Idle speed correcting
 

My Porsche manual says you can correct the idle speed on my 85 Carrera by using a bridge test wire on test socket in engine bay. I see how to do it but doesn’t the CPU ultimately tell the motor what the idle should be. Will this adjustment bring up the idle a small
Amount? Thanks.

That all depends on the current adjustment. The Computer tries to maintain a specific idle speed, but the manual adjustment can have an effect. The intent of the base idle setting is to establish the air bypass rate in the throttle body, so that the IAC is close to its center position. this then gives the computer the ability to trim the idle air to maintain the desired idle speed. if the adjustment is out of spec there can be instances when the computer runs into the limit of the IAC and still can't establish the correct RPM.

What is the problem you are having with your idle?

Yes and forget the bridge test. Just turn the idle air bypass in or out to what feels right. It's really not that complicated.

My understanding is that the whole point of the bridge is to take the Idle Control Valve (ICV) out of the equation when adjusting the idle. That is, the ICV will affect the idle if it adjusts while you're trying to dial in the value. By taking the ICV out of the equation you can adjust idle to the proper value.

this is my understanding too. i believe installing the jumper should force the computer to hold the idle control valve at its midpoint (half way open) to allow you to adjust the base idle without interference from the computer trying to dynamically control the idle speed.

I have found that some Porsche mechanics feel that the idle adjustments do little and motronic takes over anyway.

Connecting 'B' and 'C' on the test plug is actually giving the Motronic the same signal than closing the 'full throttle switch' (C = Ground, B = pin 3 on motronic, same as WOT switch to ground).
When running on idle with this B/C bridge (no throttle applied) the idle switch and the full throttle switch are closed to ground at the same time. Apparently the programming of the Motronic is such that it then stops controlling the ICV valve (open/close signal on pin 33 and 34), I assume locking it in the middle position ?

The idle adjustment screw lets some air bypass the closed throttle plate, the more air you give it, the higher the idle will be but indeed, the motronic will try to keep it at the programmed idle by compensating through opening/closing the ICV.
I wanted my idle at 880 rpm but my 1985 chip said it should be 800 rpm. To get a smooth idle, I actually went all the way and :
- reprogrammed eprom to 880 rpm (used TunerPro) so it wouldn't 'fight' for a lower rpm
- cleaned out old ICV and the tubes leading to it (lots of gunk, oil residue in there after 35 years)
- cleaned out the throttle body/ bypass circuit

Also have a look at the excellent post : http://forums.pelicanparts.com/porsche-911-technical-forum/625599-idle-ajustment-3-2-a.html

Yes and no. Motronic does have it's limits so bridging technique is there to stay within those limits. Adjustments can still be made without bridging provided you stay within those limits.