strange idle problem
 

Hi guys. I'm a little stumped here. Hoping someone can help me out. I'm used to f'n up my car and then being able to fix the problems I created, but this one isn't one I created.

My turbo miata has been running fine for plenty of track time. The tune on the hydra hasn't been touched in about a year. After the last session on my last track day, the car no longer had a good idle. It would surge from 1k to 3k consistantly. If you upped the rpm, the signal wouldn't break up and it drove and boosted fine. It sounded to me like a vaccuum leak, and I've checked everything, and even replaced the gasket between the 2 halves of the intake manifold. I cleaned the plugs up and checked the gap.

Here is something interesting. There is a big vaccuum noise at the idle air control inlet. If I plug that hole almost all the way with my finger, the idle settles down, and the car behaves. If I remove my finger, the idle resumes bouncing. The electrical plug for the IAC is secure.

Any ideas? Does the above mean that the IAC is bad? Or should I be checking something else?

Thanks in advance.
Matt

It's common for the NB IAC to fail so you could possibly have a bad valve. Unplug the valve and see if the idle straightens out.

I saw Bill Cardell make a post on the power list that he rarely saw a failed IAC when they were removed.

When you say you put your finger on the inlet, is it just open to atmo? Which inlet?

so it is a NB IAC, but I unplugged the electrical connection and it at that made no difference on the oscelation. Could this mean it is stuck open?

y8s - I have a small breather filter that I have on the inlet right below the throttle body. If you remove the filter and cover the air intake about 90%, the idle smooths out. Interestingly, this inlet is taking in a LOT of air. More than I remember that it should. It makes a very noticable vaccuum noise when left alone.

Any thoughts?

Take the IAC off, try to make it move, clean it out. See if you can make it work.

so i've removed the IAC, and I'm not sure what a healthy one looks like. I can see the valve is in the open position. Not sure how it is supposed to be when at a rest? I messed with it a litte, and the spring is still good and it isn't "stuck" open and you can gentally move it with a screw driver.

My cheap haynes manual doesn't say anything about trouble shooting or testing these so I'm a little in the dark.

I am guessing that the problem is either the IAC, or the signal it is supposed to be recieving?

You can try this

Attachment 214021

hmmm. my resistances shows to be 11.8-12 ohm.

But I have a 1994 TB and IAC on my car. I'm assuming that they are the same?

I plugged it in and disconnected it while looking at it. You can tell its getting a signal as the small diaphram moves about 1/16th of an inch. Not sure if it is stuck and not able to move further?

Sounds like I should try and find someone with a spare in SoCal for me to try and see if its a bad part...

awesome. thanks for your help guys.

I haven't installed it yet, but I just got another IAC valve, and it is obvious when you set them next to each other that my old one is busted.

Matt that breather should (I think) go into the intake before the throttle body. I wonder if it's supposed to provide some additional resistance to the flow.

At the gap a whole bunch of years ago, I had the IAC in my car fail open. Car would idle, but..... it would bounce from 2K to 4K.... in big swings. I figured out that the was plunger inside failed wide open..... Plugging the IAC got me home, and replacing it fixed the issue......

Note it also used to do it to me while driving as well.... Really scarey going into one of those gap corners @ night without being able to slow the car down.... <G>

Sounds the same to me..... <G>

Dave,

yup. my plunger failed wide open as well. its obvious if you know what you are looking at. I just didn't realise that it wasn't supposed to be wide open at a rest.

y8s- traditional knowledge I bring from other cars is that there really isn't any difference between rerouting it and slapping a filter on it like I have done. The reason it is done is to accomidate the MAF in the stock config. What are you suggesting that I am missing?

if there's no appreciable difference routing it to intake, then i'm not suggesting anything :)

I may be having the same problem, can you post a picture of the failed IAC? or Both? Thanks.

Matt, we have the same IAC. I think you should get crazy with carb cleaner and basically soak it so the plunger can move freely (did this in my 1.6).

I don't know what Hydra looks like, but I changed my valve frequency from the recommended 45hz to 55hz, then adjusted recovery and deadband to get it perfect. You may have to do this after cleaning the valve or replacing it.