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How to adjust the idle...


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If your idle speed is erratic it could be because the motor has mechanical faults, there's a vacumm hose off the engine, the ISC valve is gummed up, or, someone has been fiddling with either the throttle butterfly adjusting screw, the idle by-pass screw on the throttle body or the position of the solenoid on top of the ISC valve. There are plenty of threads about bad idle so do a search and have a read before trying to adjust the idle.

None of the adjustments mentioned are meant to be fiddled with, and I am no way advocating that people should fiddle with them, but, IF they have been fiddled with before AND you have checked everything else AND cleaned your valve, then these adjustment may be what is needed to get the car to idle again.

Be warned though, none of these adjustments are subaru approved! Attempt them as a last resort only.

For the ISC valve to operate properly you need to have it near the centre of it's travel at the ecu's programmed idle speed. If anyone has played with the butterfly adjusting screw, the idle bypass screw on the throttle body(if it has a screw), or the idle control valve's solenoid on top of the valve, then the valve can be at the end of it's travel at idle speed and that makes them unresponsive, which causes surge and erratic idle. Only try adjusting it if you have checked everything else first.

The first thing to adjust is the butterfly. The butterfly adjusting screw is only there for one reason... to stop the butterfly digging into the throttle housing when it's shut and possibly wedging shut. The best way to adjust them is with the throttle body off so you can see what is happening. Back the screw out till the butterfly touches the wall of the housing(you may need to back off the cable while you do this if it's still on the car), then turn the screw back in again till it just starts to open the butterfly. There is no specifications given for the setting but about an eighth of a turn of the screw open should be plenty. Less woulld be better, as long as the butterfly isn't touching and isn't going to touch when the lock-nut gets tightened or the engine warms up.

Next thing if it has it is the solenoid on top of the valve. Set it to the centre of it's travel if the screws have slotted holes.

We will perhaps be adjusting this again later.

Now you need to connect the car to a laptop or scan tool. The ISC valve should have a duty cycle of 40 at idle if everything is correct. By correct I mean the motor is in good mechanical condition, there are no air leaks, the temperature sensor, throttle position sensor and neutral switch are all working properly.

If the duty cycle isn't at 40 or close to it at idle then you need to adjust either the solenoid or the bypass screw on the throttle body, or, the butterfly screw if you have neither of the other two. If you have it, it's best to use the bypass screw. If you don't have the bypass screw then you need to use the solenoid. If you have neither of the others, use the butterfly screw. Adjust till the D.C. is close to 40 with the motor warm and idling. Once it seems right, rev the motor to approx three thousand revs for about one minute then let it back to idle and recheck the D.C. Repeat the adjustment till you find the best compromise between steady prolonged idle and the idle immediatly after the reving up. Ideally the setting should be the same for both situations.

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