First of all, let me express my thanks for those who responded to my original query. This is a follow-up message to pass along what I found out, just in case it might be of benefit to other 240 owners out there.

I was having a problem with an erratic idle with my '83 B21FT. Either the engine would idle fast, or it would idle almost not at all (frequently stalling), sometimes oscillating between the two states. As it turned out, it was a combination of two problems.

My cars EFI system is the CIS type with an O2 sensor. A frequency valve is located on the air-fuel control unit and has a two-wire plug. I discovered that only one of the two wires was getting a good connection. I was the culprit here -- I had removed the control unit from the airbox and moved it over to get it out of the way to tighten the alternator belts, inadvertantly pulling loose one of the connections in the process. Correcting this problem allowed the frequency valve to behave normally again, and the poor idle problem halted. It revealed a new problem, though -- a very fast idle.

The engine was now idling at about 1800 rpm cold, sometimes it would drop to about 1300 rpm, but either way it was still too fast. This car's engine wiring harness is old, hard, and brittle. Some wires' insulation is gone altogether. Such was the case for the wire leading to the coolant temp sensor used by the CIS system. This sensor is located on the head beneath the intake manifold. I was not able to view the sensor directly, and could only view a portion of the sensor's wire, so I had to loosen and pull back the intake to get at any of it. The wire, as it turns out, was grounding itself against the block. I slipped a piece of shrink tubing over the wire and heated it up, but there remained an exposed area, right where the wire was being grounded out. So, I cut off about a 1" piece of the insulation material that surrounds the large hose running to the flame trap, split it, and slipped it around the wiring harness bundle AND the exposed sensor wire right where it exits the bundle, which also happens to be where it was touching the block.

I reattached the intake and fired it up. Bingo, problem solved. The car now idles smoothly and at the correct rpm. I suspect my fuel mileage will gain a couple of points as a result, as well.