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911SC Idle Speed Fluctuation after Warmup

I have a 1981 911SC that startsup and runs fine for the first few minutes at around 900 rpm (note that since I have has it, it has never idled higher at startup, no matter what the temperature is outside). As it gets warmer, the idle begins to fluctuate between 500 and 1200 rpm in a constant surge. Often cycling low enough to cause the engine to stall.

Any thoughts on what might be causing this?
Old 10-14-2012, 04:07 PM
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On your 81SC you have K-Jetronic CIS with Lambda (O2 sensor) control. At cold startup the O2 sensor/Lambda system is off. Once the engine reaches operating temp the O2 sensor system is switched on and you will see small fluctuations in idle as the frequency valve cycles to maintain an optimal A/F ratio. The "normal" small fluctuations in idle could be amplified into big fluctuations if you have a vacuum leak. Could also be that the sensor itself is bad.

For diagnostic purposes I would disconnect the O2 sensor, then start it up cold, let it warm up, and see how it behaves. If it does not fluctuate at all with the O2 sensor disconnected then you know it's the Lambda system that is responsible.

A good place to start would be checking for vacuum leaks (brittle/crumbling hoses, etc). Another culprit for vacuum leaks on CIS engines can be the fuel injector o-ring seals and plastic seats. Then I would test your O2 sensor, they can go bad. I have never heard of a bad frequency valve, but I believe I have seen someone on this board who had a bad Lambda control unit under the passenger's seat.

There are others on the board who are more expert than I am at diagnosing CIS issues, but I think this is a good place to start.
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Old 10-14-2012, 04:15 PM
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To verify a vacuum leak, after brief warm up, leave car on, pop the lid and remove oil filler cap, if idle does not drop, you have a vacuum leak somewhere.

If does drop, it may be your fuel filter, it may be clogged, when was the last time it was changed?
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Old 10-14-2012, 05:58 PM
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Nick,

Appreciate the advice. I will test it out.
Old 10-14-2012, 08:15 PM
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Draco,

I will give this a shot in the morning. Fuel filter changed last summer, along with WUR. Was running fine, but I just recently put some fuel injector cleaner in the last fill up.
Old 10-14-2012, 08:17 PM
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+1 on checking the O2 sensor by disconnecting it. I had a similar problem on my 81SC and it was a defective O2 sensor.
Old 10-15-2012, 06:38 AM
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O2 sensor is disconnected - actually has not been connected for a while. However, this issue just started up over the past couple of weeks.
Old 10-15-2012, 10:28 AM
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I had a similar issue with my 82 SC for a while.

The idle would start to fluctuate at random times after the motor got warmed up.

After replacing the rotted out hoses from the SSI's to the Flapper boxes back in January it has not occurred again.

Odd as it may sound but I think the hoses were leaking so bad it was causing heat to build up in the engine compartment throwing off a CIS temp. sensor or creating a hose to swell creating a temporary vacuum leak.
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Old 10-15-2012, 12:18 PM
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Wow...that is interesting. I will have to take a look at mine and see what kind of state they are in.
Old 10-17-2012, 09:00 AM
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I have adjusted my idle mixture screw (half turn out) and it is idling at about 1150rpm and it seems to have cleared up the idle hunting problem - of course this seems like a workaround to me that may not be really addressing the issue.
Old 10-17-2012, 09:02 AM
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so...here is a long shot...

you added some injector cleaner...it cleaned something, causing the control pressures to decrease (between WUR and top of Fuel Distributor). by decreasing the pressure in this area, the fuel distributor plunger will "rise more" thereby causing a fuel rich condition. a rich condition tends to cause the idle to hunt up and down. To fix this, you adjusted the air fuel mixture screw out, leaning the mixture into a zone where you have no more hunting.

remember, this is just a theory!
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Last edited by Kemo; 10-17-2012 at 09:36 AM.. Reason: rise more!!!
Old 10-17-2012, 09:35 AM
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Old 10-17-2012, 09:35 AM
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