Vacuum circuits. Guru Stu asks: Do they suck?

23 December 2016  |  Stu

Affects: Just about everything

Turbos, throttle flaps, EGR valves, air heat exchangers, swirl valves – some of the many actuators who’s movement is controlled by a vacuum circuit.



You have to remember that the actual force is provided by atmospheric pressure- the vacuum circuit removes the air from one side of the diaphragm or piston of the actuator and atmospheric pressure pushes the other side, therefore there is a limit to the force that can be applied (about 14 pounds per square inch of diaphragm / piston area)

For now though we’ll assume that vacuum pulls on the actuator diaphragm.

The circuit comprises a vacuum pump, (usually driven by the camshaft) a vacuum reservoir, electrically operated solenoid valves, diaphragm actuators and small diameter pipes to link the components together.

In order to make a mechanical actuation, a computer sends a signal to the solenoid valve which opens the valve and allows vacuum to reach the actuator diaphragm and move the component in question. The position of the component can be infinitely controlled depending on the shape of the signal sent to the solenoid valve



They can be difficult to diagnose quickly. A faulty actuator or solenoid valve will affect the operation of the component it is associated with BUT it may also affect the operation of other actuators because it can cause a loss of vacuum available to other actuators, so you can easily be mislead by the symptoms.

Some actuators only work under certain driving conditions which are difficult to re-create in the workshop. It is possible to test all the solenoid valves, their electrical and vacuum circuits and actuators but it is time consuming and requires a vacuum gauge and pump and also the relevant diagnostic computer.



A visual check of all the pipework is advisable because the pipes can rub on moving parts or get burnt. If the fault is permanent, you can use a vacuum gauge to great effect, testing for sufficient vacuum at the reservoir and testing for vacuum at the actuator, but you always have to remember that a lack of vacuum can mean more than one thing- a leak, faulty actuator or faulty solenoid valve or electrical circuit. Access to parts can be awkward, particularly some turbos, and you do need to know which way round various systems work- some turbos need full vacuum for full boost and some require vacuum to be applied to open a waste-gate, some turbo waste-gates are operated by turbo pressure and have no external control. Substitution of parts is sometimes the easiest way but be careful that the part is identical as there are a few distinctly different solenoid valves. It is worth checking that pipes are the right way round on solenoid valves – it is usually marked on the valve which is the vacuum and which is the outlet.

Experience is the best tool, followed by a vacuum gauge/pump such as a ‘Mityvac’ and manufacturer’s diagnostic computer.