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Mazda Idle Fluctuations

Q. Hi Vince, Hope you can help me. I've a 1989 Mazda 929a (18V 3.0 liter FI) with about 109,000 miles. Always well cared for oil 3,000 etc. The car is experiencing a fairly severe idle instability. When I stop say for a light the idle is pretty smooth at about 700 for a couple of seconds it then feels like it's dropping off a cam or something and gets very low, under 500, very rough like it's skipping and sputtering.

Tapping the gas pedal doesn't make it go away. Upon acceleration the car runs fine. Turning on A/C also fixes problem when the step-up kicks in as the compressor turns on. This problem seems most prevalent in cooler weather but has happened in the summer too. I checked the temp switch at the bottom of the radiator and it works the way it should. I put new NGK plugs, a new cap and new rotor on too.

Car gets about 23 MPG and has always gotten the same mileage since I got it years ago. O2 sensor has never been changed. TPS checks out OK. There is a device called an ISC BAC valve on one of the Valve Covers that has something to do with idle but I'm clueless at to how to check it out. At times after the car is fully warmed the idle looks good in gear but goes to 1000 RPM when shifted to park. Timing has been checked too and is spot on. Are there other sensors I should check?

Thanks for any advise and this is a great site.
Richard

A. There are a few distinct possibilities for this condition. A bad O2 sensor, a sluggish Air Flow Meter (AFM), a bad coolant temp sensor or a bad or dirty BAC valve. The O2 sensor and coolant temp sensors are easily replaced and are relatively inexpensive so replacing them would be a viable option. The AFM can best be tested with a scan tool to insure it is operating properly. What you can do is to take the outlet hose off the AFM and move the flap inside of it. Make sure it moves freely without binds or hitches. If it does, chances are the AFM is okay.

To test the BAC:
BAC SYSTEM INSPECTION:

Start the engine and run it at idle.

Ground the test connector (Green: 1-pin).

Disconnect the BAC valve connector. NOTE: With the BAC valve disconnected the idle speed will be slower than normal.

Pinch the air hose and note the engine speed.
Cold engine: Engine speed drops.
Warm engine: Engine speed remains unchanged.
Connect the BAC valve connector.
Warm up the engine to normal operating temperature and run it at idle.
Check that the idle speed is 650±20 rpm (M/T: Neutral; A/T: Park).
Disconnect the BAC valve connector.
Check the ISC valve operation to make sure the engine speed decreases.
Reconnect the BAC valve connector and disconnect the ground wire from the test connector.
BAC VALVE

Disconnect the air hoses from the air funnel.

Blow through the BAC valve from port "A" and check the air flow.
Cold engine: Air flows
Warm engine: Air doesn't flow

Disconnect the BAC valve connector.

Connect an ohmmeter to the BAC valve Terminals.
Check the resistance. It should be 10.8-12.3 Ohms.

That engine is particularly sensitive to unmetered air entering the engine. Even a small vacuum leak will cause an unstable idle. Any air that enters the engine after the AFM will cause an unstable idle, such as a loose or missing oil cap or a dipstick that is not completely seated or a crack in the air inlet tube. You will have to check carefully for all of these things before you do any switching or testing.