imloggedin

first of all what purpose does the initial set coupler serve? and when i turn the air adjust screw on the bac valve it doesnt change the idle at all. what does that mean? and without the bac valve how do you control the idle?

scathcart

Must be a TII if you have the BAC adjust screw, but in the future, please post year and model (N/A's don't have the BAC bleed screw.)

The INS (initial set coupler) turns off the ECU signal to the BAC. The reason you need to set the INS is that the ECU tries to use the BAC maintain a steady idle. So, if you adjust the BAC screw with the INS not set, the ECU will alter the freuency of the BAC to maintain that idle. You can also use the secondary throttle stop screw if you do not have a BAC to set the idle.

The BAC is what its name implies: a Bypass Air Valve. Idle is standardly set via the airflow going through the slight gaps at the throttle plates. This position is constant at idle, so airflow at the Throttle body cannot change.
However, parasytic loss on the engine can change. Moving the P/S from side to side makes the p/s pump work, which increases drag, and so does turning on the air-conditioner; both of the pumps take energy to turn. Without a way to increase the airflow through the throttle plates, these extra loads would cause the idle to drop.
So, mazda introduces the BAC. It takes metered air and simply routes it around the throttle plates; it would be the same as slightly changing the throttle plate position. Now, you cannot simply have a pipe around the throttle body, you need some way of controlling it for it to monitor idle. What it is is a valve that flaps rapidly according to ECU signal. The faster it flaps on and off, the more air it will flow around the throttle body.
The screw on the BAC is non-related to the BAC operation, it just allows changing in idle by metering in the amount of air allowed to bypass the throttle body via a small, other "pipe". In N'A,s this is done directly on the throttle body. That screw is basically like the tap on a sink, open it wider, and more air will flow through, and thus into the intake and around the TB, increasing idle.

Note: the BAC is non-operational above 1200 rpm, and when the ISC is set.

So, you can see that you need to jumper the ISC when you alter the BAC screw, since they are not related in function. If you unscrewed the BAC screw, and let in more air, the ECU would just decrease the BAC "flapping" frequency, and keep the idle normal.


Probably a bit more than you wanted to know...

NZConvertible

But damn good info anyway.

imloggedin

that was the best explanation i have recieved yet.. very good. so if the screw isnt changing the idle then i either didnt have the jumper set right or the bac valve is bad huh.

HAILERS

****then i either didnt have the jumper set right or the bac valve is bad huh.****

Or the idle was/is so high that screwing the bac screw isn't even noticed. Or the bac signal from the ECU is kaput.

YeaH. That was the best explanation of the idle/bac ever on this site.

hugues

nice detailed explaination, scathcart!

thanks.

hugues -

imloggedin

why does coolant run through the bac?

Rotory

iTrader: (1)

just somewhere to route it.. i dont think the coolant plays any part in the BAC operation

scathcart

It is to keep it from freezing shut when the weather drops below zero celsius.

imloggedin

thanks for the good answers scathcart

NZConvertible

I have my doubts about that theory. When the engine's cold, the coolant's cold too, so it can't prevent freezing. When the engine's hot, the manifold is too hot to touch, so it can't freeze.

But then Mazda seem to have gone to a lot of trouble attaching the hose to the BAC valve for it to just be convenient routing. A clamp or bracket would've done the job.

scathcart

The BAC is needed as soon as the engine kicks out of the 1500 rpm warm-up; as soon as the thermowax is fully extended, so it would have to be defrosted by this point. I am unsure whether there is significant heat soak through the manifolds at this point (only been running a couple minutes), and I believe this was Mazda's thinking.

whiterx7TII

ok> dont want to get off topic but 2 things..
1-that was one of the best writeups i have ever seen on the BAC valve operation
2-since it is really only used to compensate for the P/S and A/C, is it really necessary on a car that no longer has power steering and A/C. Can it just be blocked off and the coolant rerouted to the water pump directly. On my car, i no longer have either P/S or A/C and the BAC valve is the jspec version so there is no adjustment anyway.

scathcart

You could remove it, but why would you want to? It doesn't take up that much space, and if you want to reroute the coolant hose, go ahead, as seen above, it doesn't do that much anyway.

Some other changes affect idle, such as load from the alternator, and engine vaccum from the power brakes. You can remove it if you are really keen on it, though.

hugues

What about the dreaded 17 seconds of accelerated warm-up phase, where the BAC is involved as well as that other valve (air bypass solenoid valve on s4 and aws valve on s5, I believe, that's how they're called).

I think HAILERS tried to disconnect his AWS valve and still got the fast warmup, that's why, I say the BAC is involved in that accelerated warm-up but I could be remembering wrong.

but, now that I think of it, I think the accelerated warm-up system might be offline (or at least the BAC portion of it) when the temperature is too low. I am not sure about this at all though.

Hugues -

NZConvertible

The P/S is actually compensated for by the Air Supply Valve on the back of the intel manifold, not the BAC valve. It opens whenever the P/S used.

But like Sean said, as well as the A/C, it also compensates for alternator loads. The ECU tries keeps idle speed constant as electrical loads increase by increasing the BAC valve duty cycle.

Some people can happily live without their BAC valve, but during winter driving I can have the A/C, interior fan, rear demistor, headlights, foglights, brake lights, wipers and amp'd stereo all on at once and still have the same 750rpm idle I have with them all off. Try doing that without a BAC valve.

hugues

that is true for the turbo model, but not for the NA. On NA, there is power steering siwtch that gets avtivated when wheels get turned and the BAC duty cycle increases. Above for s4, don't know about s5.

Hugues -

NZConvertible

Ah, forgot about that. Thanks.

HAILERS

Ya all forgot to mention the bac's role in the Start proccess. Tsk, tsk.

hugues

I gues everybody was waiting for you to chime in on this one.

At start-up (cold engine), meaning cranking:

the BAC valve injects "large quantity of air".
the AWS is also on.

Once started, during the 17s accelerated warm-up thing:

the quantity of air injected (via the BAC) is based on "feedback control" (outside temperature could be a factor then).
the AWS is still on.

The above is from s5 FSM (relationship chart in "fuel and emissions system"). It should be same for s4 but could be different.

But, still, what happens when outside temp is very cold, does the BAC still inject "large quantity of air" at start-up. That seems odd to me.

hugues -

HAILERS

Well.......when the car has cold soaked overnight in 27 degree temps, then the car is started for the first time late in the day when the temps are 48 degrees, yes, the bac's piston moves to it max position as long as the key is held to START. Once the car starts the bac immediatley goes to its duty cycle which in the beginning is in the fourty percent range then as the car warms up the duty cycle goes down, down to I forget what ....something like the thirty percent range I *THINK*.

In other words...the bac moves to it's full open (only moves about 1/4 inch from full close to full open, on the hottest day and the coldest day of the year.

hugues

ok, thanks for clarifying that START behavior. thanks.

Hugues -

fstrnyou

and what does it mean when the idle rpm is at about 900, the INS is jumped and i can turn the screw all the way in and all the way out and nothing happens. at one time, the idle jumped up and down for a few seconds and then stopped, but now while adjusting it.
a few times i revved the motor and when it revved down, it stopped at 1k for about 2 sec. then dropped to almost a stall, then went back up to about 900. i tested the BAC with a DMM but those results are probably inconclusive to whether it works or not.

Untillater

awesome writeup on the BAC, too bad it means my tps is the culprit :/