So I've been having bad idle problems, usually it's just really high idle and the adjuster screw is all the way in, anyways I took out my Haynes manual and it said to check the BAC solenoid valve. I took that thing off and one of the holes was completely clogged. Idk exactly what this thing does, but i'm pretty sure it's supposed to be air going through there seeing as it's the bypass AIR control valve... So yeah, do I just need to replace this thing or will cleaning it out solve the problem?

 

Yay nay?

 

Coolant residue. Lots of it.

Another thing. The coolant not being able to pass through that tube will prevent the Fast Idle/Thermowax unit from working properly so that would help to explain a poor idle, especially when the engine is cold.

 

Mmkay...but that tube is supposed to be for coolant then I'm assuming? And I did the test were you wire one end to the positive battery terminal and ground the other, and didn't hear it click like youre supposed to. And wasn't getting resistance out of it. I think I just need a new one lol.

 

That tube seperates from the BAC, its just a metal coolant tube. If the BAC didn't click when energized, it's dead.

 

When you place voltage to one terminal and a ground to the other of the BAC to test it, the voltage must go to the terminal which has the Black/White wire running to it. Doing otherwise will damage the solenoid.

 

Okay, I will make sure to test it again making sure I put it to the right terminal. But it also failed the resistance test, so I'm assuming it's just dead haha. Thank you everybody for your knowledge!

 

If by chance you did the voltage test first and accidentally fried the BAC then it might explain the resistance test results. On the other hand, if you did the resistance test first then it's a moot point.

 

I did resistance first

 

There are 2 Philips screws holding that clogged tube in place, you can remove the tube from BAC and you will see that it is totally independent.

 

let me know if ya need another one

 

If it's in place, but not hooked to any hoses or tubes - not talking about the wiring side, what happens?

 

The BAC solenoid helps keep the idle from falling under 750 rpm when the engine is under different types of load attributed to A/C, headlights on and so on. It helps with the starting of the car. It's used in concert with the Air Bypass unit on an S4 to create the 17 second 3000 rpm start up idle.

It needs to have the tube connected which allows it to inject more air into the throttle body to work as stated. The coolant passage hosing just helps the solenoid function in times of cold weather so as to keep it warm but is not absolutely required as the flow of the coolant through the unit is required to supply the Thermovalve which houses the Thermowax which governs the movement of the Fast Idle Cam.

 

So if it's not plugged in, it won't help - is it an ignition-required unit?

As well, if the unit, itself is in place, but no hoses connected, will it hurt operation (i.e. coolant, etc)? Coolant flows to it, then to the waterpump housing or...?

 

If it's not plugged in, you will have to manually hold the idle on cold start or it will shut off and die or if you use the AC or add electrical load it will die if you don't manually adjust the idle to something around 1,000 rpm.

The hard coolant lines are seperate from the BAC and can be detached via 2 phillips screws.

 

It assists in the starting of the car by making it easier for the engine to start but when mine is disconnected the car still starts and runs fine just at a lower idling rpm and this is w/o load.

The coolant flows from the water pump to the solenoid via the hose closest to the front of the engine and then the coolant exits the BAC at the rear of the unit so it can supply coolant to the Thermovalve.

 

Gotcha. I was asking the coolant-flow direction, b/c the hose that is usually plugged into the WP housing is discon'd and going to my turbo (I guess that's backward?) per suggestion from on here.

 

pm'd