This is my first automatic RX7. Should the BAC up the idle when shifting into gear? Seems like the idle drops a couple hundred RPM when in gear as opposed to P or N. Idle in neutral is 800 give or take. TPS and timing are spot on. The AC seems to idle up to about 1000 when the clutch clicks on, but then drops. Maybe the BAC is weak? It just doesn't seem right for the idle to drop to 5-600 when in gear.

I grounded the check wire by the leading coil and unplug/plug in the BAC and hear it working. I cleaned it out when I did my rebuild btw. It wouldn't surprise me if the BAC was weak. I had to rebuild the 6 port plungers, AWS solenoid is stuck shut, and VDEI tumbler had to be chiseled out/honed out and replaced from 7 years of sitting.

 

I've never owned an FC with an auto trans, so your question got me curious. I took a look at my '89 FSM edition, and it appears what you're describing might just be normal. I reviewed the block diagram for the entire idle control system for the NA on page F1-40, and here's what I deduced. For normal idle operation, where only the BAC is driven, it shows no inputs that would tell the ECU that the AT shifted from P or N to drive/reverse, so there's no way the ECU would know when to raise idle to compensate for the load caused by the AT. There is however an AT inhibitor switch input, which it shows connected to the EC-AT controller, which in turn connects to the ECU and appears to only be there to drive the Accelerated Warm up Solenoid (AWS) function, which is when you first start up the car. Does your FC do the high idle thing for a few seconds right after it starts? If so, then I think the AWS function is working normally

Anyway, if the drop in idle when shifted into gear is significant enough where the car fells like it's about to stall, it probably wouldn't hurt to clean out the BAC valve. My usual procedure for that is to remove the BAC, and soak its innards/passages with a strong de-carbonizing solvent, like Seafoam. Let it marinate for a while, drain the Seafoam, then get a 12VDC source and while squirting brake cleaner thru the BAC valve passages, rapidly click the BAC solenoid on & off a bunch of times.

 

AWS isolenoid is seized closed. I don't miss it, but may replace it at one point just to know it works... I cleaned the BAC valve in the Ozzy tank at work when I cleaned all the disassembled engine parts during my rebuild. I connect 12v and ground to it and it opens a few mm and can blow air through.

I had to literally chisel out the ceramic tumbler in the VDEI. Had to take a cylinder honer to the bore to get another tumbler from another intake I've had for years to turn freely. Plus the AWS is stuck shut, so it wouldn't surprise me if the BAC was just weak. I have a spare from an old car that I deemed as bad and it acts the same way.

Perhaps I'll just take the one from my GTUs that I know is good and try it.