AboutMp2 Expertise I prefer to answer questions relating to the Miata since this has been my area of specific
expertise since 1991. I can answer questions relating to the other Mazda models however
some limitation of expertise may reduce my accuracy.
Experience Mazda technician since 1987, shop owner and specialty are regarding the Miata since 1991 covering mechanical, electrical, trim and body. Specializing specifically in drivability concerns.GKHS 1985, BVC 1987, MNA 1989, ASE Master Technician Mechanical 1995, ASE Master Body/Paint/Collision 1996
Education/Credentials GKHS 1985, BVC 1987, MNA 1989, ASE Master Technician Mechanical 1995, ASE Master Body/Paint/Collision 1996
Question 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.
