Lightweight Flywheel Idle help

[s13b]

Hi,
I have installed a lightened flywheel in my car (m20b25). Problem is that the idle when warm is now too low and when coming off the revs drops <400 and almost stalls...

I have read it is possible to adjust idle in 2 places, one on the throttle body and two via an air con wire near the ECU.

Please can someone let me know which is correct and how I can tweak it up by ~200 rpm?

Many thanks

Information mainly from this thread
http://www.e30zone.net/modules.php?name ... djust+idle

[zaust]

personally I would not touch the tb. You can do it via the ac switch wire but I would check that everything is ok first ie no air leaks anywhere. I can't see a lighter fly dropping the idle 200 rpm. The only difference it made on my old m20 was quicker to rise and fall through the rev range.

[sweep]

how much did you lighten it by?

[Brianmoooore]

A lightened flywheel may make the idle uneven, but it shouldn't affect the idle speed. That is controlled solely by the engine ECU via the idle control valve.
If the idle speed is outside the specified limits, then there is something wrong with this closed feedback loop, either because of deliberate misadjustment or a fault.
As I said in the thread that is linked to, the idle speed can be increased a little to help smooth out any unevenness by making the ECU think that an air con compressor may cut in at any time.

[s13b]

Lightened by a couple of kgs on the lathe. The idle is definitely uneven

Will check the ICV...

[StuBeeDoo]

The flywheel on my 2.7 is very light. I've never weighed it, but when I bought the engine it was alledged to be 5.6kg. Since then it's been faced, so is probably nearer 5.5kg. IIRC, this is about as light as an M20 flywheel should go.

The idle is fine (850-ish on the rev-counter), if very slightly lumpy.

HTH

[Brianmoooore]

Uneven idle and low idle are two different things. BMW chose the flywheel weight they did to prevent an uneven idle, and if you have lightened the flywheel, then you may have to raise the idle speed to prevent overshoot when coming off the throttle.
This can only be done electronically without returning part of the engine management back to the '70s, so it's either connect up the aircon. pin, or modify the software.

[s13b]

Noted thanks Brian.

Stu, do you get much overshoot when coming off the revs?

[StuBeeDoo]

What do you mean? Drooping too far, then going back to the right level?
None at all.

[Silverfang]

I had to reset my idle cause the ICV couldn't cope with the large drop in revs, which was done via the idle screw on the throttle body, you'll only need a half turn at most to just give it enough to allow the ICV to do the rest.