Lately, in the teen temperatures up here in PA, my 84 500sel has been stalling after idling very low for a minute or so. The idle is extremely low (like around 500rpm) at startup after its been sitting for more than a couple hours and the engine sounds really weak at first. Then, it almost seems like the gas stops feeding the engine and the engine dies. If I try to restart it right after it dies, it'll turn over but will die immediately if I don't hold down the gas pedal. Once I let off, it'll die again. That is, unless I keep feeding it gas until the engine is warm. Once the engine is above 60C, it idles and runs fine.

What could the problem be? I just changed out the idle control unit and it didn't seem to really help. Could it be the idle control valve, or maybe the coolant temp sensor?

I would start with new idle valve hoses. They are made for air by-pass like the 420's.

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As an update, I still haven't been able to solve this problem, and it seems to be getting worse the colder it gets. I'm about to take it to the dealership to get their diagnosis, but can anyone confirm or disagree with my fuel filter or coolant temp sensor theory first?

Why would a fuel filter problem be temperature related? I'd rule that one out. It sounds like problems in the cold start loop of your idle circuit. I don't know your model, but I think most of those generation had an auxiliary air valve to maintain idle until the car was warmed up.

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Is the auxilary air valve you're speaking of another name for the idle control valve, or is it something else?

The Auxiliary Air Valve (AAV) is a device to aid the engine when cold by opening a small port to increase the engine's idle speed. The fast idle control is achieved by the port being held open by a bi-metalic strip that when heated by it's own heater element, or via natural heat soak from the engine, the port closes. The voltage supply to the air valve is the same as the feed to the fuel pump and the warm-up-regulator. If it is found that the idle speed will not reduce and that the speed is maintained artificially high when warm, clamp the rubber pipe between the air valve and the inlet manifold. If this action causes the engine rev's to return to normal, the fault is within a sticking auxiliary air valve.
It is worth cleaning the valve, lubricating it and re-test it's operation. The internal heater element can also be checked for continuity using a multimeter.

my car is w123 230E 100kW 1984 and the AAV is mounted at the back of the engine. If you need pictures let me know.

But since my car has the same symptoms I'm guessing the WUR (warm-up regulator) is the culprit to your problems. It's at front of the engine near distributor. That is what controls cold starting and regulates the control pressure according to engine temperature. Check electrical connection on the WUR to make sure it gets current to the bi-metallic strip.

I'm not too knowledgeable on the locations of the items you're speaking of in my engine, so would you be able to show me some pics of the locations of the items you're speaking of and what they look like? Btw, thanks for your comprehensive response!

Moisture in the fuel line freezing?

How would I go about addressing this issue if it is the culprit?

here you go buddy
http://i46.tinypic.com/i3xool.jpg

on that pic as you can see the electrical connection to the AAV is heavily corroded. I cleaned it but haven't checked it with multimeter yet. But after assembling I still have the same issues you have. I have yet to check the WUR (I suggest you do the same). If you disassemble the inlet fuel line to WUR (at WUR side) you should see a tiny filter in WUR which also may cause problems when full of dirt. But you should also check the cold-start injector electrical connection. That injector is also a common culprit for difficult cold starting problems.

Check the local auto part outlets for something like DryGas. There are many products on the market that promote these additives.

OVP i've fixed this problem many times. The idle air bypass is powered by he OVP. A 201 540 08 45

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I've replaced the ovp recently and it's been tested to work correctly, so I don't think that's it. However, the previous owner may have adjusted the eha valve (CO-adjustment) to compensate for the inoperational idle control unit which I recently replaced, so how would I go about checking that the CO-adjustment screw is not incorrectly set? And could an incorrectly set eha valve even affect the problem I've described?

Based on what's been discussed, I think my cars exhibiting signs of an AAV problem. It idles low when cold and high when hot. However, I can't seem to find it in my 5 liter V8 engine. Does anybody know exactly where it is, or have pics of its location in my m117 engine?

hey buddy, your question regarding the AAV location has been answered here:
http://www.benzworld.org/forums/w126-s-se-sec-sel-sd/1499426-where-aav-m117-engine.html