I've had this issue for a while now but with winter and cold starts becoming a problem I want to try solve it. When I switched over from the OEM ECU, I had zero issues starting/idling.

I've followed the suggestions by many forum members about how to determine the PWM Idle duty range. Most people say they get a range somewhere around 20-60%. However, if you look at my log, I don't get any reaction from the IAC Valve until around 70% PWM Idle Duty when the RPM starts to increase very slowly. Then when I get to around 92%, each 1% increase after that results in a dramatic 200-300+ RPM increase. This was all done in Open Loop (warmup) mode. Also, you'll notice at 2% PWM Idle Duty, or when I unplug the IAC valve for that matter, the idle RPM is sitting around 800 RPM. Also, coincidentally, around 70% my IAC Valve starts to get a loud buzzing sound that I can hear from the cabin, and by 90% it's extremely loud. I can get a soundclip later if that helps. And if it matters, I'm using VE Table 3.

Any suggestions on what to try next? Thanks.

 

What MS variant is this?

 

Idle valve frequency of 306Hz seems waaay too high. I'm running 75Hz.

 

This is an MS3 Basic built by Reverant. If it matters, I believe it is an 'older' version before Trackspeed Engineering started selling them. I bought it used from another forum member here.

Interesting, I'll try a frequency around that area next and report back. I thought I was told by someone that the NA IAC valves liked something ~300Hz, or maybe that's just because that's what the MSPNP base map uses. This thread (https://www.miataturbo.net/megasquir...equency-48120/) has some more conflicting info about what frequency I should be using, but I'll try something low like 75Hz next.

If anyone else has any suggestions please let me know.

 

If this is a 90-93, the correct idle valve frequency is 240Hz, and min/max duty is 30-70%. 94-97 use 300Hz, again 30-70%. 99-05 use 500Hz, 20-80%.

Based on your problem, it sounds like the flyback diode has failed. You may want to try adding an external 1N4001 to the idle valve wiring and see if that stops your problem.

 

Gotcha, I have a 1994.

From reading posts online, I believe there are two wires on the idle air control valve. A white/red that supplies 12V and a blue/orange that I think must be the PWM signal. So to test if my flyback diode has failed, just connect a 1N4001 in between the blue/orange connector and the IAC valve? Also, will a 1N4003 be fine? I've got those laying around already. Thanks.

 

A 1N4003 is fine. Connect the banded end of the diode to the 12V wire.

 

Cool. I'll give that a try and report back.

 

Actually one more question. So I understand I need to bridge the 12V (white/red) wire with the signal (blue/orange) wire using my diode. What is the best way to do this? Is there any way besides cutting back the factory wiring near the connector itself? I'd prefer not to open up the factory wiring if possible. I'm not sure if there's a better way near the harness adapter itself (I see the Idle Valve on the DB37 pinout) or some other simple solution I'm missing to put the diode in externally as a quick test.