Car: 90 vert with JDM Tii freshly rebuilt
New TPS
no vacuum leaks
idles perfectly smooth at 750rpm,
all emissions intact
RP down, mid and Bonez cat with Corksport catback
ACT Streetlight flywheel
Rtek1.8 with SAFC Neo

Car will occassionally die when dropping to idle. Have replaced dashpot with brand new one but havent tested yet.

When you turn the a/c on the idle rises momentarily which indicates the BAC is working.

Read Section F2 of the FSM. For setting the idle they only touch the BAC. There is no procedure mentioned for setting the idle screw on the linkage. I have a perfect 750 idle but when installing the dashpot I noticed the idle screw does not touch the linkage plate.

Is there a published procedure for synchronizing the idle screw and the BAC?

Here's my theory. I know the ECU uses the BAC to tune the idle speed by opening a valve in pulses (the duty cycle). I assume the more the BAC screw is open, the more air leaks in during a pulse. So my theory is that I should close the BAC screw all the way down and then use the idle screw to open the butterflies to get an idle of about 700. And then open the BAC screw until the idle comes in a 750.

All with the engine warm and the ISC connected.

Sound about right?

Thanks,

Jim

 

The BAC screw acts independently from the ECU controlled portion. The screw basically sets the minimum amount of air that will flow into the manifold, then the ECU adjusts duty cycle to determine how much extra should enter.

I'm assuming by idle screw you mean the throttle stop on the TB, right? I just did what you're suggesting recently to get my idle to behave. I turned the stop screw about a half turn clockwise, then warmed the car up to see where it idled. It settled at about 1400 RPM when fully warmed. Since the BAC screw had previously been all the way open, I just backed it off until the idle speed dropped to where I wanted it.

 

Is your thermowax still in tact?

 

Davidov - Yes, everything on the intake and emissions is intact including the cold idle system.

88 - Yes, the idle set screw on the TB. Last night I put our PVC pipe intercooler on and grounded the ISC. Then after the engine was warmed up (and our new oil pressure sensor said we had plenty of pressure! Woot!) I turned in the idle screw and raised the idle to about 1100.

Then I tried turning in the idle screw on the BAC all the way in to reduce the idle... and nothing happened. Screwed it out all the way and... nothing happened. Hmmm...

So I turned on the A/C and the rpms rose so the electrical part of the BAC is working.

I'm wonder if my homemade BAC gasket is blocking the screw's port. I was thinking about switching to the BAC that came with the engine (a JDM). I picked up a replacement because the water nipple was broken when it arrived. Turns out the JDM ones dont have the idle screw.

Either way, I backed down the TB idle screw until the R's were just about 800. Then I recalibrated the TPS (seems to be required when you mess with the TB's idle screw) and now with the new dashpot installed it doesnt seem to come close to stalling anymore when you let off the gas.

So it's all good. Thx for the info.

 

dont forget your idle enrichment needs to be adjusted from winter to summer i was told.

JUst barely though, SOmetimes it dont even need it. But mine had no emissions nor thermal pellet. so i was told i would have ot mess with it like once a year

 

If you have s5 T2 JDM manifolds your BAC screw does nothing. There's no passageway in the manifold for the BAC valve to add in extra air controlled by the screw. I've discussed this in this thread: https://www.rx7club.com/3rd-generation-specific-1993-2002-16/idle-screw-adjust-854571/ near the bottom of post #6 and the dialogue continues from there.

JDM BAC turbo valves don't even have idle adjust screws on them.

 

Ah, that makes sense. Yeah, I have a JDM UIM and the BAC is USDM. The water nipples were broken on the JDM BAC.

So the only way to adjust the idle on a JDM is with the TB's idle adjustment screw.

Thanks

 

btw, I was wrong. It would still die occassionally. That's the fun of intermittent problems, eh?

I up'd the SAFC Lo-Thr settings for 500, 1000 and 1500 by 20% and so far that seems to have fixed it. My Lo-Thr threshold is 30% and I reviewed the SAFC manual to confirm that the 30% is for 30% throttle and below.

Anyone know why Mazda engineered the USDM idle to be a combo of the TB idle screw and the BAC but didnt for JDM?

 

I guess it's just something they felt like doing to S5 JDM engines (S4 JDMs actually have the BAC screw). Just another makes-no-sense change like removing the variable resistor from S5s.

 

Yeah, even from simple economics, "Cost of Goods Sold" , spares inventory mgmt, cost of assembly and cost of documentation perspectives it seems like the differences they made for their two biggest markets were just silly.

 

There may have been an accessibility issue on a right hand drive car. Steering column in the way? And I've wondered before, do the s5 T2 JDM TB's have an idle adjust screw (akin to the BAC screw) that we don't know about? On the FD, the BAC air adjust screw was moved to the TB.

 

I dont know. I would have never thought to look. The car is with the kid at school so I have no way to check.

 

Perhaps I'm not nimble enough, or not thinking around corners very well. I can't see a damn thing on the back side of my engine ('87 fc na). Shy of pulling the hood off, or finding a good mechanics mirror and fifth hand, how are you folks seeing the screws on the firewall side of the UIM to adjust the idle? Or have you done it so many times you know it by feel?

 

It's extremely easy to find the throttle stop screw on turbos. Once the intercooler is off, the screw it right on top in plain sight (the TB is upside down in relation to an NA). But NA owners are not so lucky, and will have to fish around back there with a mirror to find it.