Car specs for reference: 1994 Miata, 1.8l, 101K on the clock. Plugs/wires about 15k ago, timing belt, water pump, radiator and major belts/hoses are all fresh within the last 10k miles. The issue: occasionally, I'm getting an extremely low (like 200rpm) warm idle. It's only occurring with no load, usually when the clutch is in, pulling up to a corner. So far, it hasn't dropped enough to cause the car to stall, so it's more of an annoyance than a big issue right now, but I want to try to chase it down. I haven't noticed any issues with the car when running under load.
At this point, I'm just looking for ideas. Truthfully, I'm not much of a mechanic, but it's not a daily driver, so it's something I can tinker with until I can run it down. What I'm looking for is an approach. I've already done a bit of internet research and found every possible source from plugs/wires, coils, injectors, vacuum leaks, bad gas, etc., etc. I've got a little time over the next couple of days, what I'm looking for is a plan of attack. Unfortunately, the issue isn't constant and, predictably, it usually happens when I'm out somewhere. I'm not afraid to replace parts if needed, but don't really want to thrash around buying stuff without knowing what the issue is.
No, the white phone.
Sorry. Had an Airplane moment.
Possible early indication of CAS failures? Have you pulled codes?
In reply to Sanchinguy:
Not currently throwing codes.
Bump, still looking for info. Please don't make me go to "the Miata Forum which shall not be named". 
Well, you posted right before a holiday 
My first thought is that the idle speed control valve is getting sticky. You're describing significant idle droop. That term might help with the googles.
Set the base idle speed. Use a paperclip in the diagnostic connector to jump GND and TEN. Then adjust the idle speed screw (on the side of the throttle body) until you're at 850 rpm. If you have to bring it up, that's quite likely your problem. Test drive.
Once that's done, if the car is still showing this problem, try backing out that idle speed screw a quarter turn at a time until the idle speed is just about to rise. This is a workaround. Supercharger owners know it well, as idle droop is a hot-side supercharger problem. This is the bodge solution.
Best but not as easy: Pull the idle speed control valve off the bottom of the throttle body and clean it with carb cleaner so that it moves freely. Do NOT clean the throttle body butterfly.
My 4Runner had a very similar issue that was caused by the Idle Air Control (IAC) valve. Maybe that is something to look at on your miata? I also have a vague memory of the little throttle plate damper failing on my 90 Miata and that causing some idle issues. The throttle plate would snap shut too quickly and cause the motor to bog for a little bit.
'94 doesn't have that damper, FYI. I suspect the newer engine management of the 1994 cars can react a little faster.
Keith Tanner said:
... Do NOT clean the throttle body butterfly.
Keith, what's the reasoning for avoiding cleaning the butterfly?
Apologies to the OP for the thread jack. I'm justifying it on the basis that the answer to this question is relevant to your question even if it's probably not helpful.
There's some stuff in there that does the final sealing of the butterfly. Looks like oil buildup. People clean it off and all of a sudden they have a high idle due to the leaks.
IIRC, throttle bodies have some magic coating on them that various solvents can remove. Supposedly there is "throttle body cleaner" that is safe, but I've never used it.
Keith Tanner said:
There's some stuff in there that does the final sealing of the butterfly. Looks like oil buildup. People clean it off and all of a sudden they have a high idle due to the leaks.
Thanks - I can't say as I've ever seen that phenomenon. I appreciate the knowledge.
I had a similar issue recently with my 99' it has a similar engine but not sure about the transmission or ECU. It gave me the symptoms you described to a tee.
It may be the neutral position switch on the transmission if there is one on that particular year - This Part - according to that link there is one.
If the switch is faulty the ECU assumes the car is in gear and it just figures you are off the throttle in gear coasting and has no need to step in therefore it allows the idle to drop really low before it realizes something is up and takes over.
This switch is on the transmission, not the one on the clutch pedal. If that is the problem, I hope you have access to a hoist, or to something that gets the car really high off the ground. It is damn near impossible to change with a jack and stands but super easy and a 20 minute job with a hoist.
EDIT** it is to the driver's side rear and slightly above the reverse light switch.
Similar wiring in this regard between the 1999 and the 1994.
An easy way to check that would be to see if it acts any differently if you pop the car into neutral vs putting in the clutch as you approach a stop. Just make sure you try each of the variations at the same time - say, 2000 rpm or whenever you usually hit the clutch. Since it was mentioned that this happens when the clutch is in, that would point more to the clutch switch than the neutral switch.
Update: The base idle was a rock-solid 850 rpm when fully warmed up. I took it for a 40 minute "Italian tune up" around a local lake and it ran and idled like a champ. My son drove it yesterday and said the idle was drooping badly the whole time he was driving. (Ambient temp is about 15 degrees lower today, is that a possible factor?) The only idle droop I noticed at all today was when I went from no accessories running to running the HVAC fan. (Is there any way that alternator load could be affecting the idle?) That never took it down anywhere near the 200-300 rpm if was drooping to before and it quickly recovered to 850.
When it was running poorly, I did a clutch vs. neutral test and found no real difference whether putting the clutch in vs dropping the car into neutral, releasing the clutch and coasting in to the corner. It was still drooping and not recovering.
I still think that IAC is sticky. I'd clean it. There's no harm!
Alternator load does definitely affect idle. Your car is having trouble "catching" the engine speed when it transitions to idle mode. Driving style - when you hit the clutch - can have an effect, and a greater electrical or AC load means you need more compensation from that valve.
Sooooo . . . jumping into the IAC cleaning today. With a lot of time and the required blood sacrifice from my thumb, I was finally able to get the throttle body off. Unfortunately, I'm not able to separate the IAC from the throttle body. (If you ever find the engineer from Mazda who decided to put these together with phillips screws rather than bolts, please throat punch him) Looking at the throttle body, I understand the "don't clean the stuff of the throttle body" instructions. What I'm wondering is whether there is a way to clean the IAC while still attached to the throttle body without damaging the throttle body itself?
Help!
It’s a lot easier if you have JIC screwdrivers. In fact, a set is useful if you’re working on anything Japanese. You’ll discover how much time you’re spending fighting screws.
now, on to the problem. If you can’t get it off, you could try squirting carb cleaner in the IAC inlet -that’s the pipe below the throttle body. IIRC you can see the piston down that pipe and may be able to push it back and forth with a screwdriver. Or hook the two electrical pins on the IAC to 12v and ground and cycle it that way. Shouldn’t be polarity.
APEowner said:
Keith Tanner said:
There's some stuff in there that does the final sealing of the butterfly. Looks like oil buildup. People clean it off and all of a sudden they have a high idle due to the leaks.
Thanks - I can't say as I've ever seen that phenomenon. I appreciate the knowledge.
That's some crazy stuff, right there. I had no idea.
Well, a friend stopped by with an impact driver and we got the throttle body and IAC separated. We got the IAC cleaned out and re-installed. So far, so good. Idling at a solid 850 both when warm and on cold start. Not ready to declare victory yet, but I'm hopeful.
Ransom said:
APEowner said:
Keith Tanner said:
There's some stuff in there that does the final sealing of the butterfly. Looks like oil buildup. People clean it off and all of a sudden they have a high idle due to the leaks.
Thanks - I can't say as I've ever seen that phenomenon. I appreciate the knowledge.
That's some crazy stuff, right there. I had no idea.
If you need to renew it because you cleaned it off, you can simply shoot a little bit of spray paint on to the closed butterfly and let it dry before you open the throttle. A very thin coat, obviously.
Just an update: 1 week in, still idling like a champ. As usual, Keith had the answer for "The Answer".
