1G 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 2 connectors, one next to the interior fusebox, one next to the battery, then turning the BISS until the car idles at 750rpm (for 1G manual trans cars). Grounding these 2 connectors removes ECU control of idle, and centers the ISC in its range. Once you have set the idle speed to 750rpm, you then disconnect the grounds to allow the computer to control the idle again.

The following conditions should be met before setting the BISS:

    Disconnect the female connector from the ignition timing adjustment plug. This is a pic from over the battery, showing where this plug will be taped to a wiring loom.

    A closeup of the ignition timing adjustment plug.

    The ignition timing adjustment plug pulled from the wiring loom, female cap still on. Do NOT lose this cap, it is on there for waterproofing protection.

    The cap off of the ignition timing adjustment plug. The terminal you need to ground is circled. Get a set of alligator clip wires (Radio Shack sells various assortments of alligator clip wire sets, everyone who works on cars should have a set) and clip one end on this terminal, the other to the negative battery terminal.

    This is a pic from a GVR4, showing where this plug will be taped to a wiring loom near the center of the car at the firewall.

    This is a view of the fusebox under the dash. Note the black rubber cap over the plug attached to the right side of the fusebox. This is the diagnostic plug (the TMO Datalogger uses this plug) that you need to ground a terminal on.

    A closeup of the diagnostic plug with the cap off.

    A closeup of the diagnostic plug (removed from the fusebox for clarity, you do NOT have to remove it). The pink circled terminal is terminal 10. Use an alligator clip wire on this terminal, clip the other end to ground. If you have the TMO Datalogger, you don't need to worry about this connector, as the datalogger has already grounded it.

 

    Start the car and let it idle. Check the tach and see if it is idling at the proper speed (again, 750rpm ± 50 for 1G 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 idle position switch (IPS) may need to be adjusted (check your Shop Manual on how to do this until I add a VFAQ on it). If the IPS is adjusted properly, it is probable that there is leakage resulting from deteriorization of the fast-idle air valve (FIAV), so the throttle body will need to be replaced.

    SO WHERE'S THE BISS???
    It's under the rubber cap on the top of the throttle body, shown circled here.

    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.).

So turn the BISS until the idle is correct, then shut the car off, remove the wire from the diagnostic plug, remove the timing plug jumper (reinstall the cap!), and start the engine. Let it idle for about 5 minutes, and see if the idle is normal.

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Last modified: Aug 7, 1998
Copyright 1998, Tom Stangl
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