E21/E12/E28: Bosch K-Jetronic Fuel Injection System Diagram / Explanation

Two types of fuel injection system are fitted. E21 3er 323i models are equipped with the Bosch K-Jetronic system, whereas E30 3er 320i and 325i models are fitted with the Bosch L-Jetronic system. This article covers K-Jetronic injection for E21 models, also injected E12 and early E28 5er. For L-Jetronic in E30 models, also later injected E28 5er, see THIS ARTICLE.

K-Jetronic Operation:
Fuel is drawn from the fuel tank by an electrically-operated pump and is then fed, via a pressure reservoir and filter unit, to the fuel distributor-unit. The fuel is then pumped direct to the fuel injectors which are located near the inlet valves, but mounted in four induction-manifold branches. The system is continuous acting type and the amount of fuel admitted to the injectors, by the fuel distributor, is governed by a pivoting baffle-plate mounted in the air-intake. Air drawn into the engine lifts the baffle-plate and this action moves a metering shuffle-valve in the fuel distributor. The ratio of air to fuel is thus controlled to a fine degree. After the mixture is drawn into the cylinder, the normal four-stroke combustion cycle takes place.

The fuel is maintained at a constant pressure by a pressure-regulator incorporated into the fuel distributor-unit, excess fuel being diverted through a return-line back to the fuel tank.

For cold starting, a solenoid valve injects fuel into the air-intake for a period determined by the coolant temperature. In addition, a temperature sensitive (warm-up) regulator, working in conjunction with an additional air slide-valve, is connected to the fuel distributor-unit and has the effect of increasing the amount of fuel delivered by the injectors. Later models are fitted with a throttle bypass-valve in place of the additional air slide-valve. On some specific versions this valve is coolant-heated.

A normal type throttle butterfly-valve, mounted between the baffle-plate and the cylinders, is connected to the accelerator pedal linkage. These constant flow injection systems work in a similar manner to a carburettor, vaporising fuel in the inlet-manifold and metering it into the engine.