AAV or AAR problem???

Some of the expertise from the "CIS update" thread might be applied here.

My 83SC starts nice, runs great, and idles correctly from cold until FULL temperature. Then (at full temp) it smoothly revs up to about 1400rpm and back down to about 1000rpm every couple of seconds with amazingly constant frequency. I have cleaned up the throttle housing and cannot see any problem with the vacuum tubes (can anyone tell me what size and how much of each size I need to replace them all) or air box. Anyway, the constant frequency implies that its either some malfunctioning control box causing this (I don't think there is one on this car) or there is a small hole in the vacuum system somewhere that allows vacuum to build up and then collapse. What do people think? Can anyone provide any tips on how to fault find this problem.

Thanks, Dean 83SC

Disconnect your O2 sensor. When the O2 system gets old on the K-Lambda system it causes exactly what you describe. Leaving it disconnected will not harm anything, and will make hot idle smoother and the car run cooler. You may want to have your mixture reset with an EGA after disconnecting it. Generally they will still pass emissions tests with the sensor disconnected.

Hope this helps.
Nick.



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Nick Shumaker
1982 911SC Coupe
nickshu@yahoo.com
PCA -- Rocky Mtn. Region

Thanks Nick,

I'll give it a go and see if it cures the hot idle. Am not really comfortable leaving it disconnected though without knowing more about the possible consequences. Will let you know what happens.

Cheers, Dean

There really are no consequences on this system. Disconnecting it just pegs the frequency valve bypass system at about 60% open, allowing the fuel distributor to do its job uninhibited by the O2 sensor system. The O2 sensor system was basically and "add on" to the US market cars. It functions to bypass the fuel pressure set by the fuel distrubutor and "lean out" the mixture when it detects what it thinks are excessive emissions. Disabling it just makes the system function like the 78-79 K-basic CIS, which are exactly the same system without the O2 sensor add on. You cannot do this on pulsed injection cars such as the post-84 911's, because on these systems the O2 sensor is critical in determining mixture.
Any mechanic who works with CIS systems with O2 sensors will verify the benefits of leaving it disconnected.

If you want to know more about the K-lambda CIS on your car, let me suggest you get Charles O. Probst's book called "Bosch Fuel Injection and Engine Management" published by Bentley. A great book with chapters on theory of operation as well as troubleshooting and repair. Got mine at my local B&N.

Good Luck.
Nick.

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_ _ __ _ _
Nick Shumaker
1982 911SC Coupe
nickshu@yahoo.com
PCA -- Rocky Mtn. Region

[This message has been edited by Nickshu (edited 05-23-2001).]

Nick, your suggested cure seems to have done the trick and it only took about 30seconds to make the fix (I have lived with it for 9 months) - thanks!!

It idles steadily a touch high at 1050rpm when at full temp. Warm idle is 800.

Do you know if this be tuned to hot idle and warm idle at the same 800 rpm?

Thanks again, Dean

Mine idles at about 1050 at cold (stone cold) startup in the winter and settles to about 950 once the AAR closes and the WUR settles in. It doesn't seem to be too reliable on a set idle speed during warmer "cold" temps. Sometimes even when cold it starts at 950 and stays. I guess I'm just happy it always idles stable, I don't care too much about a little variation from day to day.

I believe spec is 950 RPM's. My dash tach is slow by about 200 RPM's, so I set it with one which is on my digital timing light, far more accurate. So when I idle at 950 the dash tach shows about 750. Not sure if yours would be the same, but a real tach attached to a spark plug lead (like one on a digital timing light) is the best way to go.

Good Luck.
Nick.

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_ _ __ _ _
Nick Shumaker
1982 911SC Coupe
nickshu@yahoo.com
PCA -- Rocky Mtn. Region