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Does the air pump control idle?

Old 07-11-11, 01:22 PM
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Does the air pump control idle?

On a S5 NA RX-7 (1990), does the air pump have anything to do with controlling the idle?

I'm building an SCCA road race car, so I removed the air pump and wired the 5th and 6th port and VDI open -- now the car won't idle. It starts and runs fine over 3,000rpm, but under that it sputters and stalls. I'm trying to figure out if this has anything to do with the air pump removal or if I knocked another hose off while I was pulling all this stuff.
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Old 07-11-11, 04:08 PM
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The air pump should not affect the idle. There is an idle control valve - do you still have it?

Mine has a rough time not stalling one it gets up to temps, but with a light foot on the throttle it will sit at 1k and run just fine. If your's cannot idle stable under 3k then you have another problem. Vac leaks are probably where you want to start.

-bill
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Old 07-11-11, 05:37 PM
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So this is where I begin to sound really stupid....

Is the idle air control valve the thing that you eliminate with something like this block off plate from Racing Beat? If so, then yes, I apparently took that off. The unit I removed was connected to the air pump via a tube, so I pulled everything and used a block off plate.

Outside of that, I thought it could be a vacuum problem, so I went through and capped the lines I'd forgotten about...that didn't make much difference.
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Old 07-12-11, 12:57 PM
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So I poked around the engine compartment this morning and found the idle control valve (or BAC, as the FSM seems to call it). Regardless, it is on the car. I also discovered that the air pump did feed an air line to the stock intake box, and according to one thread I found on a different forum, it provides about 2.1psi of pressure. But none of that gets me any further.

Reading that your car doesn't idle when warm but will stay running with your foot slightly on the throttle, I decided to warm the car up. It sputters and pops the most when cold and under 2,000rpm, but once the engine is warmed up, if I rest my foot on the gas it will idle at 1,000rpm.

So the main problem seems to be a cold idle problem. I'm not sure what that means, or if it is related to the tube that used to run from the air pump to the intake, but at least I now know I didn't screw things up too bad -- and now I have something else to research.
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Old 07-12-11, 02:26 PM
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You have to understand 80s EFI systems and the larger emissions issues with older rotaries to truly understand how and why the air pump works the way it does.

Any rotary engine before the Rx-8 Renesis has peripheral exhaust ports, meaning that they are on the rotor housings. This creates a natural EGR (exhaust gas recirculation) effect at low loads, causing combustion fluctuations that can lead to misfire. To prevent misfire the old engines run a rich AFR at idle (richer than 14.7:1 or lambda=1 for gasoline).

The problem with running richer than 14.7:1 air fuel ratio is that it results in higher fuel consumption but also increased HC and CO emissions. It also hurts conversion efficiency in the catalytic converter oxidation stage, the part of the converter where HC & CO are turned into CO2, H20, and O2. To address this and several other emissions issues, Mazda has a secondary air injection system.

On the FC, the secondary air injection system consists of:

1) A vane-type airpump driven by a belt
2) A port-air system which directs air to the exhaust ports, prior to the O2 sensor
3) A split-air system which directs air to the catalytic converter via the split air pipe
4) An anti-afterburn system which directs air to the intake manifold to prevent backfiring on deceleration
5) An air relief system which vents secondary air instead of sending it to the engine

Items 2-5 are mostly found within the Air Control Valve which has a series of valves that are mostly vacuum-operated and controlled through solenoids. The idle control system depends a little bit on what model FC you have, but it has these basic components:

1) a two-stage, 3 valve progressively opening throttlebody operated by a throttle cable. This has a few stop screws that are adjusted from the factory

2) a mechanical fast idle system which increases the primary throttle valve opening angle when cold (fast idle/thermowax system). This is controlled by coolant temperature via the TB coolant hose. It also has several adjuster screws that have been set from the factory.

3) a duty controlled, plunger-type idle air control solenoid valve to supply air past the throttle plates (called BAC valve on the FC)

4) an air adjusting screw to set a fixed amount of air bypassing the throttle plate, independent of the fast idle system or the idle air control solenoid valve

5) dynamic adjustment of ignition timing and mixture at idle to keep the best and most lean idle possible (only s5 does this on the FC)

This whole system works together.

When you start pulling stuff apart, you are going to have to do some work-arounds. First you need to verify no air leaks past the airflow meter. Then you need to make sure your fast idle system is working properly; if that's not working you will have to open the primary throttle valve stop screw on the TB. You also need to check your BAC valve isn't gummed up with carbon. Wiring open the aux ports and VDI probably isn't helping. Decreasing the runner lengths [by wiring open VDI] changes volumetric efficiency at idle.

Since this is an actual real race car, I would verify no vacuum leaks and other mechanical problems. Then I would remove the fast idle system, BAC valve, ACV & secondary air system. Adjust your primary throttle stop screw and make the car idle at about 1200-1500 rpm warm, which will realistically be like 700-800rpm cold. Since it's a race car you just need to keep it alive.
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Old 07-12-11, 03:15 PM
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Well that certainly gives me a lot to look at -- thanks for the help. This morning I played with the fast idle system and BAC a little, and thats when I got the car to idle with a light foot on the throttle. I'm decently sure that the remaining components on the car must work, because the car idled fine before I started removing the air pump -- they possibly just don't like the fact that I've removed the air pump and are freaking out (like you said, they're designed to work together). I'll do some more hunting for vacuum and air leaks and then maybe try adjusting the throttle stop screw.

And you're completely right -- I don't care how the car idles, I'd just like it to idle (but at the end of the day, even that isn't totally necessary).
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Old 07-12-11, 05:00 PM
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I would leave the BAC in there - it's independent of the ACV and the air pump and doesn't do any harm. It makes the engine a little better behaved.

-bill
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Old 07-12-11, 06:49 PM
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Once I found that I still had the BAC, I pulled the pipes to see what happened...and it made everything worse. I think you're right about leaving the BAC in to keep the motor better behaved -- after all, I passed the point of actually knowing what I'm doing quite a while ago.
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Old 07-13-11, 01:24 PM
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You can remove the BAC coolant hose. It's mostly there to prevent the valve from icing up.
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Old 07-13-11, 01:32 PM
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I played with the throttle cable this morning -- once the car had warmed up, I got it to "idle." Its idle is now 1,000rpm, then shoots to 2,500rpm, then back to 1,000, 2,500, 1,000, etc. I'm guessing that's the best it'll do since it seems almost all rotaries at the track have that type of idle. I'll fire it up again tonight and see if I've completely messed up the cold start.

Part of the difficulty with this is I can only do this twice a day in order to test the cold and hot idle, and since the car is running no radiator fan, I have 15 minutes or so before I have to stop.
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Old 07-14-11, 09:22 PM
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A hunting idle is a sure sign of a vac leak. Make sure that you have all the vac lines capped off then start looking at your gaskets and seals.

Good luck,

-bill
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