"Saggy" Idle
 

Since I bought my 89 turbo 2, it's had a bouncy idle until it warms up, nothing really severe, but a tad bit annoying. However, a new symptom has developed.

(Scenario)
The car is at operating temperature, I put it in first, and it idles down, only slightly for a second. I rev up to around a bit, push the clutch in, leave it in gear and let off the gas, and the RPM's go down to almost zero, then it raises back up to idle speed and levels out to normal. It ONLY does this when it is in gear. It's actually died on me once while doing that. I can pull the car out of gear and rev it all day long and not experience the same problem.

So I decided to do some inspecting. I pulled the BAC off to see if there was any excessive carbon build-up, and upon looking into the intake manifold, I noticed this:

http://i615.photobucket.com/albums/t...n/IMG_1606.jpg

It looks to me like there is something broken off, or missing. Whether this is a contributing factor to my problem is unknown.

Any suggestions? To the low idle bounce or the "broken" piece?

It's a slight chip in the aluminum on the intake manifold, that's all. Inside the manifold (through the big hole) there are supposed to be two passages. One goes to the front primary runner, one goes to the rear. That hole to the upper left of the big hole goes connects to the passage in the BAC valve with the air adjusting screw. It sets a fixed amount of air to entire the intake manifold regardless of what the BAC valve is doing, electrically. First, spray a bunch of carb cleaner into the BAC valve. Then take some jumper wires and use them to run +12V and GND across the terminals. Do this rapidly to help work the carbon out of the BAC valve. What you should do then is jumper the initial set coupler and adjust that screw according to the diagnostic procedure in the fuel and emissions control section of the s5 service manual.


FYI: your idle speed is determined by

throttle plate opening angle + bypass air mechanically set through air adjusting screw + bypass air determined by BAC duty cycle.

Throttle plate opening angle is determined by the throttle body adjustment (the stopper screws which are set from the factory). The throttle plate is held open a little bit on deceleration by the dashpot. It's also affected by excess tension in the throttle or cruise control cables. Finally, the throttle linkage is rotated open by the fast idle system which is set through some adjuster screws and operated by engine coolant. All these complicated settings are completely eliminated on modern Drive-by-Wire systems. Rx-8's (which are DBW) don't have anywhere near the idling problems as these outdated systems.

That makes sense, I'll give it a try.

What doesn't make sense is why I'm only having problems when in gear.... Is there something that tells the engine that the trans is in gear? Even though the two aren't physically connected because the clutch is depressed?

Also, the service manual states not to mess with the screw on the BAC... perhaps I'm confused?

yes you are confused. on the driver side under the hood near the trailing coil there is a green test plug with to pins.

1. make a jumper wire to jump the connector

2. start car and warm to operation temp

3. connect make jumper from step one

4. adjust BAC valve sqrew to desired idle

5. remover jumper wire

take the car for a drive and see if it idles correct if not repeat also may want to try to adjust the TPS but this does not seem like your issue just something to think about

Well the car idles just fine. Only when it's in gear do the RPMS drop for a sec when I push the clutch in while the revs are coming down. Then it returns to normal idle.....