Faultfinder

Understand your classic’s Bosch mechanical fuel injection system

K-jetronic injection debunked.

The mechanical fuel injection systems developed by Bosch enabled the hot hatch revolution, starting with the Volkswagen Golf GTI in the mid-seventies. K-jetronic injection was soon adopted by other manufacturers to fuel their performance models, with Ford, Mercedes, Volvo, Renault, BMW, Saab, Lotus and many others joining the party. Numerous classics run beautifully with K-jetronic to this day.

The principles of K-jetronic are quite straightforward – and simpler than later electronic systems. A high pressure pump feeds fuel from the tank to an accumulator, which acts as a pressure reservoir. The fuel then goes to a metering unit that sends it to the injector for each cylinder. This metering unit is the heart of the system and has to perform several functions to ensure correct fuel delivery in all conditions – from tickover to redline, from throttle closed to wide-open and from stone-cold to fully hot.

The metering unit regulates fuel pressure from the pump via a relief valve, which bleeds excess pressure back to the tank. Fuelling is controlled according to airflow. The metering unit uses a hinged plate inside a cone to measure airflow into the engine. The plate lifts as airflow increases with higher revs and larger throttle openings, moving an internal arm that adjusts fuel feed to the injectors. An electricallycontrolled valve supplies air for idling when the throttle is closed and other components enrich the mixture for cold starting.

When something goes wrong, finding the fault is generally well within the capability of a DIY enthusiast. Few parts, however, can be serviced without specialist equipment. The usual remedy is to simply replace the suspect component. Our guide explains what each part does and how you can check if it’s not doing it as it’s supposed to. There’s nothing to stop you keeping your K-jetronic system in fine fettle at home. Unsurprisingly, the secret of success is to take a methodical approach to faultfinding.