CIS warm idle low

[angello]

Hi ,

I have a 1982 911 SC with a small issue which sometimes bothers me so I would like to fix it if it doesn't involve buying some instruments or dismantling half of the injection system
- Engine cold : starts at the first crank , idles at 1800 rpm , after about 5 min. settles down at about 900-950 rpm. Therefore I reckon AAR and WUR are ok.
- Driving the car around , hot engine , no issues , idle 900-950. No backfire , no hasitation , runs good . Therefore fuel pump , fuel distributor , fuel pressure and ignition distributor are good.
- When I turn the engine off and starts it again after couple of minutes , it starts immediately , idle is still ok , 900-950.
- When I start the engine after 15 minutes , it starts great but idles a bit lower at 750 rpm.
- When I turn on the engine after 45 - 60 minutes , starts ok but idles at 650 rpm. I hear the engine doesn't like it but it runs . The fuel accumalator is ok , it starts straight away.
- If I leave the engine running, when hot again , idle is ok then.
- The next day cold engine starts again idling at 1800rpm.....

My question is what controls the idle rpm in these medium engine temperatures when it is not hot but it is warm enough ?
I know AAR is open only with cold engine .
Could it be something with OXS , lambda , FV etc ?
Thanks for your help , regards from Croatia

[2jmotorsports]

Hello Angello,

Read the first few posts of this thread.

Then test and post the following fuel pressure test data for your car:

1. Year of engine:
2. US or RoW (Rest of World):
3. WUR model number:
4. Ambient temperature at time of test (in degrees C):
5. WUR Resistance (in Ohms):
6. System Pressure (in bars):
7. Cold Control Pressure (in bars):
8. Warm Control Pressure (in bars):
9. Time delta for Cold -> Warm Control Pressure (in minutes & seconds):
10. Residual Pressure @ 5 min (in bars):
11. Residual Pressure @ 15 min (in bars):
12. Residual Pressure @ 30 min (in bars):
13. Residual Pressure @ 60 min (in bars):

Then I think the experts will have a much easier time pointing you in the right direction.

[angello]

Thanks , but I am after some simpler answer .

[tirwin]

Originally Posted by angello

Thanks , but I am after some simpler answer .

Then take a guess. Start replacing parts. You’ll eventually figure it out.

[dhanl82]

The suggested tests are being recommended so you can isolate/determine the problem which will then enable you to focus on the cause and it's needed solution. Given the nature of the CIS system, and its many components, there is no quick/simple answer to your question other than a wild guess!

[stlrj]

Seems your AAV needs some adjustment. Not too many people know they're adjustable. Just because the factory set yours with too much bypass air doesn't mean you can't set it back down to a more reasonable cold start rpm. That way you could increase your warm rpm up the way you could live with.

Cheers,

Joe
82 SC

[boyt911sc]

Originally Posted by stlrj

Seems your AAV needs some adjustment. Not too many people know they're adjustable. Just because the factory set yours with too much bypass air doesn't mean you can't set it back down to a more reasonable cold start rpm. That way you could increase your warm rpm up the way you could live with.

Cheers,

Joe
82 SC

AAV (auxiliary air valve) has nothing to do with the warm up. It is normally open and immediately closes as the engine creates vacuum. The AAR (auxiliary air regulator) by-passes the cold metered air. Maybe you meant AAR instead of AAV.

Tony

[angello]

As far I understand AAR closes as the engine warms up . It stays closed when the engine is hot ( warm) and when we turn the engine off it opens again after couple of hours cooling period.
But what controls the warm engine idle rpm ( not exactly hot ) at medium temperatures when we for example go to the shop , turn the engine off and turn it on again after 45 minutes ? Does ECU by oxygen sensor and FV regulates idle ?
This is a US spec 1982 SC with lambda CIS.

[stlrj]

Originally Posted by boyt911sc

AAV (auxiliary air valve) has nothing to do with the warm up. It is normally open and immediately closes as the engine creates vacuum. The AAR (auxiliary air regulator) by-passes the cold metered air. Maybe you meant AAR instead of AAV.

Tony

I stand corrected. (the one mounted on #5 intake runner)

Thank You

[Walt Fricke]

Well, here are some simple experiments:

When warm and idling at 900 RPM, turn the idle bypass air screw in until it drops to 800. Assuming that is a nice, consistent, idle, let the engine cool down to where you are having a low idle problem. Restart and see what you have. You'd suppose it would be even lower, but you'd learn something.

Assuming this is as bad or worse, set the idle screw back to where it was. Then recreate the restart idle problem, and adjust the idle screw to get 800 RPM (that is the Euro value, so it shouldn't be too bad), and then see what you get through the sequence.

No tools needed for this.

The Lambda system has an effect on things, though it is not designed to be an idle speed control system, I think. You can check some of it with a VOM (to see if, for instance, the FV relay is getting power), and you can feel the FV itself - with a warm engine you should feel it vibrate, and some if you have a dwell meter (which fewer guys probably have, since by the time of the SC there are no points in the distributor, and no adjustment of the dwell). Your engine has an engine oil temp sensor (unrelated to the sensor for the temp gauge)which connects to the Lambda mini-brain - it is in the right side chain housing cover (or maybe in the engine air vent cover on some models). You could check to see if that is working - it is just an on/off switch to ground, and you could trace the wiring. You may need to do some more studying - I can't recall if it grounds when hot, or the other way, or at what temp it does this, but that is the kind of thing one can look up. Try "Jim Williams Garage" or "Jim Williams" and "CIS". Someone has taken over keeping Jim's website up.

