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By tx_haggis
#47761
My engine has both a PWM idle valve and a mechanical idle adjustment screw on the throttle body.

In open loop idle with a fully warmed engine, should the valve be closed and idle adjusted with the screw? I.e. the IAC PWM duty should be 0 (or what ever setting fully closes the valve)
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#47762
The screw is probably use it is the IAC goes bad/is disconnected.

IAC used for when engine cold, or increased engine loads cause idle to droop.

That is my guess.

Stating what engine/vehicle you are using would help.
#47764
The recommended procedure to adjust idle for some stock ECU´s with PWM valve:

1. warm up engine to normal temperature.
2. disconnect idle valve.
3. adjust engine idle 50 rpm lower than normal, manually. This may be a dedicated screw or at the butterfly in some cases.
4. check for correct CO / lambda.
5. connect idle valve and check for correct rpm. Check CO again.

If you modify this procedure in step 5, you can find the base PWM setting for your idle valve. This might take quite some time, if you do not know the recommended frequency for your specific valve. Maybe measure it on a stock installation.

PS the idle valve is always triggered to prevent it from sticking. Some ECU types run it on 100% for a short time at start up, to clean out any dirt/ sludge that may build up inside. WD40 is great to clean these valves internally, run them on 100% and inject into both sides. Be careful with polarity! Some have a build in diode to protect the ECU/ kill spikes directly where they are produced. This is impossible to measure from the outside with a simple DMM. Look for polarity markings at the connector or check in the work shop manual.
By JHolland
#47768
On most of the stock engines I've worked on the screw is adjusted to give 50% PWM, warmed up with no faults, that allows the ECU the maximum range of adjustment. If your set up is not stock then 50% may not be a practical value.
#47781
Chris Wolfson wrote:
Fri Jan 22, 2021 1:42 am
The recommended procedure to adjust idle for some stock ECU´s with PWM valve: ...
JHolland wrote:
Fri Jan 22, 2021 12:05 pm
On most of the stock engines I've worked on the screw is adjusted to give 50% PWM, warmed up with no faults, that allows the ECU the maximum range of adjustment. If your set up is not stock then 50% may not be a practical value.
Thank you both. Exactly the information I needed.

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