Interested in some input here from folks that have had experience with this exact problem on their S5 NA. This car starts just fine cold and the RPMs go high like they are supposed to on a warm up cycle, however, when the ECU turns off the cold start fast idle cycle, the engine idle surges and drops, surges and drops, very rhythmically, you could practically set a clock to it. And it does so within a very specific range and only for about 3 minutes or until the engine starts to approach operating temperature. I have read several posts that are eager to implicate the BAC valve in a situation like this. One post suggests running the following test to determine if the BAC valve is faulty -- "with the engine running at operating temperature, turn your vent fan on full blast and watch the RPMs drop." The post suggests the BAC valve is faulty if your RPMs drop but do not recover. On my car, the very thing happens. My RPMs drop almost 200 RPM and never recover until I turn the vent fan off. If I hit the A/C button and do the same thing, the RPMs do recover -- this seems strange to me. So in other words, I'm not really sure I trust this method. Can anyone suggest a better method for determining whether or not the BAC valve is functioning properly? Also, just so you know, I did pull the BAC valve and clean it out. And I do know for sure the BAC valve is doing something because if I pull the connector off while the engine is running, the engine dies.

How many people that had this engine warm up cycle rhythmic revving go away after replacing the BAC valve? Thanks.

 

because if I pull the connector off while the engine is running, the engine dies.
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You just proved the BAC works. You also proved that you have not set your idle speed to approx 750-800 rpm. So you NEED to set your idle. See your local FSM download, fuel and emissions section.

Your BAC is having to work it's *** off to maintain an idle. It would not have to work so hard if the idle had been set to 750-800 rpm. The BAC is not there to CREATE and idle of 750 rpm, it is there to help MAINTAIN and idle speed of 750-800 rpm when a LOAD is put on the engine.

 

Does your car go through the rhythmic bouncing when the car is just cold started and not in gear or when the car is cold started and in gear and you have the brake depressed? What I'm questioning is there a difference between the two scenarios?

 

similar, but my car does nothing when i pull the plug, but wont start without it connected.

 

agreed! i need to scan it in, but the S5 will actively search for the correct idle speed, and it can use the BAC valve AND timing to do it.

so it uses the "base" idle speed calibration, and adjusts from there, if your base idle speed is off, it still tries to find the right speed, but its calibrations are all off, and it does weird things like surge

 

Yeah, the series five BAC has more inputs to it than my series four car.

From the first jpg, if read, you can see they jumper the initial set coupler. That act nulls out the BAC to a fixed duty cycle so it won't try to make any corrections as you turn screws etc. Persomally I just pull the plug off my BAC and set the idle as close to 750-800 as possible then reconnect the BAC plug.

I posted that first jpg to emphasize that the car in the jpg is idling with the initial set coupler jumpered to gnd and the guy is setting the idle while it's idling. Get my point? Its idling with the BAC nulled out. Be kind to your BAC and don't work it to death, set your idle speed or BASE idle to help it out.

As for the guy who's car won't start without the BAC connected.......let it be known that when you HOLD the key to START..........the BAC should be goiing full open and stay that way til the engine starts. Then it goes into its duty cycle which is the little buzzer cycle at approx 120hz.

 

ive had the bouncing around ..i elliminated with antifreeze was not getting to the thermwax it was low ...and i found a small vac leek

 

Made some headway on this finally. I need to make a correction -- I stated that my car died when I removed the BAC plug. Well, it turns out I was also inadvertently removing a vacuum hose at the same time and it was the loss of vacuum that was causing the engine to shut off, not the BAC unplugging. So, I repeated this test multiple times, making sure only to unplug the BAC. The result... ABSOLUTELY NO CHANGE IN ENGINE IDLE WHEN I REMOVE JUST THE BAC CONNECTOR (PLUG). So... time to go look for a tested and working BAC valve for an S5. Anyone? By the way, I am assuming it is this faulty BAC that is causing the surging during warm-up cycle. Yes?

 

If your idle is set properly then removing the BAC should not have any effect on idle unless your car was under load. Again, Does your car go through the rhythmic bouncing when the car is just cold started and not in gear or when the car is cold started and in gear and you have the brake depressed? What I'm questioning is there a difference between the two scenarios?

