CIS vacuum question
 

Hi everybody! My 75 911 carrera runs beautifully. The idle is smooth, and the car never hesitates upon acceleration. The CO is perfect. Before I found that my tach is a little off, I thought that my idle was a bit low, and I attempted to adjust it. The screw is unresponsive... Is it possible to have a vacuum leak that will render the idle screw unresponsive, but allow the car to run well at all rpm's? Don't know if this matters, but the car's air pump stuff has been removed. I searched the forums, and could not find this specific question answered... Thanks :)

Idle speed adjustment.........
 

Nic,

Your '75 has air pump? Do you have 3.0 liter motor in your car? I'm totally baffled by this information and interested to learn more about your car. If you could turn the idle screw several turns from either directions and don't get any idle speed change at all, I would put my money on unmetered air going into the system. A faulty (leaking) decel valve could contribute to this problem. Keep us posted.

Tony

It did have an air pump, as well as thermal reactors. All that emission garbage was taken off. It is the factory 2.7. Like you said, it seems as if the car would have a vacuum leak (unmetered air), but it idles beautifully, the CO is bang on, and it accelerates and drives smoothly. I'm wondering if the car can run well and have a vacuum leak large enough to cause problems with the idle screw. Thanks for the reply Tony!

Dustin

not to insult you, but are you turning the BIG screw that has the slot facing the left rear wheel?

how much did you turn it? a little or a lot?

I turned the screw you're talking about...It also has a spring on it. I turned it counter clockwise because I wanted to raise it (like 2 turns), but nothing happened.
Thanks for the reply :)

Carreras have 3.0 liters - I think maybe you just have a regular 911S?

Typically a car will develop a vacuum leak slowly, and the mechanic will compensate at idle by richening the mixture, so your CO at idle will be OK. The problem is when you step on the gas, the small vacuum leak is such a small part of the incoming air, the vacuum leak is not felt. But the rich mixture is. You say your car is running fine, but I bet it will run even better after you find the vacuum leak and re-adjust the mixture.

Turn it out more.......
 

Nic,

With the engine idling smoothly, turn the idle screw out more without getting the it off the throttle body. Just remember your original setting (idle screw) before doing this experiment. Try 3+ full counter-clockwise revolutions and observe any change in idle speed. My guess would be NONE (???). If this is true, I might have an idea about this case. Keep us posted. Thanks.

Tony

Rex, you're correct about the "s". The car is titled as a carrera, but the VIN says its an "s". (I just had my dealer check it) I thought that for the 1975 m.y. the US carrera had the same 2.7 engine as the "s". I learned something new today :)
It's starting to warm up slightly in OH, so hopefully in 2 weeks I can perhaps do the starting fluid test for vacuum leaks. Is just a quick spray enough on each connection, or should I spray a bit more?

Everyone's help is appreciated :)

It's getting warmer in OH so hopefully I can do this test within a week or two... My guess is that the idle would not change. Thanks very much... I'll keep everyone posted.

Am I correct in assuming that vacuum holds the injectors in place with the 911's CIS? I know that's how my 924 is. My 911's fuel injectors move around pretty freely in their place. Does vacuum from the car running suck these down, or are the injectors not supposed to move like this? Thanks!!

There are some good pictures on here recently of what holds the injectors in. There is a sleeve that is held in by a rubber "O" ring, and the injector is held in by a larger "O" ring that is visible from the top. These "O" rings can move around some.

Either or both of these "O" rings and the sleeve itself are very common places to find vacuum leaks. Some have had success by temporarily covering the injector base with silicone.

Injectors should not move "freely" They are not rigidly held in place but they slide down into a plastic type sleeve inside the intake runner and then are "locked" in place by a rubber o-ring.

The o-rings get hard over time and you can suck air by them. They may even shrink a little but I am not sure.

If you can bottom the idle screw all the way clockwise (EDIT) AND IT STAYS RUNNING you have false air, more than likely.

I am in Akron so I would be glad to help if you cant get it sorted.

Thanks for the info! I've never tried turning the screw clockwise, so I'll give that a try... Hopefully the weather will warm up enough soon so I can fire the car up and perform some tests. I greatly appreciate the offer to help - I'll definetly send a pm if I need some help. Thanks Bob!

Dustin

BTW, you need to turn it a lot. Unlike other cars where half a turn makes a big difference, these cars you have to turn several turns. Clockwise to slow down, counter to speed up.

Hi

I had a similar problem with my 3.0L, could not get the idle down to a value less than 1100. After much exploration I discovered a crack along the edge of the edge of the air box runner at cylinder 1. I replaced the airbox and it idles much better and there is no backfiring when I am decelerating, another symptom of unmetered air.
Dave

Hey,
I wrote this up awhile ago.
eric

I'll try turning it some more. How many turns do you consider several? I turned it about 2 turns counter-clockwise, and nothing happened. Once this Cleveland weather clears up a bit, I'll try turning the screw more. I'm itching to try everybody's suggestions!

The pig tail like hose (breather) that goes from the oil tank to the right side of the air box is starting to get some cracks in it...I did some research on here and found that before 1978 the oil breather hose is before metered air, thus no vacuum leak would result from these cracks, correct? Since mine is a '75, there should be no difference in idle when openeing the oil cap, which is true of my car. Can anybody verify the accuracy of my thoughts on the breather hose? Thanks very much :)

That hose only connects to the engine shroud, and is not part of the intake - so no air leak there.

You are correct again - the oil tank is vented to the air box under the air filter - no air leak there either.

The typical air leaks are at the injectors and if you have a cracked air box.

I just took the car out and got it to operating temperature. I turned the idle screw in 4 turns and the idle dropped enough to probably die if I kept turning the screw in. I didn't want to kill the engine by turning the screw in more. I'm gonna check for vacuum leaks with propane, just to be safe...

Thanks so much for everybody's help! I appreciate it :)