I'm working on an emissions systems reference to help with troubleshooting and for people who start removing those systems without knowing what they're taking off. So far I'm basically sticking to info from the FSMs. I have a few questions:

Is the 3,000rpm startup provided by the BAC mainly to warm up the catalytic converters? The training manual says the ACV stabalizes idle; how does injecting air into the exhaust ports stabalize idle?

Also, if you see any inaccuracy in the reference, let me know. I'm updating it as we speak, so if you try to modify it, I'll overwrite you!

 

There are a couple of really good ones around that I found by doing a search for the emissions system removal. One is at rotaryresurrection.com, a couple of good ones at Mazdatrix.com and something at fc3s.org I believe

Like Mulder says, "The truth is out there"

 

Oh, sorry, missed your questions.

First, I'm pretty sure it's not the BAC that causes the car to rev to 3000 rpms, I'm pretty sure it's the Thermowax driven cold start assist. I have a BAC, but no thermowax, and my car doesn't rev to 3000. It raises the idle so the car will run, because without the cold start (Not sub-zero start!) then (mine at least) won't hold an idle until the car heats up to operating temperature.

The BAC provides a higher idle to compensate for things that would drag the idle down, like A/C, power steering and stuff.

Maybe my car is different, but the only thing on the exhaust ports is the exhaust manifold. The ACV injects air into the lower intake manifold (the intake ports are directly over the exhaust ports).

 

Ok...your wasting your time. If you had searched you'd find that we already have some extremely well written writeups. Richter is right. The BAC has nothing to do with the accelerated warmup system.

 

I'm not sure what the 3,000rpm startup (AWS) is triggered by (not the BAC), but it's useless. There was a TSB to remove it. It was only put there to help pass U.S. emissions testing. It was something like for new cars they got their emissions tested after 30 seconds from being started cold. The rotary sucks for emissions when cold. But, for modern testing (Cali.), it's not needed.

Keep the BAC and Cold Start Thermowax.

 

The BAC most definitely does raise idle to 3000rpm (for the first 17 seconds on a cold start):



The thermowax's fast idle cam also raises idle, but it raises idle to 1500rpm, and gradually falls off to not adding anything to idle at all. I am wondering if the 3000rpm is to give the cats a "shot in the arm," which is what I believe, but the FSM doesn't say (unless I missed it), so I was hoping someone well versed in the subject might give some input. My reason is, if part of the BAC's function is emissions related, it should be in the emissions writeup. This particular writeup is part of the community maintainable reference project, meaning if there is an improvement, anyone can update this exact reference page, so people don't have to scan hundreds of threads or multiple off-site how-tos. For example, if I was wrong about the ACV being able to inject into the exhaust ports, Richter could log in and update it without private messaging me to do it. For more info on the project, check here.

 

Ok, maybe the BAC is what raises the idle there, but it's mainly for maintaining the idle when P/S and A/C are used.

The thermowax raises the idle to 1500rpm when cold so you don't stall and gradually falls off because it's not needed once the engine is warm.

Yes, the 3000rpm is to heat the engine/cats up. It's for emissions only. Like I said, there's no reason to keep that piece of **** system active.

 

Thank you sir, I'll note that the BAC factors into emissions in the overview. So you know, the project has a pretty good idle overview already, but if you see obvious flaws, feel free to update. For instance, the air bypass solenoid isn't mentioned there, so I'll go ahead and fix that.

 

Alright, now to get to the bottom of the BAC question, here's the reference image (from the training manual) I was going off of:



I believe this shows that the intake manifold connection (blue) is for the anti afterburn system, while there is an exhaust port connection (red) to divert the air pump to. It would make sense because then the air pump could inject oxygen into the exhaust manifold/furnace to burn off hydrocarbons before they reach the cats.

 

Quick update: This is probably the passage the ACV uses to inject into the exhaust ports. I poured water into the port where the red arrow is, and it came out the exhaust port.

 

The BAC system includes the BAC valve and the Air Bypass Solenoid valve (AWS). I believe it's specifically the solenoid valve and its respective relay, not the BAC valve itself, that raises idle to 3k rpm. The training manual is unclear about that. I'll check with a multimeter for proof, but it makes sense, because the solenoid valve wouldn't be good for much else otherwise.

 

Alright, I didn't want to talk like it was gospel in case I was wrong, but the port you're showing where the red arrow is located BELOW the exhaust port. The ACV is bolted directly to the lower intake manifold (I have a blockoff plate there now). The two holes below the exhaust port also feed into the exhaust manifold, as to design or purpose I have no clue.

