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Hi folks,

I have a 92 NA6 with some idling issues.

My two main symptoms are;
- Low idle during cold start (about 500-600rpm, it will sometimes stall if I allow it)
- Lots of hesitation when moving off the line when cold, until about 2000 - 2500rpm
- Low idle with the A/C on
- Idle is fine at all other times

Here is what I have attempted so far;
- Reset base idle with TEN+GRND bridged (warm idle is now at a steady 850-900rpm with and without TEN+GRND bridged)
- Cleaned intake plenum
- Replaced air valve (though haven't tested it on the car yet as it hasn't had a chance to cool down yet)
- Ran the 'click test' on the ISC, which it failed (no click, idle speed did not change)

Based on all of this, especially the failed click test, it'd be safe to assume the ISC is at fault here, right? The only doubts I have is that it idles perfectly fine when warm with the AC off. Does that mean that the ISC is stuck closed?

There's also this miscellaneous plug coming off the thermostat that isn't attached to anything...should it be?


The only fault codes the car pulls are both related to the o2 sensor (that'll be replaced soon too).

- what is the condition of the battery?
- clean the AFM electrical plug
- have you tried adjusting the idle screw on the throttle body?

tails wrote:I have a 92 NA6 with some idling issues.

My two main symptoms are;
- Low idle during cold start (about 500-600rpm, it will sometimes stall if I allow it)
- Lots of hesitation when moving off the line when cold, until about 2000 - 2500rpm
- Low idle with the A/C on
- Idle is fine at all other times

Here is what I have attempted so far;
- Reset base idle with TEN+GRND bridged (warm idle is now at a steady 850-900rpm with and without TEN+GRND bridged)
- Cleaned intake plenum
- Replaced air valve (though haven't tested it on the car yet as it hasn't had a chance to cool down yet)
- Ran the 'click test' on the ISC, which it failed (no click, idle speed did not change)

Based on all of this, especially the failed click test, it'd be safe to assume the ISC is at fault here, right? The only doubts I have is that it idles perfectly fine when warm with the AC off. Does that mean that the ISC is stuck closed?

There's also this miscellaneous plug coming off the thermostat that isn't attached to anything...should it be?

My NA6 was doing much the same thing before I removed & cleaned the idle speed control valve. The engine would idle fine at operating temperature, but too rich/slow when cold and too slow when the aircon compressor engaged - often stalling. The article http://www.miata.net/garage/isc.html was a huge help. Note that I didn't do the 'click test', though.

As for that unattached plug, mine is the same. I've looked at my wiring diagrams and cant find it, either.

Steampunk wrote:- what is the condition of the battery?
- clean the AFM electrical plug
- have you tried adjusting the idle screw on the throttle body?

- what is the condition of the battery?
I couldn't tell you, I don't have a multimeter to check. It starts fine every morning though. Is there a way to check it without a multimeter?

- clean the AFM electrical plug
I'll give that a try.

- have you tried adjusting the idle screw on the throttle body?
I have, I reset that while bridging TEN and GRND to match it to the ECU.

tails wrote:

Steampunk wrote:
- have you tried adjusting the idle screw on the throttle body?
I have, I reset that while bridging TEN and GRND to match it to the ECU.

What I meant was JUST adjust the screw without jumping the terminals as sometimes that's all it needs.
Also check for any loose, deteriorated air/vacuum hoses.

I'm almost certain that loose electrical plug is connected to something beneath the throttle body as it looks very familiar when I removed my intake pipe several months ago, but I just had a good look at can't see exactly where it attaches. Most likely to the idle stepper motor, if so then there's your culprit

Steampunk wrote:

tails wrote:

Steampunk wrote:
- have you tried adjusting the idle screw on the throttle body?
I have, I reset that while bridging TEN and GRND to match it to the ECU.

What I meant was JUST adjust the screw without jumping the terminals as sometimes that's all it needs.
Also check for any loose, deteriorated air/vacuum hoses.

I'm almost certain that loose electrical plug is connected to something beneath the throttle body as it looks very familiar when I removed my intake pipe several months ago, but I just had a good look at can't see exactly where it attaches. Most likely to the idle stepper motor, if so then there's your culprit

Wouldn't adjusting the idle screw just raise the idle the whole time though? I don't want a high idle when it's warm.

