CIS GURUs

[kltarga72]

My 1980 SC 3.0 when cold starts correctly and warms up correctly for the first two or three minutes. I always give the engine about two minutes to warm up sitting in the driveway and idle speed is around 1100 rpm. As I leave the neighborhood the rpms will start to rise, around 1700. Now this high rpm at idle (when I stop at stop signs) will last until I'm 7-10 miles down the road. After the engine is thoroughly warm, when I'm at a stop sign the idle is back to normal 950 rpms.
If I stop the engine and it sits for 30 minutes or more the engine will start up immediately but my high idle is back. Drive down the road, get the engine hot and the engine will idle properly again!

So, my first question is can't I rule out a vacuum leak? I say this because if I had a vacuum leak my high idle would not disappear after the engine is at operating temperature.

Second question. I can also rule out the WUR "warm up regulator" because I have no issue starting the engine hot or cold.

I think that the culprit is the AAR "auxiliary air regulator". I have read about how easy they are to test and make repairs but every time I look at the AAR I think how difficult it would be to remove from the engine without dropping the engine. So, before I do that, I wanted to test my diagnosis before proceeding.
I think that the AAR bi-metal strip is not working correctly and the heat from the engine is heating the bi-metal strip, this is what's causing the vane to open (or close).

This is the only gripe I have with the 911, the engine runs great all other times except for the high idle as I described.

Thanks
Lash

[john walker's workshop]

Try tapping on the auxillary air regulator when it's at the high 1700 idle. They can be sticky.

[Schulisco]

A high idle can be caused by several reasons. If not proved also a "temporary" vacuum leak can't be excluded as well. To assure that there's no vacuum leak caused by any rubber or plastic parts you need a smoke generator. No other chance.
The same is for the WUR - if not proved with a pressure gauge you cannot exclude this as well. The reasons for a WUR failure vary and may be also leaks in the fuel line under several circumstances. Will say - you can't trust any part of the CIS unless you checked it for function with the tools and gauges required, mostly taken out of the car and tested it separately on a bench.

I agree with your supposition that the AAR may be the culprit which is - if it failed - strictly speaking may also be a vacuum leak if it remains open... they fail often due to carbon deposits and can be cleaned, you'll find lots of threads talking about this. But normally the AAR is opened up on (cold) start and get's closed while warming up the engine. Then they remain shut. A sometimes open or closed AAR I've never heard of.
Next is the start valve and the deacceleration valve (both mechanic or better saying pneumatic activated only) which your car is supposed to have as well as it is a lambda controlled US model which I suppose it is the Weissach.

But at first glance I would give the usage of a can injection cleaner in the gas tank a go and driving the car some hours before making the next steps. Except the AAR (or vacuum leaks in general) also the plunger in the fuel distributor might hang from time to time which can cause similar symptoms as well. If this fails you can proceed with the steps described. And drive the car more often - do I am right that the car sits longer between driving episodes?

Thomas

[kltarga72]

John Walker
Great suggestion, I will do that.

Schulisco
I don't think it's a temporary vacuum leak because the high idle is consistent each time I start a cold engine. I drive the 911 at least twice each month and drive around 100 miles each time. I only have the idle issue when the engine cools down. This idle problem started intermittently last spring but now the last five months the problem is consistent.
I have used a injection cleaner several times and the cleaner have no effect on my idle issue.
I should have mentioned that the top end of the engine was rebuilt three years ago with all new rubber seals replaced. I have checked all the rubber seal clamps to make sure all clamps were tight. All the injector housings and seals were also replaced.

I have the smoke machine and correct pressure gauge.

Again, I only have this high idle when engine is cold.

[Alan L]

I had the same issue once in my SC - about same rpm under similar conditions. Go to fill at gas station and on start up, 1700rpm . Was idling fine arriving at gas station. Drive down the road a mile or so, gone. Can't remember if blipping the throttle fixed it. BUT - the problem went away, so I have no idea what caused it.
Sorry - not a whole lot of help - but you are not alone.
Alan

[al lkosmal]

I believe that the 1980 SC Distributor has two vaccum hoses connected to it....one is for advance and the other is for decel.

If the decel hose is is not connected or open or the advance and decel hoses were swapped....the idle can stick high, especially if you blip the accel during starting which may cause the mechanical advance to actuate......

regards,
al

[PeteKz]

The others have covered the bases. I think one of those will be the problem. However, CIS is very sensitive to vacuum leaks. Since you have a smoke tester, test it. Then rule it out.

[Bill Douglas]

If all else fails... Mine was going a bit whacky like yours. I drilled the rivets out and checked the insides. It was kind of gritty, as if there was a tiny bit of sand in there. I gave it a good clean and put it back together using little nuts and bolts and it's been fine since. Good luck.

[Alan L]

If it only happens on a cold engine then I would be thinking vacuum leak. Cold engine = richer mixture. Add more air = more revs. Thats how the AAR works.
Like Pete Kz says - rule out something - first on the list - vacuum leaks. he is right - SCs hate them. Major cause of most poor running conditions.
Mine happened on a warm engine. Wish I knew what it was. No idea what changed to fix it.
Alan

[AndrewCologne]

If the hoses of the dizzy where switched then a high idle would be the result an all engine states no mater if warm or cold, as simply the "advance" hose gets vacuum as the throttle is closed. So I dont guess that this is the culprit.

