). They also form on the tips of the injectors, restricting fuel flow, inside the passages of the EGR system, on the backs and stems of the intake valves, and in the combustion chambers. Modern gasolines do have detergents, which slow the formation of deposits, but they can't keep the intake clean by themselves. There are just too many deposit-forming components in gasoline to allow additives to be 100% effective. Hence, the need for these services.