I have searched and I'm not sure if this has been addressed.

Scathcart mentioned that guess and test isn't the way to go when diognosing common problems.

My Idle is a little rough here and there. I'm trying to get it smooth all the time. The TPS is adjusted correctly at 1000k.

So I checked the resistence in the BAC and found it to be 12.6 ohms. The factory manual specifies this resistence to be between 10.7-12.3 ohms.

I guess I could just swap them out with another BAC I have that's between the spec. But, since I am certainly not a professional rotary mechanic, I don't want to just go swapping stuff out and possibly cause more problems. As Scathcart rightly points out: "Why" instead of "Guess and Test" is a less than desirable method of approach. Because the truth is even if I swapped them out and it worked I would still not know Why or why not it was a solution. Its time to stop the madness.

Is there any other way to determine that the BAC isn't functioning correctly besides an Ohm mesurement?
The BAC valve does click when 12V is applied.

 

If operating properly, electrically, I would ckeck to see if the inner seal is operating (sealing) properly. Either remove Bac and you can just blow through large air intake to see if any passes the seal. I guess it would be possible to put a hose the id of the BAC, then blow through that. If the inner seal doesn't seal properly, the engine will be sucking additional unwanted air, hence rough idle.

 

The difference in ohms could just be the resistance of the leads on your ohm meter or the meters accuracy itself.

Go here and read the part about the bac http://www.teamfc3s.org/info/articles/idle.html

Or even simpler, go to pin 2Q on the ECU. Put one lead on a good ground and the other lead up the back of the wire at 2Q. Run the car at idle fully warmed up. Read the voltage at the pin/socket 2Q. Let's say it says 10volts as an example. Now turn the headlights on and the heater fan and air conditioner. IF the bac is getting signal from the ECU, you will see a voltage drop down to say about 8 volts. That means the thing is a working. And NO, that voltage drop isn't just the battery voltage dropping down because there's a load on the alternator.

A better bet about the irregular idle....might be the ACV. It has a diaphram in it that is called the anti-afterburn valve. After several year/decades of usage.....they leak air into the intake manifold when they should not. That causes an irregular idle. Pull the large hose off that runs b/t the airpump and acv at the airpump end.. Spray starter fluid into the hose towards the acv. Did the idle change dramaticaly??

 

Thanks T,

I will definately have to see if this is problem (at least one of them ) Sometimes it idles alright, and then other times it seems as if the engine wants to jump right out of the hood and turn itself into grinded metal. I wouldn't have thought about a BAC sealing issue at all.

Since it is a s4 block in a s5, and I tried to make it compatible for my own experimental reasons, it just may never run correctly no matter what I do. Like Mr. landers said, just put the right freakin engine block in it to begin with. But at least it does run while I'm trying to rebuild the s5 block.

Thanks again.

 

Hailers, Thanks. When I see you post I always get alot of helpful info.


Exactly why I didn't just assume it was the BAC like I normally would. Its only what? 3 mOhms of diff. Something else must be amiss.

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Go here and read the part about the bac http://www.teamfc3s.org/info/articles/idle.html

I'm going to study it.

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Or even simpler, go to pin 2Q on the ECU. Put one lead on a good ground and the other lead up the back of the wire at 2Q. Run the car at idle fully warmed up. Read the voltage at the pin/socket 2Q. Let's say it says 10volts as an example. Now turn the headlights on and the heater fan and air conditioner.

I nixed the air conditioner and PS when I swapped the s4 into the s5. I like two, if not one belt all together.

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IF the bac is getting signal from the ECU, you will see a voltage drop down to say about 8 volts. That means the thing is a working. And NO, that voltage drop isn't just the battery voltage dropping down because there's a load on the alternator.

I'm going to try it anyway....Or not a good idea without Air conditioning?

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A better bet about the irregular idle....might be the ACV. It has a diaphram in it that is called the anti-afterburn valve.

Is that the valve that's almost the size of a nickle that sits on the LIM between and the ACV gasket? You know, when I was messing around in there the other day I could have put in backwards. Stupid question; does it matter which way its put in? Alright that's a real stupid question because I believe that it only lets air travel in one direction.

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After several year/decades of usage.....they leak air into the intake manifold when they should not. That causes an irregular idle. Pull the large hose off that runs b/t the airpump and acv at the airpump end.. Spray starter fluid into the hose towards the acv. Did the idle change dramaticaly??

definately going to do this. All I can say is I hope I didn't put that valve in incorrectly.

Thank you very much HAILERS. You supply a lot of helpful suggestions, and I really appreciate it.

 

I don't know about the nickel size. There is a thin check valve that is about the size of a half dollar b/t the acv and the manifold. It's hard to install backwards.

No, the antiafterburn valve is internal to the ACV so you can't really look/examine it. That is the only part of the ACV that will allow air into the intake manifold, and should only do that when under deceleration. If out of starter fluid, you might try removing the large black hose b/t the airpump and the acv AT THE airpump, and plug the opening on that hose where it connects to the airpump. That will prevent the airpump from pumping air towards the acv and MIGHT give you a clue if that is the problem (air leaking past the internal diaphram in the acv, into the intake manifold).

About the other queation....even if you don't have a/c, if you turn the headlights on and the heater fan....the voltage should drop on pin 2Q IF there is an output from the ECU to the bac. If the output transistor in the ECU that controls the bac is fried, you won't get diddly in a voltage drop. P.S. I'm assuming your idle is below say 800-900 and close to 750 or so.

 

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