I also recommend doing a proper check of the AAR. Do the fridge and oven tests and check how after these the AAR closes and opens properly. If not, beside checking that everything inner mechanism moves freely, you can do the "knock on the plug" method to adjust the position of the bi metal strip which affects. Just have a search in here there are several hints/pics how to do that.

[kltarga72]

First on my list is to tap on the AAR as John Walker suggested.

I will than move to checking for vacuum leaks with the smoke machine, I think I can do this without dropping the engine. I do have a friend that will help me with this test.

I appreciate the information and suggestions from everyone. I plan to check things out this afternoon, I will keep you posted.

[Alan L]

The other thing to keep in mind is a possible mechanical hang up. I had something similar happen a week or two ago when reinstalling my 930 engine. Turned out I had routed a small vac hose close to the return spring. Also the pedal linkage to the engine goes via a lever that the clutch cable rests on.
Pay attention next time whether something like this may be at play.
Alan

[Schulisco]

Originally Posted by kltarga72

First on my list is to tap on the AAR as John Walker suggested.

I will than move to checking for vacuum leaks with the smoke machine, I think I can do this without dropping the engine. I do have a friend that will help me with this test.

I appreciate the information and suggestions from everyone. I plan to check things out this afternoon, I will keep you posted.

The vacuum hoses were discussed here as well, andas AndrewCologne correctly raised objections against the switched vacuum connection. But the dizzy may also be the culprit in another way: If the internal adjusting mechanism hangs machanically from time to time things could cause the same behaviour. Therefore I also would check if the dizzy's advance and retard with a digital ignition timing gun as the next step after the steps described.
The hard part on a CIS troubleshoot like this is that every single component of the engine management, not just the CIS, may be the culprit causing this behaviour.

Pretty curious about your testing results.

Thomas

[proporsche]

Lash ,long story short
here is the test index for you...

[kltarga72]

Started engine again this afternoon. Same scenario, engine starts up 1000 rpms idling, after two minutes engine will rev to 1700 rpms and after seven to nine minutes engine will drop back down to 950 rpms.

I tried tapping the AAR (forcefully) when engine was idling at 1700 rpm, no change.

I took a can of carb cleaner with me and sprayed intake runner rubber boots at the air intake box and where intake runners attach to the engine. i sprayed each fuel line connector at the fuel injector. I also sprayed a couple other vacuum hoses. There was no change in high idle. I waited a minute or two and the rpms started dropping, back to 950 rpms. I used a laser temperature gun and the AAR case and it read 143 F. Engine temp was showing normal operating temp on the dash gauge. I keep thinking the AAR is the issue, I'm going to try to remove 12vdc connector to AAR to make sure I have power.

Waiting on my friend to help me with smoke machine and checking fuel pressures, I will keep you posted.

[mike sampsel]

Originally Posted by kltarga72

I think that the culprit is the AAR "auxiliary air regulator". I have read about how easy they are to test and make repairs but every time I look at the AAR I think how difficult it would be to remove from the engine without dropping the engine.

Thanks
Lash

Its not that difficult to remove on a '78.

Pull the black E plug first, it pulls off.
Then disconnect the hose
There are two 6 mm allen bolts to pull.

Then it comes off.

Nice if you have an old wiring harness so you can easily connect 12 volts to the AAR.
Put it in the freezer for an hour, then hookup 12 volts and you can observe if it closes.

for install I like to use a dab of weather strip cement on the allen head so they stay on the wrench. you want to have 6 inch alien to reach IIRC.

[john walker's workshop]

WUR and AAR are on the same circuit.

[kltarga72]

mike sampsel

Great information
I figured I could remove the AAR, it was the reinstalling the unit I was concerned about. I have seen a couple other threads about checking and testing the unit so now it's my turn.

I will keep everyone posted.

[Alan L]

You can check it in place. Take the outer hose off and with a torch/mirror look down the throat. When cold you should see a partial opening on the disc down the end of the throat.
That is the extra air being let in on cold start/idle. Now with it plugged in and key on you should have 12V at the unit from mermory. It should close that opening off over the next 5 min or so.
Yes - not too hard to pull - one of the allen screws is a bit tricky from memory - but a long T bar allen key helps.
The fact it only happens when cold suggests something in the warm up system for sure, but it is not easy to rationalise.
The dizzy suggestion was a good one - check the timing at 'idle' while this is happening - altho with 1700 rpm you are probably going to have some mech advance going on. Chicken and egg.
But one other thought would be to the vacuum limiting switch. It closes the manifold pressure to the dizzy on a cold engine. I wonder if the dizzy is advancing when this kicks in allowing advance. You could try disconnecting the elec connections next time and see if it still behaves like that.
Alan
Edit - the vac limiting sw is under the heater blower - easy to get at.

[kltarga72]

Update I think the 1980 SC is on the road to recovery.

Okay, Its been two months since i posted last. After checking fuel pressures which were correct I started testing the vacuum system with smoke machine. I found a leak at the pop-off valve where it was glued into the air box. Using a special glue (Loctite EA 11C) I was able to seal the leak. Tested vacuum system again and no other leaks discovered.

When I started the engine it ran very rough and I could tell that the engine was running to rich. Reading old threads and books on Bosch CIS systems I turned the fuel mixture screw CCW a little at a time and engine started running better. Over the last two weeks I have made adjustments to eliminate the idle from hunting and eliminating rough engine idle. I did this by adjusting the fuel mixture screw CCW to lean, Idle screw and ignition distributor. I drove the 911 twenty plus miles yesterday and did not have any high idle issues.

Thanks guys for your inputs and suggestions.