The extra plug is for the Power Steering Pressure switch. It's just taped back to the loom on models without P/S. Part No.6 in this picture.(not an MX motor I know but the pump and switch are similar)

It also bumps idle when the pump puts too much load on the motor during slow mauvers like parking.

Did you put new gaskets on to the Throttle body, Air Valve and Idle Control Valve when you checked them. Extra air drawn past old gaskets can wreak havoc on idle.
Part numbers for the three gaskets are,
B6S7-13-ZC7 O Ring Air Valve
B551-13-W89 Gasket, IDLE AIR VALVE
B6S7-13-655 TB Gasket

Rolley wrote:The extra plug is for the Power Steering Pressure switch. It's just taped back to the loom on models without P/S. Part No.6 in this picture.(not an MX motor I know but the pump and switch are similar)

It also bumps idle when the pump puts too much load on the motor during slow mauvers like parking.

Did you put new gaskets on to the Throttle body, Air Valve and Idle Control Valve when you checked them. Extra air drawn past old gaskets can wreak havoc on idle.
Part numbers for the three gaskets are,
B6S7-13-ZC7 O Ring Air Valve
B551-13-W89 Gasket, IDLE AIR VALVE
B6S7-13-655 TB Gasket

Thanks for clarifying the random plug.

I haven't removed the TB yet and the air valve's rubber gasket was fine. I was having issues before I removed that anyway (I only replaced it yesterday).

Given the issue occurs when the engine is cold, have you replaced the coolant temp sensor used by the ECU at the rear of the head? OEM ones had a green top from memory - it's a very common for them to go on an NA6, & when they do cold idle is very poor. However, once warmed up idle is ok.

93_Clubman wrote:Given the issue occurs when the engine is cold, have you replaced the coolant temp sensor used by the ECU at the rear of the head? OEM ones had a green top from memory - it's a very common for them to go on an NA6, & when they do cold idle is very poor. However, once warmed up idle is ok.

I have seen this pointed out. I'm hesitant given what a pain in the ass location it's in.

Would you recommend changing this before looking further into the ISC given that the ISC failed the click test?

Removing the coilpack improves access, but by all means follow-up on the ISC as they are also sometimes a problem. And CTS seem to be less of an issue these days than they were, perhaps due to the replacements lasting longer. Then again, if yours in an original maybe replacement is worthwhile.

Thanks for the tips folks.

I've managed to source a spare ISC locally for nothing. I have my hands on a multimeter now, so will test both the one in my car and the spare one to see if there within spec. I'm expecting the one in my car to fail given that it didn't change when I unplugged it which suggests that it's stuck closed. If it shows resistance within spec I'll give it a clean instead of replacing it.

The 02 sensor wouldn't have anything to do with this, would it? I would normally ignore it for such an issue but the fact I'm pulling a couple of fault codes related to it means it needs replacing anyway.

Apologies for the double post but I just tested the ISC with a multimeter and at 12.5ohms it's within spec.

I guess the next job is to clean the bugger.

tails wrote:The 02 sensor wouldn't have anything to do with this, would it? I would normally ignore it for such an issue but the fact I'm pulling a couple of fault codes related to it means it needs replacing anyway.

hks_kansei sorted a hesitation/miss issue on his NB8A by replacing the O2 sensor, so given the car is throwing O2 fault codes replace it before chasing other stuff, especially as they are somewhat of a consumable.

93_Clubman wrote:

tails wrote:The 02 sensor wouldn't have anything to do with this, would it? I would normally ignore it for such an issue but the fact I'm pulling a couple of fault codes related to it means it needs replacing anyway.

hks_kansei sorted a hesitation/miss issue on his NB8A by replacing the O2 sensor, so given the car is throwing O2 fault codes replace it before chasing other stuff, especially as they are somewhat of a consumable.

It's booked to get the o2 sensor done in a couple of weeks. I don't have the means at home to get under the car.

In the mean time, for my own curiosity, I checked the plug that leads to the ISC while switching the AC on to make sure the ISC was even getting a signal from the car. With AC off and engine warm the plugs reads about 12.7v. When switching the AC on the volts drop to about 11.5v.

Am I correct in my thinking that this is what is meant to happen? I figured if it defaults to open when unplugging it, then dropping the volts would partially open it.