NSW928

I'm currently tending to some long overdue jobs including an intake service, timing belt change and changing the cam cover gaskets. Having the manifold out is allowing me to get behind the engine block to deal with some other issues I knew of, the most challenging of those is dealing with the vacuum system in this vehicle.

The vacuum system was basically disconnected. Hoses and connectors were broken, missing or simply just not going anywhere. I've browsed numerous threads on the topic on this forum and I'm confident that I can get it all up and running, but I'm just confused on a few matters and would appreciate any help:
1) The heater valve - is it correct that this is the only item in the engine bay which has its own dedicated hosing coming from the main console? Its seems that the hose for this should come in from the firewall under the wiper motor, where is it meant to go from there?
2) With this in mind it seems that it doesn't matter which hose goes to which item (within reason) for everything else in the engine bay, as long as the system is closed. Is this correct?
3) There is a black vacuum hose which comes through the firewall about half way across and about halfway down. Some posts suggest that the main hose from the console is also under the wiper motor but I'm assuming that this is it in my vehicle?

Thanks in advance.

'87 S4 Auto Cassirot

FredR

There are two vacuum systems- the passive system and the active system.

The passive system has a reservoir located in the rear of the driver's side front wheel arch. This is where the vacuum is derived that feeds the flappy valve via the solenoid valve on the top front of the motor. The other feed from this reservoir feeds the HVAC system via a single nylon line that runs through the firewall. This connects to a distribution manifold under the central console sort of behind the radio/cassette module. There are several electrical solenoid valves that are switched depending on the programme required. The heater valve is one of them and the signal is carried in a white vac line back to the heater valve.

The active vac system is located at the back of the inlet manifold through a multi way connector. This sends the active vacuum to the fuel pressure regulator/dampers and the auto box [on automatics] plus one other user [the petrol tank vent system [I think].

Adk46

That's a good way to describe it, Fred. Would it also be correct to call the passive system the "constant vacuum" or "utility" system, and the active system the "manifold vacuum" or "vacuum control" system?

My feeble understanding is that the former is used merely as source of motive force - for various actuators that need a steady vacuum, and are unconcerned with the engine's running state. The latter is used to communicate the engine's running state to actuators that need to react to it (or to actuators that don't need a steady vacuum - e.g., the dampers?).

So many details I don't know - how the brake servo is hooked up, the venturi ....

Anyway, to repeat with different words, there's a line that emerges from the firewall that goes directly to the heater valve from the HVAC control stuff inside the cabin - which is not working on my car, so I have the valve hooked up to the "passive" or "utility" system during the summer to keep it closed. There's another line nearby that disappears underneath to the auto transmission - that's hooked up to the "active" or "manifold" system, and is used to modulate shifting speed - making shifts more gentle when you're not hammering, when the manifold vacuum is high.

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Ever stare at the word "vacuum" too long? There is just one other common word with a uu: "continuum". Now I'm obsessing about how they are pronounced differently.

FredR

Curt,

My terminology is what it is- nothing formal per se but I feel it reflects the intent reasonably well and perhaps more importantly, helps other owners differentiate between conditionality [i.e. does something under certain conditions] and in the passive case is simply there to drive something.

The venturi is present for one reason only- to provide a continuous source of vacuum to the brake servo at all times whenever the motor is running. The airflow through the venturi is metered air but it completely bypasses the butterfly and the ISV wherein the latter modulates to provide "additional air" to stabilise the idle condition at the setpoint idle speed [675 rpm for the S4/GTS]. The venturi is a simple but very effective piece of engineering that produces a deep vacuum downstream of the internal nozzle at a position known as "the Vena Contracta"- a point where the stream lines are at maximum convergence, the velocity is at its highest so the reaction produces the deepest vacuum locally. Under normal circumstances there is no flow from the servo which is kept at a state of readiness, thus is primed ready to support braking action the split second it is needed. The little blue check valve is there to protect the braking function in the event vacuum seal is lost in the passive system- if such happens and the check valve fails [or was simply not present], there will be little to no vacuum to support the braking function - not good!

Do not look for logic or consistency in the English language- remember it is a bit of mongrel with a little bit of everything in it hence why it is so illogical at times. We started out with the Celtic languages [whatever they are, then the Romans popped in for a cup of tea, then the Saxons [Danes], the Vikings [Scandanavia], the French and all had their impact not to mention a few others sprinkled in for good measure. If you want structured logic in a language you need to try Latin or Arabic. Did you know "Algebra" is an Arabic word? Now we get treasures from you chaps such as Hamburger, MacDonalds, Pepsi. Googled and my cultural favourite "Fubarred".

Rgds

Fred

NSW928

Thanks for that, I've now replaced and reinstalled everything as far as I know correctly. As I've got half the engine out I can't fire the car up to test it, but just a couple more queries:

1) There is a 4 way junction which (as some posters have described it) looks like a spider. I know which lines to connect 3 of these to, the 4th spot on is for a check point, what is actually meant to be on this to close the system? (I'm actually just using a 3 way junction instead)
2) Where the 2 lines go into the fender for the reservoir and the cruise control, which is the white one and which is the black one? And if I need to get in there to replace one of them how does one do that?

Thanks again.

FredR

The 4 way connector is in the "passive" system and is located in front of the brake servo. The connections are:
1. Resonance flap
2. HVAC system
3. Vacuum reservoir
4. To the brake servo check valve/connector nipple [this short line must also have the small blue/black check valve installed in it],

The line that goes through the fender wall is item 3 above. As I am aware there is no vacuum line to the cruise control module- that was only on earlier models unless some of the early S4 models are different to mine. The vacuum reservoir is accessed from the wheel arch after removing the wheel well liner.

DuncanF

NSW928

Yes mine definitely has vac controlled cruise control and 2 lines going into the fender. I think I have everything hooked up the right way now, except for one line:

There is one sole vac line that comes from underneath the throttle assembly and it says it goes to the 'secondary air valve'. Where would that piece of equipment be located?

DuncanF

NSW928

No that one has various names, commonly referred to as the 'carbon cannister valve', it also has it's own dedicated line but from the top of the throttle body.

I've since established that if the secondary air pump isn't functioning (as is the case in my vehicle) the secondary air valve might be non existent and the associated vacuum line is capped. I'll leave it as it is for now, if one of the other vacuum outlets isn't working when I fire the car up I might switch over to that line. So It becomes my spare outlet.