84 911 "floating idle"
 

I have an 84 3.2 - my problem is a "floating idle" - on start up the idles floats from 100 rpms to 1200. Use to do it for a minute or so, now almost continuous. At some points, it shuts down completely and I have to re-start. Seems to not matter how long the car has been running. Just replaced the total ignition key module, at least now I can re-start it.
Any suggestions on where to start? Car has just over 60k on it.

look for air leaks in the intake system -- unmetered air will cause the motronic system to adjust and readjust causing this issue -- there are some good posts you can search the forum for on using smoke, starter fluid, etc to check for air leaks. Also search the forum for posts on how to clean/check the Idle Control Valve (ICV), which helps stabilize idle on a properly-running engine.

The first check is to make sure when the throttle is all the way closed the micro-switch on the throttle body is closing. This signals the computer that it needs to stabilize idle.

In your case if the fluctuations are that large it could be a vacuum leak somewhere on the intake. Did this set in gradually and got worse over time (points to a leaky deteriorating hose) or from one day to the next. (hose snapped off, switch doesn't close, defect in DME or ICV)?

Mileage (only 60k) isn't really the driving factor here. It is the age of components such as the rubber hoses that harden with time and develop cracks. They don't like the modern fuels with ethanol either and that sometimes compounds the issues.

Cheers,
Ingo

Or too rich. Which is it? Trust your nose.

I'd clean the Idle Air Controller and make sure the elbow that connects it is in good condition. Then inspect all the vacuum lines as well as intake gaskets and the rubber boots that connect the two intake manifolds. Honestly though there are a lot of things it could be but that would be a good place to start.

I had the same issue. It good as bad as rpm swing would cause the engine would stall at idle. I tried to cleaning the icv, adjusting idle. Finally decided to replace the icv alltogether. Wish I would have done this sooner as the idle problems are history. It was a pricey part but well worth replacing.

Make sure your ICV was installed right side up. Mine came arrow up (backwards) when I got it and caused all kinds of idle problems for years until I noticed the arrow.

All the above and check base idle CO before cat. Bentley manual tells how and lots of threads here.