Idle screw adjustment: relative or absolute?

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Idle screw adjustment: relative or absolute?

Postby TorqueHorse » Mon 24.10.2022, 04:58

Hey folks, I just got the engine fired up after the rebuild and have been trying to dial in my idle speed. I'm pretty close now, but I'm wondering if the adjustment of that screw is absolute, or is it relative to the current ECU config? i.e., if I disconnected the battery and then plugged back in the ECU, would it still be "set" the same? Or what if I swapped out a different ECU; or what if I mess with it while it ISN'T in Service Mode?
(my assumption is that it is a mechanical adjustment and isn't tied to a pot. or sensor per se, so it should behave more like an absolute kind of adjustment)
Thanks!
'90 FED M100 -mid resto
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Re: Idle screw adjustment: relative or absolute?

Postby dapinky » Mon 24.10.2022, 09:40

The idle speed is both absolute and relative, is so much as the reading is going to be different between service mode and running mode, when the sensors are throwing their inputs at the ECU.....

..... but the actual position of the screw (bleed bvy-pass) will remain static (unless it moves due to vibration etc).

Trying to set the screw with the engine running in 'normal' mode isn't possible, as the sensors will simply adjust other things to get to the target 950rpm, and a whole range of positions will "work".

As a rough start, if you screw the adjustment all the way in, then back it out by about 1/2 - 3/4 turn, then set the idle in service mode to get it right.

Obviously, it isn't possible to have both a paperclip and an elanscan lead plugged in at the same time, so (many years ago) I fitted a switch in the glovebox spliced into the 2 wires on the back of the ALDL plug so I can plug in the elanscan and then flick the switch to enter or exit service mode whilst looking at the live results on the screen - this is one of the few jobs where it makes it so much simpler to have the switch!
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Re: Idle screw adjustment: relative or absolute?

Postby TorqueHorse » Mon 24.10.2022, 14:02

Ahh, thanks for the info and tip!
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Re: Idle screw adjustment: relative or absolute?

Postby Simon_P » Mon 24.10.2022, 19:07

Confusingly we think of it as idle/tickover adjustment.

It is absolute - It is a fixed air bleed to calibrate the the IAC position.

It has no effect on idle speed, it is just that matching the idle speed is the means of calibration.
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Re: Idle screw adjustment: relative or absolute?

Postby Chas54 » Tue 01.11.2022, 16:33

I can thoroughly recommend fitting a DAPINKY switch in the elan scan cable . It's simple to do and will allow observation of parameters in the service mode. If you are adjusting your idle air screw, you can see immediately that the idle air control valve has been driven to count 20 before you start any adjustment.
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