Idle speed

["checksix3" <jetjock11@xxxx>]

>>The Idle Speed Motor does a fine job of setting the idle speed, and 
of maintaing it. But as I found on my car, only when you drive 
gently.<<

Thenn something is wrong with your car and you've compensated for it 
by using the manual idle.

>>If the engine is at a high rev (usually about 4K or more RPMs), then
when you push the clutch in to drop (or let off the gas on an auto
from my experience), the RPMs will fall VERY quickly.<<

The CIS has nothing to do with this. Again, you have other problems. 
I suggest you check your decel valves or throttle linkage setup.
My car (an auto) experiences none of these problems.

>>Now I had to compensate on my car to bend the bracket for the Idle
Speed Microswitch, and extending the stop bolt on the throttle
assembly. This now causes my Idle Speed Circuit to trip early, and
activate the ICM so that it will open early enough so the engine
doesn't dip too low, and I keep my electrical system stable.<<

Again, you need to find ths source of the orignal problem, not bend 
things to fix it. How many other owners have had to do what you've 
done?

>>However, there has of course been a trade off. I now have no gradual
powerband. Gently pressing my acellerator pedal, The ISM then tries to
compensate for a "fast idle" by choking my engine, rather than
allowing me to add the extra airflow to my acelleration. Once the
Microswitch is deactivated, my RPMs then jump to 1,500!<<

You clearly have something way out of whack and you're going about 
fixing it the wrong way. Also, your statments imply you don't have a 
full grasp of how the CIS system operates.

>>The other problem with the Idle Speed Circuit is that it doesn't 
like to work well with a motor that's not properly tuned. If you do 
have a vaccum leak, or you're running rich, then the idle 
seeking/hunting will of course occur (that's the LAMBDA system 
cycling).<<

Well, you need to tune your engine. And I said the engine must be in 
good health with the CO set properly. The Lambda will cause a very 
minor idle hunt of less than 50 rpms if everything is setup 
correctly, or none at all if you chose to set it another way.
All the symptoms you described indicate other problems, your car 
should do *none* of those things.

>>However, I do feel that as great of a system that it is, that it 
needs to be supplemented.<<

Absolutely incorrect. It will perform flawlessly if set up properly, 
as long as the engine has no major problems. I have mass flow 
measured the CIS system know this to be true.

>By setting the initial Engine Idle Speed manually with the 3 brass
screws, we create a "pillow" of sorts for the engine speed. The idle
should stay far more stable.<

Yes, thats true (see below) but you're not setting it up correctly if 
you use the brass screw to do that. It will work however but you 
shouldn't be using the balancing screws at all.

>When the RPMs drop hard, the engine isn't going to have to wait for 
the ISM to activate, and open up to feed the
engine air.<

The CIS valve sets to it's basic position as soon as you turn the 
ignition on, before you start the engine, as long as the idle switch 
is made. It only beccomes closed loop after ignition/tach impulses 
arrive.

>>Now, if there is any downside to utilizing the Manual portion of the
Idle Speed Circuit, then please, let me know. I myself really don't
see any harm in using it. Only benefit.<<

No harm but as you say, you shouldn't need to do it. There is no 
benefit to setting idle speed manually. It has no "improvement" over 
a properly tuned CIS. You clearly have problems with your car.

>>>If you will follow the link to the web site, it will show you how 
to set the idle. I would set it 950. 775 is to low. About middle of 
the page.<<

John, your procedure is ok for manual idle but useless for CIS. 
Again, you're chossing to set idle speed manually because of an 
inability to correct for CIS problems. Sure, setting the idle 
manually as you state can be done and 950 is fine for open loop 
control. You will lose the benefits of closed loop control however. 
As for 775 being to low, this setpoint is hardware built into the CIS 
ECU and can only be altered by a component change. We're talking 
apples to oranges, I'm talking about making the CIS work as it should 
in full close loop. It *will* do this if set up correctly. But I'll 
agree a higher idle is beneficial at times. My ECU has been modified 
to increase idle rpm to 950 when the headlights, A/C or cooling fans 
are on.

I see I'm dealing with shadetree mechanics and not engineers. The 
point I'm making is to stop viewing this as a car part, the CIS is a 
simple flow control loop. If you want a decent idle (assuming the 
engine is in good health and your CO is set properly) you need to 
prevent the CIS from controlling *all* the air that enters the 
engine. CIS conductance is not effected by CPR control pressure and 
that makes a big difference in mixture management.

By allowing the CIS to control all the air entering after the 
throttle body you increase it's response (it's proportinal band, for 
those of you who understand control theory) and this canlead to 
oscillation. By allowing it to control a window around metered air 
you decrease it's response to overshoot. Even the Volvo and Bosch CIS 
setup procedures make this clear and it makes perfect sense. It's why 
you set manual idle *higher* during setup than the CIS provided 
setpoint. It's difficult to adjust or troubleshoot a feedback loop of 
any kind when it's closed, thats why you need to open it for setup.

If you want to remove or plug the CIS path into the engine then yes, 
you can use the brass idle set screw on the throttle body to set idle 
speed. But if you prefer to leave the system in place a better way is 
to force the CIS motor to it's default position. This is done by 
removing the rpm input to the CIS ECU. The idle speed can then be set 
by using the throttle plate stop screw while still allowing the CIS 
motor valve to return to it's basic position whenever the throttle is 
closed. It leaves the CIS system intact, albiet not closed loop 
active. Either method is effective and either will give you a stable 
manual idle, as will correct setup of the CIS. Why people disconnect 
the system is beyond me, there are no benefits other than to hide the 
root cause of problems.

I can spout all the theory on Earth but this isn't rocket science.
It's your choice to run your car anyway you wish, I offer this only 
from a desire to demystify the CIS system and rebuke the long held 
notion it doesn't work as designed. If your car hunts during idle it 
shouldn't...period. Course, what do I know. Like I said, my D idles 
and runs perfectly in all regimes and so does the other one I 
maintain for a friend. 

And what is all this I keep reading about vacuum leaks caused by bad 
valve cover gaskets? There is no negative pressure with regard to 
ambient under the valve covers, on the contrary...measure it and 
you'll see.