Also, I just finished doing a complete emissions removal for my car. Now when I start the car, the engine will NOT idle when it's cold. I misread some information about this and reinstalled the BAC and the Split Air Solenoid valve, thinking this would help my car to hold an idle until it warmed up. Before reinstalling the BAC and the split air solenoid valve, I checked them out using the procedure in the FSM and both are working perfectly. I reinstalled them. Not only does my car still not idle cold (which is handled by the cold start thermowax), it also does not hold itself to 3000 rpms, no matter whether you're in gear, in neutral, pressing the clutch, pressing the brake or whatever. My TPS is spot on. Therefore, the BAC is NOT the thing that raises your idle to 3000 rpms for 17 seconds.

I cannot however, vouch for the ACV in this regard, because mine is still removed. I only know that everything on my car has been tested and found working. The motor is a fresh rebuild, and other than the initial surge from the engine starting, it does not hold 3000 rpm (or in fact, ANY rpm) even with the BAC and split air solenoid valve connected.

I will admit though, that my understanding of the ACV valve is incomplete. I do know that it is mounted to the LIM, so I presumed whatever air it had would enter that. I hear there are a couple of good, detailed writeups on it, though.

 

Holy misinformation....

To clear a little bit of this up...

The 3K startup, AWS, ACV and BAC systems are totally different.

The 3K startup is primarily created by the AWS. This is based on an oil temp sensor in the oil pan, and the thermoswitch on the bottom of the radiator. During cold start, the AWS opens and allows a lot more air into the intake manifold past the throttle bodies. This is responsible for the 3K startup. I have also heard that the BAC contributes to this by opening up a bit. Can't say for sure as I have never tested. There was NEVER a TSB to remove this system. The TSB is for the cold start assist system (ie. passenger side firewall resevoir). I don't know why people keep confusing the two.

The ACV is responsible for directing air from the airpump to either the cats, exhuast ports, air silencer or rear rotor. The exact time all these happen is covered in the FSM and Haynes.

The BAC is responsible for maintaining the engine's idle under varying loads. For example when you turn on the headlights the electrical draw from the alternator increases which tries to load down the engine. The BAC responds by opening up a little, allowing more air into the engine and increasing the idle. If you turn the power steering, then both the BAC and AWS open up as the power steering presents a much larger load on the engine that the BAC cannot handle by itself.

The thermowax is a small pellet of wax in a case that is mounted to the rear of the throttle body. I don't remember if it retracts or extends when it's cold but either way this moves the cold start cam which keeps the throttle plates open for the 1500 RPM cold idle. As the engine warms coolant circulates around the thermowax, thus moving the cam and grandually lowering the idle.

 

Richter12x2: I'm thinking there's a passage from the ACV through the center iron to the port I pointed to - there are passages everywhere (i.e. the TB) so this sounds reasonable to me. Unfortunately, I don't have one of the irons on me, I only have that rotor housing. I believe (check my last post, I may have posted while you were working on the above) the BAC's BACV doesn't handle 3k rpm idle, but the BAC's air bypass solenoid valve does. I'll multimeter the AWS relay to see, because I don't have an ACV, thermowax, or air bypass solenoid valve anymore myself.

Aaron: Excellent, so my info is correct except that I didn't know the ACV can direct air to the rear rotor. Thanks for clearing things up! (although the BAC system includes the BACV and AWS)

 

Another quick update: according to the S4 and S5 FSMs, both the turbo and n/a ACV directs air to one of the three places I listed before: main converter, exhaust ports, or air relief. It's the anti-afterburn (on top of the ACV assembly) that directs air to the rear rotor during deceleration.

 

Aaron, I bet you're right, the BACV probably does add to the 3k rpm, because check the graph: the AWS can only be on or off, but there's a second stage of increased airflow just after the 17 seconds. So there have to be two systems adding to the 3k rpm startup, and the only systems I know of that the ECU can use to adjust idle speed are the BAC's BACV and AWS.

Someone had a 2.5k rpm startup problem before, and I bet if we know which valve was adding which portion of the 3k, pinpointing the problem would have been much faster. I'll scope the BACV and AWS and see which cuts out first. See? Collaboration is already paying off, and we'll end up with a better reference because of it. I would have missed the second hump of the graph even though it's right in front of me.

 

About those housing pictures. Yes, on a series four non turbo the acv feeds the air from the air pump into the intake manifold where it exits at the top of each END Plate (front and rear side housings), where it travels DOWN in an internal passage in those housings to the bottom and exits into the place you pointed to in the rotor housings where it then goes into the exaust ports and mixes with the exaust gas PRIOR to the catalytic converter.