Also, while your engine is going through its bouncing stage and you grab a pair of pliers and squeeze close the BAC hose does the bouncing continue or not? If the bouncing continues then the BAC is not the likely culprit, but introduces into the equation components such as the vacuum diapragm perhaps and even the thermowax. In addition, what is your idle set at RPM wise? The idea that the BAC is problematic for a few minutes after a cold start and then the problem goes away is a bit peculiar to say the least. Answer these questions and you'll provide insight into helping to solve your problem for the more that's know the easier the solution will be!

 

"vacuum diapragm" = Double throttle diaphragm.

 

There is a slight difference between unplugging the BAC to set the idle and jumpering the initial set coupler. When the jumper the initial set coupler the BAC valve is still doing something (fixed duty though). On the FD for example when you set the idle and use the initial set mode, the BAC duty cycle is fixed at 38%. On a 90s Honda when you set the idle you unplug the BAC valve (called an IACV, same thing) and the idle drops. Then you set idle to about 450-500rpm (yes it will idle that low) and plug the BAC valve back in to get back to 750-800rpm.

 

Sorry to tell you this........................the valve itself rarely fails EVER. What drives it does fail. Called the *transistor inside the ECU*.

Can I ask what rpm was the engine idling at when you pulled the BAC plug???? IF the rpm was close.....real close to 750 or above, I'd expect no noticable change. BUT if the idle was say 500 or below and there was no change...................trouble is brewing.

I can tell you one of the simplist ways of telling if the transistor inside the ECU is good or bad. You reach under the car and pull the small trigger wire off the starter. Then remove the two nuts that hold the BAC on and pull it off its studs where you can see the internal guts. Have someone HOLD the key to START. The diapharm in the BAC will move full open..........which by the way is not more than a quarter inch movement or therebouts. IF the guts move open,,,,,,,,,,the transistor is good. IF the guts do not move at all............transistor in the ECU is bad UNLESS one of the two wires in the BAC elect plug is *pushed back* not making contact with the contacts in the BAC's jack.

Nomal operation of the BAC results in it making a buzzing sound which is it opening and shutting repeatadly. Called it's Duty Cycle and that cycle is close to 120hz

 

Going out right now to "retest" the BAC valve. I truly appreciate your advice Hailers, it cuts through the confusion and gets right down to it. These cars are worth the effort -- an absolute blast to drive and worth the sacrifice in my opinion.

Here is more information to answer some of the questions directed to me:
1. The rhytmic surging only occurs for about 2 minutes during the warm-up cycle and at no other time. All accessories are turned off, the car is in neutral. In other words, the car is not under any additional mechanical or electrical load.

2.. When I pulled the BAC connector, the car was at about 750-800 rpm.

As a side-note, I am getting codes 12 and 18 which is not surprising as I know for a fact that my TPS narrow and wide are out of range and need to be dialed in correctly which I plan to do in just a second here.

Will report back with my results and any additional questions I have.

BTW (diff subject), anyone have the link to the thread that has the instructions on how to rewire the trigger wire to the starter with the use of an additional wire and relay? I'd really like to implement this mod to help with hot starts.

 

Well if the rpm was as you say...........then its inconclusive 'cause there will be no change 'cause that is the level a BAC would try to maintain........and it's already there. I'd do the thing I mentioned about pulling the small trigger wire off the starter........undo both BAC nuts and pull it off where you can see the guts..............have someone hold the key to START and see if the guts move. Should move if the transistor is any good in the ECU.

 

You could also try squeezing the hose closed during the cold start surging and this will tell you what is and what is not playing a role in your problem.

 

I tried again yesterday and have traced it all back to the TPS switch. When it was reving up and down I unplugged the TPS switch and it went away. Then, I plugged it back in and began dialing in the TPS adjust screw just until it made the bouncing idle go away. After that procedure, I fully warmed up the car, set the idle according to FMS, then set the narrow band on TPS right at 1vdc. I shall go out soon and fire it up again to see what happens this time after these adjustments. Fingers crossed people.