How- To: Idle Adjustment and Tuning

[OB]

This has been discussed in several threads over the years, so I figured it might be worth making a permanent thread about it. There are a few reasons to tune your idle adjustment. The main reason is to take up the slack in the factory setup and tune throttle tip-in. The purpose of tuning tip-in is to decrease lag and maximize throttle response. Another reason for making idle adjustments is to smooth out tip-out/off-throttle performance. With the right settings, throttle response can be increased, and off throttle behavior can be optimized.

This adjustment is only possible on models equipped with an idle adjustment screw. To my knowledge, ATX models do not have this screw and cannot be adjusted. I've also heard that older MTX cars did not come with the screw. It is also possible that cruise control equipped cars did not come with it either. Anyone who has more info on this, please let me know so I can edit accordingly.

STEP 1: Locate idle adjustment screw.
The screw serves as a throttle stop plate, and can be found on the throttle body, just below the throttle cover.

STEP2: Adjustment.
The screw can either be adjusted from the bottom with an external TORX E4 bit, or from the top with a 10mm wrench. The adjustment can be made with the throttle assembly intact. Turning the screw clockwise (running it down) will decrease idle speed. Turning it counter-clockwise will increase idle speed.

STEP 3: Fine Tuning.
The ideal adjustment should not raise idle more than about 100RPM. Using a scan tool to monitor exact revs is ideal, but this can easily be done by ear and by referencing the tachometer. Be prepared to use a trial and error method to find the sweet spot. Adjust, test drive, then analyze, and repeat.

STEP 4: Evaluate behavior while driving.
During a test drive, note throttle tip-off behavior. When cruising with the clutch pedal depressed, idle should drop to just above 1000-1200RPM. Once stopped, idle should progressively and consistently drop to it's normal setting. If you overtune, idle could settle as high as 1500-2000RPM. While this setting may be preferable for your driving habits (i.e. instant tip-in response after coasting), there is a good chance the PCM will set a check engine light with a code for "Idle Speed High." This is normal and can be fixed by tuning the setting down, resetting the PCM, and retesting. I have found that a setting of somewhere around 1200RPM off throttle out-of-gear is ideal for street driving, without setting a CEL.

The reason idle is higher during out-of-gear cruising is in anticipation of throttle tip-in. Commonly referred to as the Dashpot in the carburetor days, this characteristic can be beneficial in a performance application. The Idle Air Controller (IAC) is capable of compensating for a significant amount of adjustment range, and serves as the modern day dashpot. Having a high dashpot idle makes throttle tip-in seamless and responsive. In-gear cruising will also be affected, and will feel smoother and more linear. However, since in-gear revs are directly proportional to transaxle gearing and wheel speed, the tach cannot be used to gauge adjustment. All adjustment parameters should be based on still-idle and out-of-gear dashpot data.


Please post up any and all questions and/or information you want to share. Thanks for reading

[occasional demons]

There is also another post, where they put a tie wrap around the pedal end of the cable to eliminate the delay caused by the spring there. You can actually see it compress, and then expand when the pedal is pushed.

This would allow for a little more tweaking.

[OB]

There is also another post, where they put a tie wrap around the pedal end of the cable to eliminate the delay caused by the spring there. You can actually see it compress, and then expand when the pedal is pushed.

This would allow for a little more tweaking.

I've seen that, and long before that post came around I had that worked out too. I used a wire nut and made a custom spacer to take up 100% of the slack in the throttle cable. Deleted the spring altogether, making for a direct connection with zero slop. The wire tie idea is good, but cannot be tailored to fit perfect; the slack removed is going to be equal to the width of the tie.

[Swordfish2Cowboy]

It's awful hard to see up under the pedal. I've just got a wire tie in there now, but it seems ghetto. I might try to crane my neck and take a lot at it again.