CIS issue... oscillating idle right after startup

[Groovy911SC]

I believe that my auxillary air valve may be the culprit to my problem. I have very liitle experience with CIS components so I would like to get an opinion or two before I buy a new part.

The problem is that after I first start the engine, all is well and it idles perfectly... after a few minutes... say 4 or 5 minutes... the engine begins to slightly oscillate. The oscillation grows after another 3 or 4 mintes to the point that the engine dies.

I have read many posts on this site and I have determined that from my reading the auxillary air valve can cause the oscilation. Does anyone have any ideas to help me test this idea out?

Any help will be appreciated!

[Groovy911SC]

Another bit of information is that if I drive the car and keep the idle high for about 5 more mintes and not letting the engine die, the engine then behaves normally until I turn off the engine... let it cool...

[T77911S]

i dought it, it either works or it doesnt. sounds like it may be too rich. always eliminate the basics first. start with a tune-up.

[psalt]

Part swapping CIS usually results in expensive mistakes. Your system uses many separate components that do not speak to each other and need to be tested separately to determine what is wrong. The AAV should only let in extra air as the engine starts and should be out of the picture when it is running. To test it simply pinch off both hoses and see what happens. The AAR is a different component and it controls idle speed when cold. Your problem is probably mixture related, perhaps caused by a vacuum leaks. First set the ignition timing and idle to spec, next check the mixture. You can get a rough idea what is happening with a dwell meter at the test connector for the FV. Cold it should be 58 dwell, then fluctuating between 40-58 dwell when it goes closed loop. The injector sleeve orings are a source of vacuum leaks, check for leaks with an unlit propane torch.

[boyt911sc]

Groovy,

Have you checked your fuel pressures (cold & hot)? Your erratic idle speed could be caused by unmetered air (vacuum leak) or WUR beginning to malfunction. Double check the vacuum hoses of your AAR.

Do not replace CIS components unless they are confirmed defective. WUR, CSV, TTS, AAR, fuel pump, etc. could be tested individually and will give you a better idea where the problem is coming from. Avoid guess work trouble shooting and you'll save a lot.

Tony

[john walker's workshop]

have a shop with an exhaust analyzer set the mixture for you. you might want to leave the oxy sensor unplugged and set the hot, at idle CO to around 3-3.5%. then see what it's like next morning.

[daniell911]

I have a similar problem. My mechanic checked my vacuum unit and I have a leak on the left side. I just ordered some parts from our host including a new Vacuum unit. Also I will change the O-ring seals of my injections(I have no clue how to remove the line yet) Just pull a bit I guess as per the Technical Articles.

We dropped little water on the line and it suck it right away so a air leak there also. After that I assume the car will drive FASTER




Daniell911

[T77911S]

pull on the injector not the line. try an open end wrench under the injector for some leverage.
curious....what vacuum unit?

[daniell911]

1-237-122-763 Pelican Part

I have 1983 911sc

Yes you are right not the line.

I saw a thread about it using a wrench and a flat screwdriver.
I will pop out. Not screw right?

Little rookie here!

But did some deep work before
See my page below

Daniell911

[Richard Burns]

Do not change the AAR until you have checked everything else. I had the same problem with my 77 Carrera 3 and after trying several things I took it to the local Porsche guru and he fixed it in 30 minutes. I was charged labour only so the AAR was not changed.

[Groovy911SC]

Thank you all for your input! I would love to fix the oscilation myself only to learn more about the CIS system. I rebuilt the engine about 18 months ago and did not replace any CIS components except for the rubber intake sleeves on the manifold. I use the car as a track day only vehicle so fiddling does not worry me so much... I just do not want to cause myself more self inflicted expense.

[RarlyL8]

Do you have the equipment to check your CO?

