Engine: Troubleshoot Idle

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Normal Idle speed for these cars is 700rpm +/- 100rpm, meaning anywhere from 600 - 800rpm is normal. Aside from improper tuning, many people have minor idle problems that are easily fixed.

If you are having problems with your idle too high or low, try adjusting the idle adjustment screw to see if you can get it to idle properly. If this too does not work, try cleaning your ISC/IAC Servo. Here is a link for detailed instructions on doing so:

ISC/IAC Help from Stealth316


Is your idle surging dramatically? Check this link out, if you want to learn a little about ISC/IAC.


Below is an exerpt from it:

Q: So what is an ISC and what is it good for?

A: The ECU attempts to control the idle speed by opening or closing the ISC. The valve on the ISC allows extra air to bypass the throttle plate. Since a stepper motor does not provide any feedback on its actual position, the computer simply counts steps and assumes that the valve lands in the correct place.
During normal operations, the ECU uses the ISC for some very interesting things besides simply adjusting the idle speed. The ISC is completely shut when you kill the engine to make sure it stops quickly. The ISC is opened completely at startup to provide plenty of air. It is also opened during normal driving to prevent the car from bucking when you take your foot off the throttle to shift. If you run the air conditioner or turn the steering wheel, the ECU compensates immediately by opening the ISC a little to keep the revs up. This also happens during cruising to mask the effect of the increased load on the speed of the car.

Q: So what, exactly, is idle surge?

A: Idle surge is the condition that occurs when something in the ECU idle speed feedback loop fails to operate as expected. Since the ECU is not quite as smart as you are and has a very microscopic view of its world, its behavior may seem very strange to you, but seem perfectly normal to the computer. The result is an idle speed that oscillates from ~1200-1500 RPM about 2 times a second and no error codes are ever displayed.

Idle surge is a result of the following situation:

  1. The idle switch is depressed (foot off the gas)
  2. Too much air is getting through the throttle body (ISC stuck open)
  3. O2 sensor registers lean mix (too much air)
  4. ECU adds fuel to achieve correct mix
  5. RPMs increase due to increased fuel/air supply
  6. At 1500 RPM the ECU goes into decel mode and cuts fuel (*)
  7. With only air coming into the engine, the RPMs drop rapidly
  8. At 1200 RPM the ECU turns the fuel back on
  9. Goto 2.
(*) In our cars, the ECU cuts the fuel supply completely when the car is decelerating. It determines that you are trying to decelerate because the idle switch is depressed and the RPMs are above 1200. This is done to prevent an over lean condition and to assist in engine braking.

--3Sfever 21:01, 24 October 2007 (MDT) (surging info addition)

This article originated from a thread or post on 3000GT/Stealth International. (1931)