My '99 911 has a crazy idle...at rest, tach needle swings from 1 to 2,000 rpm. The local tire shop pulled the codes and came up with p0506...IAC malfunction. After doing some research, the culprit sounds like a faulty Idle Air Controller. That's a new one on me, but I think the tire shop hit the nail on the head. Does this seem likely to rennlisters?

relinuca

 

I believe '99 was the only year with an IAC. Sounds plausible.

 

I haven't dealt with the IAC on these cars, but on many other cars I have. Often times the IAC gets gummed up and its performance degrades. Just a thorough cleaning will often bring them back to life. Just a little elbow grease is free.

 

Definitely the IAC (or idle control valve). Remove throttle body and then take off the IAC. Be very careful of the thin paper gasket underneath. Clean the IAC with carb cleaner, then use Q-tips and alcohol to clean the inside. It will look black but it is supposed to be shiny sliver. You can operate (open and close) the valve with a 9V battery and some leads. It should move easily and smoothly.

 

If you have a really clean Throttle Body and a properly functioning engine/ air fuel ratio, (no vacuum leaks). you can disconnect the IAC wiring and the engine will idle at the base idle speed of about 500-600 rpm and stable.

The Idle speed goes up from base speed with the extra air of the IAC.

IF unplugging the IAC results in the engine stalling out, it is either not getting enough air around the Throttle Plate (dirty) or there is a vacuum leak causing very poor lean air/fuel ratio. (try adding some propane from a unlit propane torch to see if RPM increases dramatically)

Start by cleaning the TB and IAC very good, Also clean the MAF with CRC MAF cleaner.

My guess is you got a vacuum leak causing a lean condition and the engine is almost stalling then overreacts by energizing the IAC too much.