1985 380SL, rough idle until warm
 

Desperate for help! My wife's car has 53k. It sat in the garage for 6 months. After the initial start, it was idling at 1,800 RPM. Worked a few weeks and the issue was the Idle Control Computer. Replaced it and now idles at 650-700.
When the engine is cold, it is rough idling with black smoke. I drained the fuel tank? It seems to go away after it is warms up. I can unplug the idle control valve & it will run at 1,500 rpm.
Don't know what to do at this point.

your ICV is either stuck or non operational , remove and test , there are some tricks to fix but start there

So disconnecting the power at the end of the ICV and getting a different RPM does not show it is working?

Ok, I will take it out and clean it with brake cleaner & retest with a 9 volt battery. Is this the correct procedure?

so lets call this correctly. YOurs is A kJET . so we are talking about the auxiliary air valve or AAV If it does not open the the cold idle will not rise and therefore the idle will be slow and rough. The valve should be open at first start , closing as the engine warms. A simple test remove from car , place it in a freezer, when you remove it the t
valve should be open . other issues could be present, if this checks out

leaking cold start valve.
binding air flow sensor
gummed up injectors
control pressure too high

Pretty sure the '85 M116 V8 has the electronic idle control, not the thermostatic valve that the I6's kept for a lot longer. There's an electronic solenoid behind the distributor that controls idle speed. If it is unplugged or if the idle control module behind the glovebox fails, the idle will be very high (1500-1800 RPM). Once the engine warms up, it pulls closed and varies to maintain idle speed ~650-700RPM.

It sounds like the idle speed is being governed correctly, a very cold start will idle ~1500 RPM, then drop to ~1000, then finally drop to ~700ish after a minute or two depending on temperatures.

Black smoke is overfuelling plain and simple. You have a fuel problem somewhere. Does the car run rich once it warms up too? You should NEVER see black smoke. Even my non-emissions (no cat) Euro-market 500SL never smokes, even when cold.

hmm you may be right , I assumed a Kjet with a AAV

It's definitely K-Jet, but as time went on they added more and more electronic gadgetry to them. The US models differ a bit from my Euro since they have O2 sensors and some sort of frequency valve to control fuelling, I'm not familiar with that part, but been there done that with the idle control issues.

ICV tested in freezer after I cleaned it. I now have a faster idle when the engine is cold. Still have some black smoke on take off & engine is not running smooth. Seems worse with the faster idle. I'll check each of the spark plugs tomrrow morning to see if they all look the same. Any other idea?

leaking cold start valve.
binding air flow sensor
gummed up injectors
control pressure too high
MeltedPanda, how do I test the above? Thanks again for your help

PM your email and I will send you information that may help

Does this car have an EHA? I know the 560SL's had them but I'm not sure about the '85 380. High idle speed can also be caused the the over voltage protection relay. If it's dead or the 10 amp fuse is blown you will have high idle speed. Most of the time you will also have the ABS light on in your dash. This year car has ABS so if that light is on that could be your problem.

No EHA. ABS was optional in '84. Not sure about '85.

Could be the WUR. Get a fuel pressure gauge and check th pressure during warm up.

What is the WUR?

WUR is the warmup regulator. It's the square looking thing with fuel lines going to it on the front of the engine next to the passenger side valve cover.