2G BISS Adjustment

Setting the Base Idle Speed Screw

If you've added any mods to your car that increase airflow through the engine (especially IC pipes or a frontmount IC), you may have a problem with the car stalling, or the idle oscillating up and down when coming to a stop. One of the most common causes is the BISS screw being out of adjustment. Another reason to change the BISS setting is if you have moved to a higher or lower elevation and you are experiencing the above symptoms.

The BISS helps the ECU maintain idle. The ECU uses the Idle Speed Control (ISC) stepper motor to hold the idle speed as constant as possible. The ECU counts on the ISC being in the center of its operating range to have as much control over the idle as possible. While the BISS may have worked fine on a stock car, as soon as you start increasing airflow, you can push the system to the point where the BISS is not bypassing the proper amount of air past the throttle, so the ECU has to push the ISC stepper motor past the halfway point to maintain proper idle speed. Stalling happens if the BISS is set wrong, the ECU pushes the ISC to full lock, and can't push it any farther, and full lock of the ISC is not giving enough airflow to keep the engine running.

Setting the BISS involves grounding a connector on the fuse panel mounted Diagnostic plug. According to the Shop Manual, you need to use the Scan Tool, as it holds the IAC motor at a certain step level while you are setting the screw. Unfortunately, the Scan Tool runs about $1800, so hopefully you know a shop that has one you can borrow quickly.

The following conditions should be met before setting the BISS:

 

    Connect the Scan Tool to the datalink connector (16-pin). NOTE - when the scan tool is connected, the diagnostic test mode control terminal should be grounded.
 

    Start the engine and run at idle. Select Item No. 30 of the Scan Tool Actuator test. NOTE - this holds the IAC motor at the basic step to adjust the basic idle speed.
 

    Start the car and let it idle. Check the tach and see if it is idling at the proper speed (again, 750rpm ± 50 for 2G manual trans cars). If it is NOT, check the following conditions before messing with the BISS:

    • The engine may be 20-100 rpm low for a new engine (300 miles or less). Adjustment is not necessary
    • If engine stalling occurs or the engine speed is low with an engine with over 300 miles on it, the throttle body may need to be cleaned.
    • If the engine speed is higher than proper even though the BISS is fully closed, the fixed SAS may have been moved, and may need to be adjusted. If the SAS has not been moved it is possible that there is leakage resulting from deteriorization of the fast-idle air valve (FIAV), so the throttle body will need to be replaced.

    What if the SAS has been moved? Here's how to adjust it:
    NOTE - the fixed SAS should not be moved unnecessarily; it has been precisely adjusted by the manufacturer. If the adjustment has been disturbed for any reason, readjust as follows:

    • Loosen the tension of the accelerator cabel sufficiently.
    • Back out the fixed SAS locknut.
    • Turn the fixed SAS counterclockwise until it is sufficiently backed out, and fully close the throttle valve.
    • Turn the fixed SAS clockwise until the throttle lever is touched (i.e., the point at which the throttle valve begins to open). From that point, turn the fixed SAS clockwise another 1 1/4 turn.
    • While holding the fixed SAS so that it doesn't move, tighten the locknut securely.
    • Adjust the tension of the accelerator cable.
    • Continue with the BISS adjustment.

    SO WHERE'S THE BISS???
    It's under the rubber cap on the top of the throttle body, shown circled here (this is a 1G pic, but the 2G plug will be similar).

    Here'd the cap off of the BISS - use a screwdriver to turn it in or out until you get the idle adjusted properly. Turn it gently, you don't want to bottom it out too hard, as it can screw up the air passage, and you don't want to turn it out all the way, have it pop out, and lose it, as they are hard to find (Mitsu part # is MD614948 and it's called, "Adjusting Screw, Throttle Body". You need a #5 O-ring (3/8x1/4x1/16, available at most hardware stores), now available as Mitsu part # MD608806. This goes right under the head of the screw in that valley that looks as if an O-ring should go there.).

 

    • Press the Scan Tool clear key, and release the IAC motor from the Actuator test mode. NOTE - unless the IAC motor is released, the actuator test mode will continue for 27 minutes.
    • Switch the ignition off.
    • Disconnect the Scan Tool.
    • Start the engine again and let it run at idle speed for about 10 minutes; check to be sure that the idling condition is normal.

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Last modified: Apr 12, 1999
Copyright 1999, Tom Stangl
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