Warm up idle question

[universeman]

My SC idles at about 1900 RPM when it's cold outside (under 40F). It does this until the first white mark on the temp gauge, and then after some up and down hunting, finally drops to 950. This usually takes 10-15 minutes of driving.

As the engine gets warmer, the idle drops proportionally -- from 1900 down to 1800, 1700, etc. as it warms up, until about 1200 then it oscillates a bit, before settling down to 950.

Is this normal?

[Bill Douglas]

Normal - not really.

But summer is just around the corner. Why worry.

My SC had some slightly strange warmup things going on so I opened up the warmup regulator and cleaned it, and now it works quite well. But I think just leave it.

[universeman]

I'm posting to this thread again b/c the problem is still happening. On days where it's <55degF outside, the idle stays high for a long time. On warm days, it comes down almost immediately to where it's supposed to be.

Tony rebuilt my WUR so I don't think that's the problem.

Are there other easy checks in the cold-enrichment loop? I checked 15C switch, it's good. Problem only happens in cold so I don't think it's leak-related.

[Bob Kontak]

Auxiliary air regulator can be checked with a mirror with the hose pulled off of the snoot. It's the thingie near the #5 intake runner. Look in there when cold with a flashlight. You should see a partial hole in a disc. As car warms up the plate will rotate and seal off the hole. Idle should drop.

['76 911S 3.0]

Sounds like it may be electrical with regards to the AAR. The 2 wire connector applies voltage to the AAR and heats the control element internally. This works in conjunction with engine and ambient temperature to give the idle flare.

Seeing as how it acts relatively normal when warm and takes a while to come down when relatively cold, I would check across the 2 pins of the connector for 12v when the key is in the run position. If 12v is present, you can check for continuity between the windings of the AAR itself. Perhaps the internal circuitry has failed and it is only operating based on ambient/engine temperature.

[boyt911sc]

Originally Posted by '76 911S 3.0

Sounds like it may be electrical with regards to the AAR. The 2 wire connector applies voltage to the AAR and heats the control element internally. This works in conjunction with engine and ambient temperature to give the idle flare.

Seeing as how it acts relatively normal when warm and takes a while to come down when relatively cold, I would check across the 2 pins of the connector for 12v when the key is in the run position. If 12v is present, you can check for continuity between the windings of the AAR itself. Perhaps the internal circuitry has failed and it is only operating based on ambient/engine temperature.

There will be no voltage reading at the AAR plug terminals with the ignition switch @ ON/RUN position unless:
a). Engine is running.
b). FP is running.
c). AFS's ground disconnected or broken.
d). Fuel pump relay terminal #30 is energized.

Simply turning the ignition switch @ ON/RUN position will not deliver power to terminal #30 to energize the FP, AAR, or TV (optional).

Tony

[sugarwood]

[Pazuzu]

Originally Posted by sugarwood

The 3.2L has fuel injection, with a computer that tracks and corrects the idle. It should sit at 900rpm all day every day if the computer is working, no matter temperature, load, or altitude.


The 3.0l SC has no computer, and no active idle control. It has an ancient mechanical valve that closes when it gets warmed, either by direct ambient engine heat or be the small thermistor inside that gets constant and unregulated power when the fuel pump is running. Therefore, the 3.0L has not only no idle control, but no real proper cold/warm start correction, nor a way to compensate for engine load or a/c useage, and it really kinda pukes on itself when the engine needs more idle air but the idle air valve has picked up lots of residual engine heat and thinks the engine is hot, when it's only warm. That makes for crappy hot starts...

When I had CIS, I assumed my idle was good enough as long as it eventually dropped somewhere below 1500rpm by the end of a 10 minute drive...that is as accurate as you get time after time in various conditions.

[Bill Douglas]

My advice is still the same.

[MotoSook]

Originally Posted by universeman

On days where it's <55degF outside, the idle stays high for a long time. On warm days, it comes down almost immediately to where it's supposed to be..

^^^this = leave it alone. Unless you are willing to replace all the soft goods in your CIS system you must understand that it is 30 years old or so and things just happen a little slower when it's cold.

[MotoSook]

It's that time of the year AGAIN! (CIS)

[universeman]

Ha MotoSook that is funny. This topic is so old, you back then were saying "Yeah these cars are TWENTY YEARS OLD" and now they're 35 years old and people like me are still complaining! You must be shaking your head.

I'm all for "leave it alone." I'm also all for "richen the mixture a bit." But -- the 2000rpm idle speed I get when it's cold out (for 15+ minutes) gets a little old around town, at stoplights, etc., so I was hoping there was something I could do to regulate it.

My thinking is these cars did not operate this way in 1982. I realize it's not 1982, but the virtue of an all-mechanical system is that it can be repaired mechanically to its original operating state (or better,) given the right parts and know-how. That's what I think this board is all about, helping people to do just that. While I understand where the "leave it alone, it's good enough" thoughts come from, and I can appreciate them, I am also in this partly for the fun/hobby factor and trying to get something working well again actually is a goal in itself.

