The underbonnet layout especially on the 24V can
be rather daunting with a multitude of strange looking valves and vacuum
controls for the various systems.
Primarily there are 3 major systems that use vacuum control.
The Variable Inlet System that is used to increase engine power at certain
revs and loads - this uses a vacuum solenoid to apply vacuum to a mechanical
valve located at the front of the inlet manifolds under the control of the
The EGR system that uses vacuum to open a valve between the exhaust
manifold and the inlet manifold to allow exhaust gas to be drawn into the
engine to reduce the size of the fuel/air mixture when running under light
loads which increases fuel efficiency and reduces harmful exhaust emissions.
Closed circuit EVAP system which retains fuel vapours within the engine
system preventing the harmful release of petrol vapours into the atmosphere.
The vents inside the air-conditioning evaporator are also controlled using a
The diagrams showing the correct connections of the various pipes are
shown below for the 2.9 24V engine.