According to the signal of the control unit the injector valve should supply all cylinders at once (throttle body fuel injection) or the cylinders individually (multi-position injection) with petrol.
While the injector of the throttle body injection is inserted above the throttle valve, the injectors of the multi-position injection (figures) are arranged at the end of the inlet manifold near the intake valves. Often they are connected directly to the distributor pipe (figure 2). Most often they are ground rated with 12 volts. Older systems inject all the fuel at once, in case there are more than four cylinders, it is done in two groups, half of the total quantity per crankshaft rotation. If the intake valve is closed for a short time, the fuel is stored.
Modern injectors inject the total quantity in the induction stroke or a little earlier (sequential) or, e.g., in two groups for four cylinder engines. The injection amount depends only on the injection pulse timing, because the opening cross section is fixed, as well as the pressure difference between injection pipe and inlet manifold. The starting point of the fuel injection is most always the same, while its end is shifted according to the demand. Injectors must be checked particularly in case of disturbances in terms of their density with running fuel pump and taken out plug.