QuestionQUESTION: My 1990 Mazda Miata idles smooth. But when it is stopped at a red light for instance with the engine running and and the gear in 'D' position (drive) and the head lights and heater fan ''ON', the engine vibrates somewhat. This has an automatic transmission. But when you acceletae a little with the left foot on the brake, the vibration stops and the engine idles well. Will increasing the idle speed be OK?
ANSWER: It seems you ICS (Idle Control Solenoid) may have failed. A quick test is to turn off all accessories, get the car up to operating temperature, idle in park. Turn on the AC and observe the tachometer, does it drop or does it flutter and remain the same or does it increase. The ICS captures the idle speed and maintains it the device is an electro-mechanical control that the ECU used to step up idle by allowing metered amount of air to bypass the throttle all based on demand. The power steering, the AC and the alternator are the primary devices that introduce a load when they switch on or off the ECU makes the adjustment via the ICS. Increasing idle speed will compensate for the short term however not properly so a new ICS may be in your future. Since they can last 15-20 years it's worth buying a new one and being done with it, they are rathe costly though. Cleaning the device internally sometimes has a remedy effect however considering the age the solenoid is likely the fault which means replacement.
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QUESTION: Thanks for the information.
I checked with a Mazda dealership and found that the IAC unit with the ICS is $673.00. You have to buy the whole unit which price is too high for me. My idle speed at Park is about 665 rpm. I understand it should be 750 -850 rpm. How do I increase to this this recommended rpm? I have no AC in my car. Can I use the screw in the throttle body to increase the rpm? How do I clean the ICS? I live in Toronto and there are no used parts around here.
AnswerBuying used is always a gamble and since that control was only used from 1990-1993 it means anything used is pushing 20 years old. The ICS is available by itself through us for $484.19. One out of every ten will respond positively to cleaning, most have solenoid failure which can't be repaired, at this time. The idle speed can be set as follows; Idle up to temperature, then the ECU must be shunted (jumper wire between GND & TEN in the Diagnostics box, located on drivers fender apron). Set your base timing between 10-14 degrees (with a good timing light, by rotating the CAS as needed, Crank Angle Sensor) and idle up to 850RPM using the throttle stop screw. (timing varies based on your sea level elevation) Once done your base lines are established, remove the shunt wire and observe how it idles static, in gear and with full load (accessories on). Without the ICS functioning it will idle down when loaded you can compensate by adjusting the throttle stop screw (shunt in place) however the trade off is high idle when not loaded which introduces another range of issues unfortunately.
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