On a series four, the BAC in combination with the Air Supply Valve cause the startup for a cold engine to go to 3000 rpm. IF the Air Supply Valve is disconnected, the rpm will still go high, but only to about 2000 to 2300 rpm for 17 seconds.

After 17 seconds the rpms on a NORMAL car will drop to about 1300 rpm because of the thermowax device on the throttle body. After the water gets hot the thermowax piston extends and pushes the *cam* off the roll pin and that action closes the throttle plates which means the idle will drop to .....idle speed which should be approx 750 rpm on a NORMAL car.

The path of the air from the airpump to the acv to the exaust diffusers is a touch different for a S5 n/a, turbo or a S4 turbo. But in the end it goes to the exaust prior to the catalytic converters.

If the Air Supply Valve plug is taken off or the Air Supply VAlve is removed I doubt it will effect emissions at the check station since you had to drive the car there and the engine is well past the cold stage.

The 3000rpm cold start should only happen when the water temp is below approx 65 degrees F. Any temp above that and it should not happen. But the thermowax will still cause a high idle until the water temp rises to normal.

 

Awesome HAILERS, so maybe I don't need to scope (or do I?), and there's the answer to the ACV passages too. Does this graph get your blessing?



It combines a few of the FSM graphs into one, with more ASV info that you just provided (FIC is Fast Idle Cam - is there a more appropriate name?). I still wonder about the original BAC system graph, because the second hump after the 17 seconds has overshoot, like it's not a slow change (fast idle cam) but a fast one (ECU cuts something off). I think I still may scope this thing to satisfy my curiosity!

 

I wrote that on the fly so watch for mistakes. There's something I don't understand about either graph on this thread. I was pretty sure the engine water temp had to be below 65 degrees for the 3000 rpm to happen at startup. I've no fsm where I am right now, but I was fairly sure of that temperature.

And I'm fairly sure the oil temperatue sensor in the pan hasn't anything to do with the startup (I think I saw that above somewhere).

Again, if the Air Supply Valve (AWS on S5) is not connected it should have no effect at the emissions station since you have to drive there and the water temp is up pretty high.

I couldn't think of the name when I wrote the first thread, but the acv air goes into the exaust DIFFUSERS to mix with the exaust gas. Couldn't remember Diffusers for the world.

By the way, if your interested in RX stuff, if you look at the SERIES FIVE FSM and the Fuel section, there are several pages towards the rear that are helpful. LIke OUTPUT DEVICES and things like that. Much of it applies to S4.

 

Good catch, I forgot to say what "hot start" meant; the 3k idle is only for coolant temps 59 - 98.6F (per training manual). I think the bottom limit protects the cats from heat shock when they're extremely cold. I'll update the reference and image with those temps. As for the sensors, so far I've only listed those the FSMs suggests, and I don't think the oil temperature sensor has been mentioned yet. Maybe for the sub-zero starting assist? This thread has veered away from my initial 3k idle question, but the result is a more useful set of references; I appreciate everyone's input. Here’s what I’ve updated with this thread’s info:

Idle System Overview
Emissions Systems Overview

The S5 FSM charts look like they'll open a lot of possibilities - thanks for the heads up!

 

Actually... that temperature range is interesting because that means at both fast idle cam extremes, the AWS is disabled. The AWS only ever kicks in when the fast idle cam is inbetween, meaning my graph is incorrect. Don't give it your blessing yet...

 

I wrote something wrong. The 3000 rpm shouildn' happen below 65 F, probably to protect the engine, what with cold, thick oil etc.

And FYI, the BAC Goes full open anytime the key is held to START. It's in the S5 manual and you can see it for yourself by removing the bac and holding the key to Start while staring at the piston in the BAC. The piston doesn't move very far for full open. But it stays full open and does not *vibrate* like it does normally.

 

Hmm, well I'll be damned. I was always under the impression that the BAC was to help balance the idle due to the load of the AC and PS only. I guess I was somewhat wrong. BUT considering I eliminated all that crap off my TII you can understand why.

 

Your right, but it does other things too, as aid in the cold start for 17 seconds and go full open during Start.

 

I updated the reason for the low temperature cutoff, and when you mentioned the ignition switch, I checked the S5 FSM again and saw that I missed the arrows from the AWS sensors into the BAC system (page F1-40) - thanks for the heads up again, that's now fixed!

HAILERS: I PM'd you an updated idle graph