I found the source of my 92 SLC's rough/searching idle. On a cold start it would chug-chug-chug and then up to 2000 RPM. It would continue to swing between 1500 and 400 RPM until it was warmed up a bit. However, even when it was warm it would still idle roughly and usually around 850 RPM. I checked the engine fault codes - there were about 7 but none conclusive.
I decided to clean my idle-air valve to make sure it wasn't sticking - no luck it moved freely. I then decided while I was under the hood to clean any oil build-up in the intake zip-tube between the PCV and the throttle-body. As I was fiddling with this tube I found the problem: A BROKEN VACUUM LINE. There is a hose (1/4" ID thick wall - like high pressure hose line) that goes from the passenger side of the intake manifold where the throttle-body attaches to a vacuum sensor attached to the front of the strut tower under the intake tube. This route also passes generally over the exhaust manifold. After 105,000 miles of heat cycles, the original hose had broken in half at its sharpest bend. I replaced it with a new hose and the car idles smoothly again!!!!
Add this to the list of things to check when your VR6 idles rough.
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