BCDD and Air regulator
 

My 81 zx NA idles at around 500 rpm and wants to die when its cold. once it warms up, its pretty steady at 950 rpm. The air regulator works fine, but But it seems like its doing the opposite of what its supposed to. I hav already done an ignition tune up and vacuum lines filters etc. Can I live without the air regulator and BCDD? I think the BCDD might be opening at idle (it shouldnt) and then when the air regulator is open, its just too lean at idle. I have the original AFM and the idle m,ixture screw has never been touched, but the idle speed screw on the intake has been.

I dont have the BCDD hooked up and I only just recently started using my Air Regulator again after a few months without it. I took it off because when I changed TB's I had no way of hooking it up, but once I acquired everything I needed I reinstalled the Air Regulator. But anyways, no you don't need either one for the car to run, and you wont even miss the air regulator once the cars warmed up anyways. As far as the BCDD goes mines been plugged off since the day I bought the car and I haven't had any trouble related to it.

i dont have my BCDD connected either and the only effect it has had is when i am slowing down using the tranny. it will "pop" or "gurgle". doesnt hurt the car or anything.

adjust idle speed?
 

You say the air regulator works fine? Is that the air flow meter?

I believe the item that is not working is this one pictured. It controls high idle when the engine is cold.
Sounds like you've got the correct idle once warmed up.

Yah, thats it, I dont know why Nissan put it on at all, it does the Opposite of what it should do for a cold engine! You want a motor to run a little fat until it warms up so it wont die. TARGA obvously didnt read all of my post. I am guilty of this on occasion. The idle SPEED adjustment is a valve on the side of the intake manifold, there might be a plastic plug covering the adjustment screw on unmolested models. Dont worry about the idle adjustment, even good techs will get to adjusting on both at the same time and mess a car all up.

Mine are unmolested, on the 2 Z's I have. And I think the main reason you can't set the idle, is because the computer adjusts it accordingly. Depending on how the car is running. So if you start screwing around with the AFM, and idle and such. It can confuse the computer, which is trying to get the car to run smoother. That is what I have learned.

From what I can tell, and Jfairladyz can probably back me up on this, thi ecu doesnt directly control the idle. The throttle position switch, idle speed screw and idle bypass in the AFM do.

There are only two things that control the idle on the 280zx. One is the idle adjustment screw. It's located either on top of the throttle body on the 79-80's or next to the manifold as mentioned above on the 81+. The other only controls it when cold, and thats the air regulator. The air regulator is meant to raise the idle by letting more air bypass the throttle to compensate for the extra fuel that is "defaulted" into the computer while it's cold. When the coolant temperature is below a certain point the ecu automatically enriches the fuel mixture accordingly and takes absolutely no input from the O2 and therefore could care less about AFR. The idle bypass in the AFM does NOT affect idle however. It only allows more "unmetered" air into the manifold to adjust the O2 AT IDLE and ONLY at idle without any input from the ecu. Timing can affect idle speed drastically but it doesn't control it. So if the ECU is having trouble reading any of the Cylinder Head Temperature Sensor this will affect the mixture ratio of a cold car and can therefore affect the idle speed.

So the problem either lies in the air regulator itself (sounds pretty confident it's opening though), the CHTS, the ECU is not enriching the mixture enough, or even a corroded wire giving a bad signal. TPS can affect idle but it would do so full time and not just when the car is cold. The only point of the TPS on the Z (as it's a SWITCH and NOT a SENSOR) is tell the ECU whether or not to switch between open and closed loops. Don't get me wrong, some cars use a TP Sensor, but the Z uses a switch.

So in a way WildmaN you're right in that the ECU does control the mixture which can affect idle. But at the same time messing with the AFM like I said won't do anything for a cold car and the computer has no idea what the car is idling at which is why it's so important to set the TPS correctly.

So the first thing to do is to verify that the air regulator is actually open. Can you verify that jm? Don't guess you need to actually check if you haven't already.

And come on guys, I got both your backs :D

Oh and coincidently all of this is why you never try to smog a cold car. It will usually fail miserably as it runs so rich.

The air regulator works, but it is leaving as soon as i am done with the body work this weekend. I would post some pictures, but dont know how.