If not there is still one thing I would try first and that is to lean the mixture.
With the engine cold and off, place the 3mm tool into the mixture screw at a position that has the handle pointing towards a referance point. Bump the tool clockwise and then turn the screw 1/8 of a counter clockwise revolution (45*). Remove the tool and start the car. Note any changes in the cold operation and as you drive and it becomes warm. The occilation should be reduced or gone. If not, repeat the above procedure placing the tool EXACTLY where you left off. Do not adjust to a total of more than 1/2 of a revolution (180*).
If this does not work or you are not comfortable with it simply return the screw to the original position. No harm, no foul.

[psalt]

Is there a reason you are recommending adjusting the mixture the opposite way that the factory recommends ? Porsche recommends always making the final adjustment in the clockwise direction. His engine has the lambda system which will display a reasonable indication of the mixture setting on a dwell meter.

[Paulporsche]

Paul,
I think what Rarly means is that the overall way to go to lean the mix is in the counterclockwise direction from the starting point.

Groovy,
On my car I would do what Rarly said. However, since you are new to this car & CIS, it would be better to either first check your control pressures w/ a gauge, or have a mechanic do it. If they are right, set the mixture per JWalker. It is very likely your mixture is too rich, as this is a common symptom, but you should know why it is first.

[psalt]

I'm just reading, not guessing. My point is all of the factory and Bosch publications state to adjust the mixture level from lean to rich. The final adjustment is to be made in the clockwise direction, opposite of his statement. The reason has to do with the screw thread and the lever hinge. I have never heard of a reason to contradict this procedure in 25 years of putting up with CIS.

The CO spec for the engine is 0.4-0.8 %, not 3-3.5%

[RarlyL8]

How are you going to adjust from lean to rich if the engine is running rich?

Read what I stated. You "bump" the screw clockwise (rich) then begin CC to lean. You can then "bump" clockwise again if it makes you comfortable to do so. I also have been doing this a few years. I don't "put up" with CIS. I tune it to work properly.

Again, if you don't feel comfortable doing this don't do it. A mechanic will gladly do it for you for $90/hr.

[psalt]

By following the factory's procedure:

"If the mixture is excessively rich, first turn idle control screw counterclockwise further than necessary and then clockwise to the nominal value.....always adjust CO level from rich to lean" (Porsche AG)

Following your procedure can result in false and inconsistent settings and I do not recommend it. I would not describe that as "properly", Henry Royce would be turning in his grave. I have been adjusting CIS injection since 1978 and would not advise solving a minor problem by creating another.

[Paulporsche]

Paul,

You and Rarly ARE saying the same thing: that to go from rich to lean you must go counterclockwise, but to finish off you should go slightly clockwise. In other words, go slightly past where you want and then come back.

The original spec for CO was based on EPA standards that had to be met for the times. For better running and horsepower, these cars seem to like a richer setting, about 3 or 3.5% as JW says, especially as they age. There seems to be a lot of leeway here, as these engines can run within that range.

Groovy,

I think the best thing is to check your control pressures first, especially noting your cold cp. It sounds to me like either your ccp is too high which results in poor starting (too lean), and someone may have tried to compensate by setting the mixture richer, or it may simply be too rich now that you have completed your rebuild. This may be because your WUR is shot, or it may be simply out of adjustment and can be reset by a process called "knocking the plug" (do a search). If all this sounds daunting or you don't have a pressure gauge, have a trusted mechanic check the pressures for you. You then have a baseline from which to start

Since you have done the rebuild, I think you can probably handle this. I have also been adjusting CIS cars since 1978 and if this were my car, I would do what Rarly said: set the CO about 1/16 turn leaner than now and see what happens. If it improves, but is still there somewhat, try another 1/16 turn. You can always put it back, and, since you live in a place where the temps are not too extreme, this may be all you need to do.

[psalt]

You seem to be missing the point entirely, removing the tool after a counterclockwise adjustment is incorrect and bad advice. Suggesting that it is correct , creates another error. It is impossible to properly diagnose a CIS lambda system if you set the CO to 4 to 8 times the factory spec. That will mask any number of problems and incite emails from Al Gore.

[RarlyL8]

Ok, I'm an idiot, my car runs bad, (Al Gore can kiss my ass) and I have no business giving advice on tuning CIS. Happy?