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Ideal Idle Revs? How Low Can They Go Before Stalling?


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#1 Darwinism

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Posted 24 June 2018 - 08:43 PM

Hi, what is the ideal idle rev range for a MPI? How low can the revs go before stalling?
I used to have high idle, 1100-1200 rpm, now it's down to 850 and I worry it's going to stall at lights and junctions. Cheers.

#2 RooBoonix

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Posted 24 June 2018 - 09:16 PM

850rpm is ideal for an injection car, they control the idle themselves and unless something is amiss that's what they should settle down to when hot.



#3 Darwinism

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Posted 25 June 2018 - 08:25 AM

Thanks man! And the point at which it would stall?

#4 Ethel

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Posted 25 June 2018 - 09:53 AM

... is dependent on the energy stored in the flywheel being greater than the combined frictional losses and cylinder pressure as it passes top dead centre for the next power stroke. Any steady idle speed is therefore ok as long as it produces adequate oil pressure.



#5 Steve220

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Posted 25 June 2018 - 10:01 AM

Test and adjust until it works for YOUR car.

#6 RooBoonix

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Posted 25 June 2018 - 12:25 PM

Test and adjust until it works for YOUR car.


Not easily adjustable on an injection car without the correct code reader that can adjust it, and if it's settling at 850rpm happily I wouldn't bother touching it!

#7 Steve220

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Posted 25 June 2018 - 01:40 PM

Test and adjust until it works for YOUR car.

Not easily adjustable on an injection car without the correct code reader that can adjust it, and if it's settling at 850rpm happily I wouldn't bother touching it!

Use the turn screw on the side of the TB.

#8 RooBoonix

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Posted 25 June 2018 - 02:31 PM

Test and adjust until it works for YOUR car.

Not easily adjustable on an injection car without the correct code reader that can adjust it, and if it's settling at 850rpm happily I wouldn't bother touching it!
Use the turn screw on the side of the TB.

I didn't think you were supposed to fiddle with the idle screw unless you can check the air bypass valve setup on the reader. If I'm wrong then I'm wrong, the OPs still doesn't need adjusting ;)

I'm used to SPI anyway, which is a stepper motor on the throttle.

#9 Sprocket

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Posted 25 June 2018 - 02:35 PM

Test and adjust until it works for YOUR car.

Not easily adjustable on an injection car without the correct code reader that can adjust it, and if it's settling at 850rpm happily I wouldn't bother touching it!
Use the turn screw on the side of the TB.

For the injection cars, that is wholly bad advice.

#10 Ethel

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Posted 25 June 2018 - 03:07 PM

I'd guess the ECU has a target idle rpm and will tweak the ignition timing if it drops too low?



#11 Darwinism

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Posted 25 June 2018 - 04:12 PM

Thanks for all the replies nice info about the flywheel and cylinder pressure. I'll leave well alone, it's one of those super rare cases that a Mini actually righted itself!

#12 Steve220

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Posted 25 June 2018 - 04:17 PM

Isn't there a throttle stop? That's what I was referring to.

#13 Steve220

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Posted 25 June 2018 - 04:20 PM

Test and adjust until it works for YOUR car.

Not easily adjustable on an injection car without the correct code reader that can adjust it, and if it's settling at 850rpm happily I wouldn't bother touching it!
Use the turn screw on the side of the TB.
For the injection cars, that is wholly bad advice.

Why.

#14 Sprocket

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Posted 25 June 2018 - 05:10 PM

If you have to ask, then it is already clear you do not fully understand how the “idle speed control” works on the injection cars.

The idle speed is controlled by the ECU. It alters the ignition timing rapidly to maintain the idle setpoint which is a value calculated from more than one parameter within the ECU. If the idle speed is too high for example, the ignition advance would be lower than 15 degrees BTDC. If it is lower than 10 degrees BTDC the ECU activates the Idle Air Control to reduce the idle air and bring the ignition advance back up to 15 degrees.

If the idle speed is too low, the ignition advance would increase. If it is more than 20 degrees, the ECU would activate the IACV to increase the idle air and bring the idle advance back down to 15 degrees.

Adding more air by adjusting the throttle stops or any other screws on the throttle body completely ruins the idle control tune of the ECU and causes more problems than it fixes.

I’m sure you’re just testing me, and you already know this, otherwise I hope you learned something today.

?

#15 Darwinism

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Posted 26 June 2018 - 08:16 PM

If you have to ask, then it is already clear you do not fully understand how the “idle speed control” works on the injection cars.

The idle speed is controlled by the ECU. It alters the ignition timing rapidly to maintain the idle setpoint which is a value calculated from more than one parameter within the ECU. If the idle speed is too high for example, the ignition advance would be lower than 15 degrees BTDC. If it is lower than 10 degrees BTDC the ECU activates the Idle Air Control to reduce the idle air and bring the ignition advance back up to 15 degrees.

If the idle speed is too low, the ignition advance would increase. If it is more than 20 degrees, the ECU would activate the IACV to increase the idle air and bring the idle advance back down to 15 degrees.

Adding more air by adjusting the throttle stops or any other screws on the throttle body completely ruins the idle control tune of the ECU and causes more problems than it fixes.

I’m sure you’re just testing me, and you already know this, otherwise I hope you learned something today.

?


Is it possible to test the ECU for faults, run a diagnostic test on it or something? If so is it something any garage can perform?




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