With A/C off, my Mazda Miata idles at 1500 RPM warmed up. It idles at 750 with AC turned on. This started happening recently, and nobody touched the car.

I have removed the air control valve from the side of the mainfold and plugged the manifold openings with duct tape, and it makes no difference. The valve looks very clean.

The temperature gauge shows the engine warming up OK, and it seems to work just fine. Coolant is full.

Could it be that whatever detects that the AC is on thinks it is on all the time, or is there some broken feedback?

 

It uses an airvalve for cold fast idle. It is seperate from the BAC /IAC valve. It is mounted on top of the engine and does not work off of electricity. It works off temp and has 2 hoses on it. If it lost coolant flow it would remain open perpetually. Or it could just be stuck. Tap it with a hammer lightly at idle. If nogo remove and clean it. Check coolant flow through your bypass hoses as they had a problem with slag from the engine casting breaking loose and blocking the bypass hoses.

 

I don't think it can be the air control valve, because I took it off, and plugged the holes into the manifold with duct tape to no affect. The problem must be elsewhere, and I was wondering if something thinks the AC is on perpetually, since the idle speed is normal with the AC on. I will try tapping it, but since plugging the holes didn't help, I doubt if tapping it can make a difference.

 

There are basicly 2 idle valves on it. The one i am talking about is seperate from the idle air control valve on the throttlebody. It has no wires on it. Anyway. Does it have good coolant flow? If you unplug the BAC/IAC valve connector and it still idles high then the problem is not electrical but mechanical..

 

If you look at what babyhuey posted here:

https://www.mazdaforum.com/m_37574/tm.htm

That is where I got the duct tape idea for the air control valve. He says it is in the back of the manifold, but mine midway from front to back, and on the side, and has no electrical wires (only heater hoses). Basically the idle does not change with the ducts blocked. Also I have tried tapping it as you suggest, and no difference.

As to electrical connectors, do you mean the black box to the motor side of the throttle stop at the front of the manifold? It has a three conductor connector. I tried disconnecting that connector, and it also made no difference.

What about the rest stop for the idle adjustment screw? Could that device be holding the throttle back?

 

Was the throttle body really really dirty? Sometimes if they get dirty enough it works its way into the butterfly rods and binds the rods up. If this is the case it will need another throttlebody. It was common on the 89-92 626 models. I have never seen it on a miata but it is basicly the same. There should be 2 connectors on the throttlebody. 1 is TPS and the other for the BAC (idle control valve). If you disconnect the BAC and the idle does not go down it rules out an electrical issue. You are down to a bad vacuum leak/ sticking throttlebody/sticking BAC valve/sticking throttle cable/sticking cruise cable.

 

The air control valve ports in the maifold looked very clean, as did the air control valve. Which is the TPS and which is the BAC connector?

The throttle is not held back by the throttle cable. The throttle cable is loose when idling.

I have factory cruise control, and there is no 2nd cable to the throttle. The black box on the engine side of the throttle where I removed the 3 wire cable was labeled DENSO. Maybe it was the cruise control? There is also a cable that comes out of the firewall next to the throttle going into a chamber that has a vacuum line going to the intake manifold. There is also a device in the back of the manifold connected to a vacuum line. Also, there is a vaccum line running to and from a sensor with electrical wires heading for the computer.

I found that the lower cable near the throttle has two wires and is much harder to reach. I will try disconnecting this cable next time I warm up the vehicle.

I think the throttle cannot close further because of something internal. It does not rest on the stop by the throttle cable.

 

The cable coming out of the firewall to the cruise actuator that has a vacuum hose on it needs to be checked for binding and adjustment though i doubt that is it. The BAC has 2 wires so the 2 wire connector is it. If the throttle is not going all the way to the stop and your cruise cable is not binding/misadjusted then i would remove the throttlebody and clean it thoroughly paying close attention to the butterfly rods going into the bore. The throttlebody is easy to remove and clean.

 

I found a 2nd return stop on the back of the throttle pivot that prevents it from returning any more. So the diaghram like device is not restricting the throttle movement. It is back as far as it goes.

There seems to be a lot of pressure in the valve covers, and disconnecting and plugging the PCV gets it near normal, but maybe the pressure is normal, and there is a leak somewhere else. Or is this a valve leak?

The small hose to the cruise control is too small to make any difference. The brake hose makes no difference when I plug it.

 

Forgot to add that I used a long nosed pliers and pulled the 2 wire connector out, and it made no difference.