idle problem
 

I have an '85 911 and love my car. For a few years the car's idle will occasionally cycle from 600 to 1,100 rpm. The problem sometimes lasts a few days or for weeks. My new theory is that the alternator isn't up to handling the after-market stereo amplifier. So far the problem hasn't occurred when I don't use the stereo, is this possible? Other possible causes or suggestions to eliminate electrical draw as the problem would be MUCH appreciated! Thanks in advance.

take out the stereo.

you can put a large capacitor on the output of the alt or at the battery tp provide the added power when needed. your local stereo shop will gladly put one on for you.

have the charging sytem checked

Thanks! Will definitely check charging sys & install a capacitor. UPDATE: had some of the same idle problem (1st time in 2 wks) without the stereo on so my best guess may not address the problem... Thanks again and would appreciate any other thoughts on what the problem might be in case it's not the stereo or the charging system...?

My 81 SC would hunt at idle after warming up and it turned out to be a bad oxygen sensor. Try unplugging your 02 sensor when the problem occurs. If your car idles good after the 02 sensor is unplugged, then your sensor is faulty.
Good luck.

Search for Carrera idle hunt, idle surging, etc. Common issue with these cars.

surging at idle can be caused by vacuum leaks, O2 sensor, throttle position switch, maybe a head temp sensor, maybe a some other things, but thats just off the top of my head.
most people jump on the ICV, but its usually the DME telling the ICV to very the idle.

i would still check the charging system since you say it did it most of the time with the stereo on, it may be getting worse.

Thanks everyone! I've certainly got a list of things to check and you're given me a pretty good idea of what order to start the process. What a great response, thanks again!!

Is it slowly meandering between 600 to 1,100 rpm or quickly up and down like an out of breath dog. Does it happen often on a cold start?

Sometimes 800-1,100, sometimes from 600... Not usually cold starts, happens most often when the car's warm. (and YES to the out of breath dog!)

Easy fix. Reduce idle speed to spec, which should be ~800 rpm for an '85.

Sometimes idle speed tends to creep up due to minor vacuum leaks such as leaking intake manifold gaskets. Reducing the idle speed will compensate for that.

So why would too high of an idle speed cause it to cycle so fast? It's has to do with the "fuel shut down on overrun" based on the throttle position switch signal and rpms. If the DME senses the engine is on overrun, it shuts off the fuel until the rpms get below 1080 rpm then it switches them back on. But in your case, the rpms jump back up to 1080 rpm due to too much idle air causing the DME to shut off the fuel once again. This back and forth happens so fast that...well, you know the rest.

So the magic number is 1080 rpm. If you can keep your engine from hitting that rpm when the DME switches the injectors back on, your idle cycling problems will be a thing of the past.



Cheers,

Joe