Weekend Workshop

How to understand and service your engine management system

Bosh Motronic injection explored and explained.

Bosch Motronic engine management hit the streets in the early Eighties, when flagship models of BMW, Porsche and Volvo became early adopters of the new technology. The system brought previously unheard of accuracy to engine fuelling and ignition timing, improving power, economy, emissions and driveability.

The breakthrough was fully computerised engine control. Sensors feed the computer – or Electronic Control Unit (ECU) – with information on the amount of air entering the engine, the position of the throttle spindle, the crankshaft speed and the temperature. This allows it to calculate how much fuel to pulse from electromagnetic injectors and when to fire the spark plugs under every conceivable combination of engine speed, load and throttle opening.

‘Maps’ of fuel requirements and ignition advance created on a dynamometer at the factory are programmed into the ECU.

Another advantage of the ECU is its ability to recognise when sensors give unexpected readings. Fault codes are stored and can be retrieved later, allowing diagnosis of common engine problems.

Motronic was widely used by the late Eighties, with Peugeot, Citroën, Vauxhall, Ford and Saab choosing the system for their performance cars. From 1993, regulations demanding the use of catalytic converters drove almost all petrol cars to switch to digital engine management. Bosch Motronic had all the answers and many modern classics built before the turn of the millennium are fitted with this system.

Because of Bosch’s philosophy of continuous development, Motronic was not a single engine management system, but an evolving technology. Early versions lacked detonation sensors and exhaust gas sensors, which became essential after the introduction of catalytic converters.

Some early versions used an extra fuel injector for cold starting, later cars getting their mixture enrichment from the ECU. There were many more detail changes through five generations of the system.

Our guide is based on a 1998 BMW fitted with the final generation of Motronic, which demonstrates all the main sensor types. Earlier classics won’t have every sensor shown here, but the same faultfinding principles still apply.