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I've been surfing the forums recently looking for info regarding the minimum idle set but haven't found anything that answered my question: What happens if I set the minimum idle higher than the 400-450rpm (+/- 50rpm) in drive according to the manual? Will it affect fuel trims/confuse the computer or will it just cause a higher idle? It may sound stupid, but when I set my min idle a few months back after installing a 52mm throttle body, I did everything according to the book except for the part where it tells you to se the idle to 400-450rpm with the IAC unplugged, and instead set it to 700-750rpm (+/- 50rpm) because the books idle speed felt a little too low. Now I feel as if some of the problems I'm dealing with are caused by the higher min idle I set...
Setting the minimum idle at 750 will cause a higher idle but should not affect fuel trims etc.
If you didn’t adjust the TPS voltage after turning up idle screw, that can cause issues.
Setting the idle that high does not allow the IAC valve to control idle effectively when compensating for changes in temperature, load etc.
When you set minimum idle to 450 with IAC valve closed and unplugged, once the IAC valve is plugged in and car is started, the ECM will control the IAC valve to maintain target idle speed to whatever is burned into chip.
Was your timing set correctly before attempting minimum idle adjustment ?
Here is info from my 86 manual.
The long and short is you can run out of ecm adjustment of the IAC.... would exhibit similar symptoms as a vacuum leak in some cases. IE it never reaches it's commanded lower idle speed. Coincidentally the same could happen if you went too far the other way I suppose.
Like said above, I'd go back and do it right. I did my 84 based off step counts because it had too much cam to do any other way without stalling. I also adjusted the ecm parameters to reflect those adjustments so it would work.
The whole idea is to set the idle too low.
The procedure will close the IAC valve, and that should result in a low idle, so that is what you adjust i too.
The IAC has an operation range and can only increase idle from the closed position. By setting tle closed idle a few hundred RPM below target idle, the IAC is in the good position in the midle of operating range.
If you want for example 200 rpm higher than stock idle, you should set that in the chip, and then you can also increase the minimum idle by the same amount.
Setting the minimum idle at 750 will cause a higher idle but should not affect fuel trims etc.
If you didn’t adjust the TPS voltage after turning up idle screw, that can cause issues.
Setting the idle that high does not allow the IAC valve to control idle effectively when compensating for changes in temperature, load etc.
When you set minimum idle to 450 with IAC valve closed and unplugged, once the IAC valve is plugged in and car is started, the ECM will control the IAC valve to maintain target idle speed to whatever is burned into chip.
Was your timing set correctly before attempting minimum idle adjustment ?
Here is info from my 86 manual.
I did make sure to adjust the tps once done messing with the screw (.54 volts when at idle). My timing is set at the recommended 6* btdc in drive. I think i’m going to set the min idle speed to spec since it wouldn’t hurt to try. I’ll post my results once I’ve had time to do so.
The long and short is you can run out of ecm adjustment of the IAC.... would exhibit similar symptoms as a vacuum leak in some cases. IE it never reaches it's commanded lower idle speed. Coincidentally the same could happen if you went too far the other way I suppose.
Like said above, I'd go back and do it right. I did my 84 based off step counts because it had too much cam to do any other way without stalling. I also adjusted the ecm parameters to reflect those adjustments so it would work.
I’ve seen lots of people mention setting their min idle by using a scanner to track iac counts. They mentioned turning the screw to achieve 15-20 counts at idle. Do you think this would be a more optimal way of going about it given my larger throttle body and stock chip? or am i better off just doing it by the book? I fortunately have access to an aldl to usb cable that lets me read live data and have thought about using it for this matter but I am unsure if it will help or hurt compared to the regular method.
I’ve seen lots of people mention setting their min idle by using a scanner to track iac counts. They mentioned turning the screw to achieve 15-20 counts at idle. Do you think this would be a more optimal way of going about it given my larger throttle body and stock chip? or am i better off just doing it by the book? I fortunately have access to an aldl to usb cable that lets me read live data and have thought about using it for this matter but I am unsure if it will help or hurt compared to the regular method.
The OP cites "I've seen "adjust the screw to 15-20 Counts" ". That 15-20 is the number of counts I use, I'm wondering if he's citing my posts?
So riddle me this: (Please read carefully for the parameters, pay attention to words in italics.)
> The idle speed is above the programmed speed, so you're chasing a "high idle speed" condition. You don't suspect that there are significant vacuum leaks. > So you monitor IAC, see 0-counts, and the mechanical stop is contacting the screw.
> You turn the screw out and two things happen: The idle speed lowers. And the IAC Counts go up. > Now the engine is idling at the programmed speed at 15-20 Counts.
> You re-check the TPS and TPS is in the .54V nominal range. Or TPS can be adjusted into the nominal range and still have 15-20 counts at programmed idle speed.
If it idles at programmed speed, at 15-20 counts (which is enough margin for the ECM to prevent a stall) and TPS is .54 nominal, where is the advantage or reason to use the "book method"?
There is no feedback loop for IAC pintle position. With key=on, the ECM sends enough counts to fully close the IAC, and then opens it enough counts to allow starting. The ECM then attempts to match the engine speed with the programed speed for the existing inputs. If speed, counts and TPS is nominal the screw is adjusted 'correctly'.
Note the idle screw is blocked from the factory. With the block removed, there is nothing to prevent the screw from moving due to vibration. Some PO had unblocked mine and I noted it was way off. I locked mine by putting a thin nut on the front end, making sure idle set was good with lock nut in place. Hasn't moved for years.
Minimum Idle Procedure
