The Bosch Mono-Jetronic injection system
http://what-when-how.com/automobile/thr ... utomobile/ is a very primitive system. It has no sensor for measuring the manifold pressure or air flow, relying on signals of engine RPM and throttle position for determining fuelling, with medium-term correction form the lambda sensor.
This makes the system very prone to over-fuelling, as any engine fault (or wear) which causes the throttle to be opened a little further will cause an increase in fuel consumption.
A fault will either be one that causes the correct fuel mixture to be burnt inefficiently, or one which is causing a rich mixture to be burned. Monitor the lambda voltage and see whether it is generally indicating rich or lean. This should allow a number of potential faults to be eliminated.