Engine vacuum

[Pentoman]

What is the purpose of engine vacuum? I'm having a little trouble getting this answered on car specific forums and In know there's some knowledgeable folk here who might help..

Why do engines have all those vacuum lines? Ignoring things like vacuum for operating the brakes, headlight adjustment etc, I am just interested in how they are used for the running of the engine.

My car for example has vacuum lines to the sensor block at the top front of the engine, to the bottom of the intake runners, and to the throttle body. Amongst other things they also go to something on the fuel distributor, and to the ignition control computer.

What's the beef?

[Mr2Mike]

Pentoman said:

What is the purpose of engine vacuum?

It doesn't have a purpose per se, it's simply a consequence of throttling an engine. When the throttle is closed, the engine starts working like a vacuum pump, trying to pull air in and creating a low pressure in the manifold. Diesel engines aren't usualy throttled, so they do not have significant manifold vacuum.

Measuring this pressure (i.e. via a MAP sensor) is one way of calculating the engines fuel requirements. As you have noted, manifold vacuum is also often used to provide braking assitance, and to drive actuators for e.g. variable inlet geometry manifolds. This is because it's much cheaper and more reliable to use a vacuum driven diaphragm to move something than to use e.g. an eletricly driven actuator.

Edited by Mr2Mike on Wednesday 18th July 11:23

[ELAN+2]

the vacuum circuits are used to sense engine load as stated, on modern cars, there are a couple of emission related bits and pieces that are vac operated, like the carbon canister for fuel evaporation from the fuel tank, and the exhaust gas recirculation valve is often opened by engine vacuum.Also some cars have a manifold pressure sensor (map sensor) so the ecu can supply correct amounts of fuel and correct ignition timing to suit the engine load. High vacuum; closed throttle, low vacuum open throttle.
Its paramount that there are no leaks as all sorts of running probs can be caused.
On your lotus, it lifts the headlights!!

hope this helps

Mark

[Pentoman]

ELAN+2 said:

On your lotus, it lifts the headlights!!

Indeed - I've heard amusing stories about the headlights dropping into the bonnet after extended full throttle openings? It's now got chinese eye headlights so no pop-up at all.

On my old Merc, could it cause soft throttle response? It responds exactly like a turbocharged car, with 0.5-1 second as power 'builds up' even though it's not turbocharged... The vacuum lines have actually all been replaced a couple of years ago by a previous owner, but they could done it wrong or something.