CIS stalling: missing oil return restrictor?

RallyJon · 03-26-2006, 12:17 PM

I have an '81 CIS 3.0 in a '71 911, using the '71 oil tank. The engine has always stalled when the oil filler cap is removed.

I was searching this forum to try to figure out why, and someone mentioned (in reference to a Motronic 3.2) that there is a restrictor in the hose that comes into the oil filler neck from the engine. Part number is 930.107.289.00. I see from the parts manual that the '81 SC has it too. I don't have it.

1) Think that's why I'm stalling, or might it be something else?

2) Can that restrictor be fitted to a '71 oil tank? If not, what else can I do?

Thanks, -Jon

javadog · 03-27-2006, 03:23 AM

Jon,

I'd say that the engine can't handle the vacuum leak caused by running with the oil tank cap off. I don't remember what size the orifice was that Porsche used in this line but if you do a search on this board you should find it, as it was talked about not too long ago. I can't answer the question about the '71 oil tank, as I don't have one in front of me to look at. If nothing else, I'd just have a brass fitting machined with this orifice size and install it in the existing breather hose that you have.

You might also want to make sure that your CO is set properly. And, if you are not running the stock O2 sensor, that could help compensate for the leak if you hooked it back up.

JR

MotoSook · 03-27-2006, 05:17 AM

There is a restrictor on the oil tank vent hose that leads to the CIS boot. The restrictor is on the boot end.

As JR suggested, check your mixture. Without an CO analyzer, test that you are not running to lean.

Here is the quick "field idle mixture test."

CIS Idle Speed and Mixture Setting Without an Analyzer

MotoSook · 03-27-2006, 05:31 AM

Jon,

I seem to recall that the aluminum fitting that attaches to the hose, which is then attached to the boot has an orifice built in it. So check that you have that aluminum elbow. If you have the fitting, and van verify that it is restrictive, then your idle misture is set too lean.

It's easy to get that fitting out. Undo the two clamps that hold the boot to the air meter housing. You might not need to take the boot off if you are familiar with what I'm talking about. You can work blind to get to the clamp that holds the fitting to the boot.

RallyJon · 03-27-2006, 05:48 AM

This? Best I could do with a digital camera reach around...

That other thread is great. I'll need to read it several times very slowly, though.

RallyJon · 03-27-2006, 05:52 AM

And this is what I thought might be the problem:

MotoSook · 03-27-2006, 06:01 AM

Jon, that first fuzzy picture is what I posted about. FYI: my 78 3.0 with CIS only has that fitting, and nothing else to regulate the air intake when the oil cap is off the filler neck. My engine does not stall....so read that idle-adjust thread and got from there.

Quick-n-dirty: remove the air filter, start the engine (warmed) and push the air plate up a millimeter or two. If your idle speed increases, you are running lean....if so, turn the idle mixture screw clockwise 1/8 to 1/4 turn after you shut the engine off. Then start it back up, race the engine a couple of times to get things settled, then test the oil cap...remove to see if the engine stalls.

RallyJon · 03-27-2006, 06:08 AM

Uh-oh. It seems that the tank-end restrictor replaced the boot-end restrictor in my year--1981.

You have 47a and 47b. I just have 47:

MotoSook · 03-27-2006, 06:11 AM

I'm pretty sure I have "47" Jon. My CIS is very bastardized. I'm running on the rich side of things....give the mixture test and setting a run...

RallyJon · 03-27-2006, 06:12 AM

Will do. Thanks for the expertise!

MotoSook · 03-27-2006, 06:14 AM

Good luck. I have a later CIS system in pieces if you need parts.

Gunter · Gunter's Garage

Some SC's have it, and some don't. Mine doesn't.
It's not a vacuum restrictor, it's there to stop some of the oil mist that otherwise ends up in your sensor plate/TB.
It usually sits not in the elbow, but in a nipple half-way between oil tank and TB-boot. It needs to be cleaned out periodically.
It's similar to the old PCV concept of holding back some of the oil mist while, at the same time, allow the vapors to get sucked into the engine for combustion.
All CIS systems will drop about 50 RPM from the 950 RPM when the cap is removed, it's normal. If the engine stalls, mix needs to be checked.

RallyJon · 03-27-2006, 08:38 AM

I have a pretty high idle already, and it dies instantly when the cap is loosened. I would think that a higher idle would be more stable, but I guess not.

MotoSook · 03-27-2006, 08:40 AM

The higher idle does not mean your mixture is correct. The idle bypass screw (idle speed screw) on the throttle body will change your idle speed, but it needs to be adjusted inconjuction with the idle mixture.

Gunter · Gunter's Garage

Souk said it.
RallyJon: A high idle-setting sometimes masks a vacuum leak somewhere.
What is your idle? Is it not running right when set to ~950 RPM?
You cannot really set correct mixture at a high idle.
Adding an oil mist restrictor will not solve your problem.
Reduce the RPM and check for vacuum leaks.