Engine: Adjust Idle Speed

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The idle speed on a 3S changes based off of several variables.

The idle speed is also controlled by several different devices.

Before attempting to adjust the idle speed you need to know what it is supposed to be.

Idle speed should be within +or- 50 RPM of these values.


  • 1500RPM is the MAXIMUM cold idle speed attained at -4 degrees F. After the car has been allowed to fully cool the engine coolant temperature becomes that of the outside ambient air temperature (+or- 1 or 2 degrees F) which means cold idle RPM will vary in a range of 801RPM to 1500RPM. The charts below will help you better approximate what your cold idle speed should be at a given outside temperature on initial engine startup after the engine has been allowed to fully cool.
  • 900RPM is the idle speed when the car is warmed up with the air conditioning on.
  • 700RPM is the idle speed when the car is warmed up with the air conditioning off.

FastIdleAirValveTempAndRPMranges-FIAV.jpg

The idle is can also affected by the following. The changes should be minor, if the RPM decreases the IAC is either faulty of the BISS screw is tightened too far and the IAC can not adjust enough

  • Turing the steering wheel activates a pressure switch on the power steering pump.
  • A high electrical load (headlights, Big stereo...) also increases the RPM slightly.

The Idle speed is controlled in the following ways listed in the Order that the factory service manual suggests checking/adjusting.

  1. SAS screw. This is the Small screw on the left side of the throttle body. This should never need adjustment UNLESS someone has already messed with this. This holds the throttle body open slightly setting the starting position for all other idle adjustments. Do NOT adjust this unless everything else has failed or you know that someone has already messed with it.
  2. FIAV. This has no adjustment, but it can mess up you idle if damaged, just make sure your car is completely warmed up before setting the idle. It increases the cold Idle anywhere from 801RPM to 1500RPM
  3. BISS. Base Idle Set Screw. This is used to set your base idle to 700RPM. NEVER try to set your idle to another value, Any base idle other than 700 RPM will confuse your ECU.
  4. IAC. Idle air controller. This is what the ECU uses to control the idle speed. It is a stepper motor that increases and decreases the amount of air that bypasses the throttle in steps to keep the idle right where the ECU wants it.
  5. Vacuum leaks. Vacuum leaks are not part of the stock car idle but they are often the reason the idle is goofy, so look for these first.

Idle Speed Adjustment, BISS Screw

  1. Before starting the inspection and adjustment procedures, set the vehicle in the following conditions.
    • Engine Coolant temperature between 176F-205F
    • Lights, electric cooling fan, accessories: OFF
    • Transaxle neutral (or park in automatic)
    • Steering wheel: Straightforward position
    • If engine is cold the FIAV will make the idle too fast to adjust.
    • Before adjusting idle you must make sure TPS switch is closed
    • Incorrect TPS adjustment or excessively tight throttle cable can keep TPS switch from closing.
    • The ECU will not know it is supposed to be idling if the TPS switch is set incorrectly
  2. Ground the self-diagnostic/data transmission selector terminal of the diagnostic connector.
    The easiest way to do this is to connect a datalogger or Scanmaster. If you do not have a datalogger then use a jumper wire to ground terminal 10 in the diagnostic connector as shown above.
    DiagnosticPort1.jpg
  3. If you don't have a datalogger, I recommend buying one.
    • But if you do not have a datalogger or scanmaster the RPM can be monitored as follows.
      1. Insert a paper clip into the I-pin blue connector as shown below
      2. Connect a primary-voltage-detecting tachometer to the paper clip. The tachometer will read 1/3rd if the actual engine speed. This means that the actual engine speed is the tachometer reading multiplied by 3.
    RPMconnector1.jpg
    RPMconnector2.jpg
  4. Remove the Waterproof female connector from the ignition timing adjustment terminal (tan colored by RH shocktower)
  5. Use a jumper wire to ground the terminal shown in picture below. Your idle should drop when you do this.
    TimingAdjust1.jpg
  6. Start engine and run at idle
  7. Check the Basic Idle Speed, Use your datalogger or Scanmaster
    • Basic idle Speed: 700 +- 50 rpm.
  8. If the idle Speed is out of adjustment adjust by turning the BISS screw
    TimingAdjust2.jpg

Note If the Idle Speed is higher than 700RPM even with the BISS fully tightened you probably have another problem:

  • The FIAV may be bad but the only way to fix this is to buy a new throttle body, replace the lower half of the throttle body where the IAC and FIAV are located (hard to do and hard to obtain), or block it off and loose your fast idle.
  • The fixed SAS screw (left of the throttle body) may be out of adjustment, this is not common.
  • The BISS screw may have a bad O-ring, It may even have fallen out completely
  • You have a vacuum leak
  • You didn't follow the procedure above.

BASIC idle speed adjustment using SAS screw on Left side of throttle body:

ADJUSTMENT OF FIXED SAS

SAS1.jpg

  • The fixed SAS has been factory-adjusted. Never attempt to move it.
  • Should it be out of proper adjustment, adjust by following the procedure given below.
    1. Sufficiently slacken the accelerator cable.
    2. Loosen the lock nut on the fixed SAS.
    3. Sufficiently loosen the fixed SAS by turning it counterclockwise to fully close the throttle valve.
    4. Tighten the fixed SAS slowly to find a point at which it contacts the throttle lever (where the throttle valve starts opening). From that point, tighten the fixed SAS further 1 l/4 turns.
    5. Holding the fixed SAS to prevent it from turning, tighten the lock nut securely.
    6. Adjust the accelerator cable tension.
    7. Adjust the basic idle speed.
    8. Adjust the idle position switch and throttle position sensor (TPS).
This article originated from a thread or post on 3000GT/Stealth International. (1870)