CIS Idle issues
 

I have a 77 911 w/ 81sc transplant. Upon startup, I will either cosistantly idle at 2,000 rpm, without being able to adjust with the screw - OR- will idle fine at 950. Motor temp does not seem to be a factor.
Does this sound like a wiring issue? Any help would be greatly appreciated.

fast idle with the screw all the way in is a classic sign that you have an air leak.

Quite often the air leak is @ the decel valve.

It could also be your AAR or any of the vac lines.

Other possibilities incl the diz and a stuck sensor plate.

Agree with Rex--classic air leak. However the fact that it does idle at 950 points to an intermittent air leak, probably from one of the cold start devices or possibly the decel valve.

A bit more investigating on your part with reports back will help in tracking down the problem.
Tell us everything you can about when the problem started, how long the problem has been going on and if any work has been done on the engine.

I have a 74 with an 83SC transplant and mine started doing the same thing. It's only on the initial start-up of the day though--it idles at 1800-2K, even in 1st gear with zero throttle it will push that RPM. I am reluctant to mess with anything though because after a brief drive, if I turn the car off to go into a store or something, when I come back out the car idles perfectly at 950 again. It's baffled me for a while now.

Thanks for the help. I'll do a little more specific observations and see where this goes.

Hotwatermusic,

That is the correct operation of a CIS engine. The AAR keeps the revs up to about 2000 upon cold startup. As the engine warms up, the AAR gradually allows the revs to go down to 950.

Or, are you saying that the revs NEVER go back to 950 unless you shut it down? If that is the case, then it sounds like the same problem as tres 911's and you should check for vac leaks, especially the decel valve and AAR, and then check the mixture.

I had heard this was normal operation, and I would accept that and be fine with it, except that it never did the RPM jump until just recently. It used to start and idle at 950 like clockwork, then one morning I came out and started it, and it was 1800RPM. It's been that way ever since. But yes, once I drive it around, stop to go eat or something, and get back in it's back to 950. So I am leaving well enough alone. I've never touched the mixture or idle screw, and knowing myself I'll over-tweak and F something up, so until I have to I am not going to.

not knowing the specific names, I found a disconnected rubber vacuum hose to the dist and reinstalled. Also found another loose. This seems to have brought the idle to normal.
Thanks for the imput all!

Great news! You might post a picture of the hoses you found and reconnected, if possible. It will help others who may find themselves with a similar problem.

You know having sat here thinking about it, my idle issue started right as we stopped having 105 degree days this summer here in Texas. Could the ambiant temparature be hot enough that, especially with the sun beating down directly on my car all day long, could it be hot enough to have my engine already be "warm" if you will upon start up? Cause every day this summer it started at 950. As soon as fall hit and we got down into the 70's, boom--there's my 1800 starting RPM. A possibility or too much Coke Zero and staring at a computer screen??

Doubt the sun would get it hot enough to affect the sensors. especially if your talking the difference between 95 and 75. maybe replace your coke zero... lol gotta love this tx weather...

Sounds like the AAR is sticky and with the cold it takes longer (hotter engine temp) to "unstink" it. Do not mess with the idle mixture just yet.

I would remove the AAR and clean it with some carb cleaner, let dry and reinstall. Even if it isn't your problem it wouldn't hurt to clean it. You can also try to plug the air path at the AAR when the idle is high and see if the idle drops. You can pull the hose off the AAR and then plug the AAR and hose to close off the circuit. Us a large enough rubber plug on each so it won't go anywhere and
then start the engine. Do this after a couple of minutes at the higher idle. Shut engine off and pull hose then plug then start again. An internal air leak ( wihin the air circuit) can cause behavior like this.

Search "annual CIS mystery"

Ok, I'm gonna ask my "should already know this answer" question of the day: What and where, is the AAR on my engine? Thanks for putting up with a noOb.

Check out this link, all your basic questions will be answered:
CIS Primer for the Porsche 911

Hot water,
Here is what I'm doing to fix the AAR. 1st: it is located just in front of the middle passenger side injector. 2nd: remove both inlet and outlet hoses. 3rd: spray in some carb cleaner, let dry and repeat ( just like shampoo! ). I'm using a shop vac to speed the air and pull out debris. I can't get the thing completely removed, too tight of spot, but believe this will do the trick.
Tres211 75. 911 88 Carrera

tres211,
With engine cold, just remove the outside vac hose, not the inside one:

Check the AAR located on the right side of the engine between 5-6 intake runner.
It's the item that has a small electric plug and a large vacuum hose connected to it.
Open the clamp holding the outside-hose.
Pry it off with the screwdriver just enough to get it out of the way so you can see into the opening.
Don’t worry about the Alu pipe; it’ll move enough.
With the help of a small mirror and a light, peek inside the AAR.
When COLD, you should see an opening in the slide shaped like a half-moon.
When HOT, the opening is completely closed.
If it isn't closed, the AAR is either kaput or doesn't get any power.
You can spray some WD-40 into the valve in case it is just sticky.
To check the power, carefully open the tiny clip on the plug with a very small screwdriver and pull it off.
The wire clip is tricky but necessary; don't yank it off.
Start the engine and check the plug for 12 V.
Just ignition ON will not get power to AAR or WUR; engine has to run.
(One wire is power, the other (brown) is ground)
With power to the AAR, it should close after about 5 minutes engine running.

Thanks all!
 

The true solve was a power issue. Without the 12v supply, the AAR acts different to all conditions. Got the power solved, car idles perfect in all conditions!

The AAR heats up through heating (your 12 vdc discovery) and/or engine heat. As you found out, it eventually closes afte enough running and the engine transfer enough heat to it.

Glad your problem is fixed.