I've checked the AAR electrical and it's getting 12V when running. I've inspected it while cold and hot and it does what it's supposed to do...it's closed when warm, open when cold:

https://www.icloud.com/sharedalbum/#B0S5yeZFhvcMhV;30E97EE6-383D-49CF-ACD9-BA2336C2AACC

The above is a picture from April of last year, showing how far open the stone cold AAR is...is that the correct amount? Some PO MF'er stripped the rear allen bolt to the point I can't get the AAR out to bench test it without dropping the motor.

This pic shows it closed/warm: https://www.icloud.com/sharedalbum/#B0S5yeZFhvcMhV;7A32044D-53CC-42CE-AE7C-E8B52AE650EA

It could be gunked up, I guess, and need cleaning. Maybe the thing to do is to try again to get it out and work with it some more. If it's getting stuck in its travels, that would cause the idle issue.

Thanks for all of your inputs, even those saying "leave it alone" - I appreciate all the perspectives.

[universeman]

MotoSook not trying to start a debate but in your other thread, you said "cold air is denser and so that will cause CIS to lean out the mixture in cold weather." Understand that theory - denser air, same fuel volume = leaner mix.

I wanted to ask you what you think about my understanding of the following: the airflow sensor in CIS is designed to compensate for denser/thinner air (as happens in altitude changes also) by raising the control plunger higher on a cold/dense morning than it would on a warm/thin morning. The plate is raised more in the same volume of air, if that air is denser (b/c more air molecules hitting the plate), thus causing the FD plunger to be raised more, thus richening the AFR.

Are you saying you don't think this is how it is designed, or that it is designed that way but it just doesn't work (or doesn't work well?) so we need to manually richen the cars in winter?

[javadog]

It's really hard to see from your picture if the AAR is completely closed. It looks like it is still partially open. Pinch off the hose leading attached to it when the engine is warm and see if the idle speed changes. If it were my car, I'd probably go ahead and do what's needed to remove it, so it could be cleaned and inspected. Whether it completely closes, or not, and whether it closes at a normal rate is hard to say. I can say that in cold weather it does take a very long time for a 911 motor to come up to temperature and in really cold weather it never gets fully warm. I can also say that some of the 911 engines (believe it or not, they were not all the same ) had cold idle speeds I thought were higher than I liked,

Denser air does move the air flow plate more than thinner air. How significant this difference in movement might be is hard to say. None of us has measured that. In addition, the airflow funnel is tapered and the taper is not constant throughout the full travel of the arm. This has an effect on mixture.

JR

[boyt911sc]

Jason,

Those pictures you posted of the AAR were still open. Remove the AAR and test it with a 12-volt power supply. At room temperature a good AAR would be closed between 2~3 mins. and some would take 4 mins. (?). Depending on the value of the heater resistance (Ohms) the closing time is inversely proportional.

My '78 SC is the oldest CIS SC model and it still run reliably regardless of weather. There is no need to adjust the mixture setting seasonally and if you do it, there is something wrong going on with your system. Each individual CIS components like WUR, FD, FP, AAR, AAV, DV, TV, TTS, etc. could be bench tested by any DIY'er like you or me.

It is the lack of understanding and knowledge that makes CIS troubleshooting a problem for some people. The fact is, it is very easy and simple to maintain a CIS engine to perform reliably if you pay some attention to it.

Tony

[1987 Porsche]

My 78SC was doing the same thing. I ended up pulling the AAR, tested
it with a 12v motorcycle battery, cleaned and lightly coated it with WD-40 then reinstalled. I came to find out the hose that connects to the AAR (where I'm pointing too) has an aluminum bend in it right where that red hose sits on it. Well the red hose in the pic was lying on the aluminum hose and after all the years of vibration put a slight hole in it causing it to run a high idle like yours. I plugged the hole with aluminum tape now it's all good. Maybe your problem, maybe not. Thought I'd add it.

[universeman]

Thanks guys, I'm gonna try to pull that damn AAR and will let you know what I find.

[gazzerr]

I found the air box rubber mounts on mine had sagged to the point where the whole system was sitting on top of the engine shroud. The vibration can wear holes in many different places.

[SCadaddle]

My 82SC used to do the same thing in the winter time. Then one day I opened the rear decklid and the idle came down to where it was supposed to be. Closed the decklid and it went back up.

????

Turned out to be the A/C condenser in the decklid was pushing down more than usual on the cruise control cable due to a loose fastener holding the condenser to the underside of the decklid. The cruise cable was then pulling on the throttle linkage arm!

So eliminate the obvious!

[universeman]

Ok guys. Damn AAR won't come out. Rear bolt holding it down is totally stripped, and there's no clearance to get any tools for drilling/EZouting etc onto it....I could remove the CIS or the engine (amazing that it will take that to remove a silly AAR but - Porsches!)

So I'm planning an engine drop. Can address a few other projects as well while I have it out.

My aluminum hose rubber is broken and frayed to bits. That ain't helping. I buttoned it up as best I could and cranked down the hose clamp. Looking at the cost of that hose, I'm hoping I can find someone to recrimp a new rubber hose to the end of what I've got...

Thanks for all the helpful advice guys, as always!