You could disconnect the electrical plug to the AAR, just to see what, if anything, that does. It should slow the progression from the (rather high?) cold start idle to the warm running idle. The AAR is a throttle bypass air device - same in principle as the idle adjust screw - which closes because of heat - a bimetal strip when heated closes a valve, basically, and opens it as it cools when the engine stops. The engine will eventually provide this heat if no electricity is supplied. But as with the thermotime circuit which leans out the WUR, the bimetal strip in the AAR has a resistance coil around it, which heats it up faster, thus closing the AAR faster.

Here is how the AAV works:


But I'm just guessing here. With a little more help from you, someone like Tony who doesn't have to refresh his CIS knowledge may be able to narrow the search for a cause. He'd like to see the various figures suggested, but those suppose you have access to the tools to measure various pressures, which may be trickier for you to access than here in the States.

[SkiVT]

The device in the picture is the AAR and not the AAV correct? Doesnt address the posters problem, just trying to be clear. I made the mistake of not plugging my AAR back in once. Started engine and drove away. Idle stayed way high and didnt come down. The engine heat alone in that case didnt close off enough of the air. You might also want to check how quickly the AAR opens back up as it cools. Maybe bimetal strip and little internal spring arent as effective in reopening. Several threads on testing and cleaning that device.

[angello]

AAR is closed until the engine is fully cooled.
So this can't be the culprit for my low idle when the engine is at medium temperatures.
Playing with the idle screw , ccw or cw as someone has suggested , doesn't change anything . After 45 minutes I restart the engine and it idles at about 650 again.
I reckon I 'll have to live with it.

[SkiVT]

When I tested my AAR it closed slowly as it heated and opened slowly as it cooled. It did not snap completely open and snap completely shut on either end of the temp spectrum. Your test of your AAR may have shown something different and posting those results would likely benefit others over time.

[keoniahlo]

I'm having a similar problem with my 911SC--warm startup has slightly lower RPM. I'm disappointed that the solution to Angello's issue was never posted or identified!

[ahh911]

Been through this, was not able to fully eliminate the lower rpm after sitting. I've written it off as running friction vs metered air. I don't think this can ever be made perfect as the AAR vs oil temperature / viscosity can never be fully mapped open loop over the varying conditions.

I optimized the AAR open window vs temperature so that it just closes at colder temperatures.
At one point I eliminated most what you're talking about by also bypassing the o2 sensor for around 1min at startup and keeping the FV at 65% in the process. This is forcing the rpm up by being rich as otherwise the O2 sensor can turn on fairly quickly and drive the rpm down as it delivers stoichiometric ratio, this is for SC with lambda.


Phil

[PeteKz]

Phil, I know you have gone through your CIS many times and in great detail. I'm sure you have tried adjusting the warm idle RPM with the idle air screw on the TB. Does that not work as expected?

[ahh911]

Hi Pete,

Angello is asking for a specific set of conditions.

Originally Posted by angello

- Engine cold : starts at the first crank , idles at 1800 rpm , after about 5 min. settles down at about 900-950 rpm. Therefore I reckon AAR and WUR are ok.
- Driving the car around , hot engine , no issues , idle 900-950. No backfire , no hasitation , runs good . Therefore fuel pump , fuel distributor , fuel pressure and ignition distributor are good.
- When I turn the engine off and starts it again after couple of minutes , it starts immediately , idle is still ok , 900-950.
- When I start the engine after 15 minutes , it starts great but idles a bit lower at 750 rpm.
- When I turn on the engine after 45 - 60 minutes , starts ok but idles at 650 rpm. I hear the engine doesn't like it but it runs . The fuel accumalator is ok , it starts straight away.
- If I leave the engine running, when hot again , idle is ok then.
- The next day cold engine starts again idling at 1800rpm.....

[Schulisco]

Angello, check this out if this could be the same issue on your car:

https://forums.pelicanparts.com/porsche-911-technical-forum/758788-cis-troubleshooting-dummies-7.html#post12341104

Thomas

[AndrewCologne]

Originally Posted by ahh911

Hi Pete,

Angello is asking for a specific set of conditions.

This behavior is not uncommon.

On cars with choke (or better said manual idle adjust like on earlier k-Jet 911 models),
with a hot engine stalled, after one hour with engine still warm you often need the choke a little pulled again, to make the idle running smooth.
On cars with AARs at this point the remaining rest heat still keeps some AARs closed if not at factory spec or even modified, ... but the engines idle already needs more a/f mixture to run smooth.

With factory calibrated AARs which do run at approx 1.600-1.800 rpm at cold idling, these should already open a bit when engine cools down for an hour, which gives the needed min. 900 rpm at idle on a warm re start after an hour stalling.
But often if AARs are out of specs or even modified to not run with such high idle rpm at cold run, then this also lets drop the idle on warm start/running states cause AAR is still fully closed.

For me personally the very high idle at cold run drove me nuts and I guess people on the street around me as well, cause in times of now its very uncommon and unknown that this was normal by design.
So I modified my AAR that it runs with 1.100-1.200 rpm at cold idle, with no stuttering driving or similar, so for me its optimal. And I can live with that side effect of low idling on warm restarts.