having some issues with my bac valve on S5na have a couple around and was looking at a S4TII bac valve and its adjustable w/diff connector<square instead of round> how diifferent are they can they be swaped?? I put in another one off a parts car and I having the same issues but with out a code.... its a ported motor with good 15lb vac at750/800rpm.. got the timing at 10 past stock.. motor wants to fluctuate under accessory load lights fan etc... still get some fast idle.. doesn't want to idle down... tps is brand new and adjusted with LED then METER. would an adjustable bac help at all or could it work at all??? don't want to fry ecu... I was throwing tps, MOP and acv but those were fixed threw the fumbling.... but now getting only acv and that o.k. with idle fluct. under accessory load...at 750>800 rpm

 

hehe me too bumper

 

The BAC is self-adjusting, in a sense, in that it modulates (via the ECU) its opening times (also called duty cycle)...

The adjustment on the S4 T2 you see is probably the idle screw (which doesn't sound like your problem in the first place)...

Best way to see what your BAC is doing (other than just unplugging it to see if anything changes) is to watch the voltage outputs at the ECU. Alternator voltage there at idle either means the circuit/valve is bad, or that the BAC is not opening due to a high idle. Usually I see about 11 volts, give or take, at a 750 idle. The voltage will fall as electrical loads are applied, this verifies that the BAC system is working as advertised (the BAC's duty cycle is increasing, in effect dropping the voltage seen on the meter).

 

I had basically the same question, I dont think my BAC is any good, I've unplugged it while the car (S4) was running and it had no effect. I have an S5 BAC lying around but it would need some modification to make it sit correctly on my S4. I'm wondering if its even worth the trouble or if its normal for there to be no change in engine operation after disconnecting the BAC.

 

If you BAC valve isn't connected isn't your idle suppose to die?

 

Well... I've heard of some people removing the BAC so based on that the car can still run without it. Technically speaking, its only there to aid adjusting the idle under different load conditions. Allowing more air to flow when different accessories are on to help maintain the idle.