RockLobster

iTrader: (7)

It seems like i deal with this question every time i am doing an install.

The FSM for S5 NA emissions systems is kinda ambiguous when it comes to the 3 air nipples on the back side of the throttle body. I know two are for the purge control valve and the 3rd i believe is for the injector air bleeds.

Thus two are fresh air (pre throttle plates) and the 3rd is vac (for the bottom side/outlet of the purge control valve).

Which ones are which?

jjcobm

iTrader: (18)

Top one - Goes to top of PCV
Middle one - Goes to air bleeds & passes through the solenoid rack...
Bottom one - Goes to bottom of PCV

(I will double check in a minute on the car).... edit: I had the middle and bottom one mixed up, corrected now.

jjcobm

iTrader: (18)

Sorry about that, I re-corrected my original post twice. What is posted there is how it is routed. I got a confused since the line for the bottom of the PCV goes under and then over the line for the air-bleeds.... The top nipple is the easy one : )

I attached a diagram that may help you out.

RockLobster

iTrader: (7)

That is the diagram that is confusing lol. I have studied it for far too long....too many times.

They hide where the middle and bottom one go behind the oil filler neck (nice job guys).

It is also ambiguous whether the top one is actually a vac source or pre-throttle plates (based on the associated schematic diagram). Seems to me it is pre throttle plates so that it has an actuation effect on the PCV based on engine load.

jjcobm

iTrader: (18)

There is another diagram in the FSM that shows the throttle body vacuum source layout. The top hose is PRE-THROTTLE BODY (sort of), it is before the second set of throttle plates in between the first and second sets, the middle one is PRE completely (fresh air), and the bottom one is AFTER (so that it can suck up all those nasty blow-by gasses back into the engine, full vacuum all the time).

You are correct, the PCV doesn't activate during idle (high vacuum, but not for the top hose), only during load when the throttle plates open up and create vacuum for the top hose, if it were stuck open at idle you would have a rough running engine...

Basically, the top hose gets vacuum when the throttle plates open, but not when closed, the middle one only gets fresh air period, and the bottom one gets full vacuum all the time.

Straight from the wiki page:

"At idle, the intake manifold vacuum is near maximum. It is at this time the least amount of blow by is actually occurring, so the PCV valve provides the largest amount of (but not complete) restriction. As engine load increases, vacuum on the valve decreases proportionally and blow by increases proportionally. With a lower level of vacuum, the spring returns the cone to the "open" position to allow more air flow. At full throttle, there is nearly zero vacuum. At this point the PCV valve is nearly useless, and most combustion gases escape via the "breather tube" where they are then drawn in to the engine's intake manifold anyway."

http://en.wikipedia.org/wiki/PCV_valve

RockLobster

iTrader: (7)

That of course makes the most sense, and is how i assumed those nipples were plumbed in the intake manifold and throttle body.

But, alas that is not what mazda FSM artists show......

jjcobm

iTrader: (18)

Yeah it's a pretty bad picture, but if you look closely its right after the first set of plates but in front of the second. I bet if you put a vacuum gauge on that port and run the engine, at idle you will get low vacuum readings, but as you increase RPM (open up throttle plates) it should start increasing in vacuum (ported vacuum).

Modern cars run some components using vacuum that only comes on during throttle plate opening. I can't remember off the top of my head right now but I believe the type of vacuum used for that top nipple of the PCV should be called PORTED vacuum, where it only recieves vacuum as the throttle plates open.

Your "normal" vacuum is MANIFOLD vacuum if I remember right (high reading at idle, low reading at higher RPM (opened throttle plates)). I will have to pull out some of my books and looks through them, it's been a few years since I went over this stuff, but that is the basics of it....

RockLobster

iTrader: (7)

But it is not connected to that throttle body runner. It is shown connected to the larger single barrel lower runner that only has one throttle plate....

The upper one is the two runner double throttle. Which is where we are saying this is actually connected.

jjcobm

iTrader: (18)

You are right (looking at it again, it is very hard to tell where the throttle plates are at), but it really wouldn't matter which throttle plate it would be connected near for our concerns, as long as it would allow for vacuum to be created as the throttle plate opens up (which is how the PCV system should activate also, we already know the PCV cannot be left open during idle, only as you increase RPM and that is the job of that top nipple port). If you have a vacuum gauge I am very confident it will provide max vacuum as you open up the throttle plates mid-range and possibly drop when they are fully open (depends on the system I believe)...

If I had a vacuum gauge I would verify this for you...

But as for the other middle and bottom nipple ports, those are pretty straight forward...

FYI: Ported vacuum is also used for vacuum advance distributors, ect...

RockLobster

iTrader: (7)

Ill just hook it up the way we agree is probably correct and when i can start the car ill check the vac readings at those different points.