Irregular Idle and now failing emissions
 

I have a 1985 380SE Mercedes that has been a dream for me up until now. I have had a problem for about 1 month where my idle will sometimes be at 400-600 RPMs when I put the car in Neutral, and at other times it revs up to 2000-2300 RPMs when I put it in Neutral. I have found no pattern for this and have checked all lines, tubes, and conntections.
And to top that, it has just failed Virginia Emissions with the following readings:

PREVIOUS CURRENT
HC ppm 15mph 68 241 Failed
HC ppm 25mph 108 253 Failed
CO% 15mph 0.38 3.47 Failed
CO% 25mph 0.61 3.54 Failed
NOppm 15mph 28 303 Pass
NOppm 25mph 31 332 Pass
RPM 15mph 612 1398 Valid
RPM 25mph 434 1210 Valid
Dilution 15mph 14.8 16.5 Valid
Dilution 25mph 15.0 16.5 Valid

I have not been able to get anyone able to tell me what the problem is. I believe the O2 Sensor should be replaced, but I need to keep repairs to only those necessary (money tight due to medical bills) to pass emissions here in Virginia.

Does anyone have an idea so I can deal with this and not make unnecessary repairs at this time?

Fast Idle
 

Replace fuse on top of OVP as a first port of call, even if it looks ok.

Need clarification
 

I have spoken to my Mercedes Part person, and they have never heard of an OVP. They are questioning whether you actually meant the Overload Protection Relay, which does have a fuse on top??? Could you be more specific? And thanks, your the only person who has assisted me out here...

Yeah, that is the Overload Protection Relay, I've always heard it called the OVP too....oh well. If your 85 is like my 86, it is located right in front of your windshield (with the hood up obviously) towards the center and it has a read top with a clear cap and the fuse is under that cap. I have no idea (need other people to help me here), but could the idle problem be related to the idle control valve or idle computer?

Fast Idle
 

As MBTJC has clarified, the OVP (Over Voltage Protection relay) is the OPR. There are other causes, including the ones mentioned by MBTJC. There is a nice bunch of guys here at the forum eager to assist given a little time to come across your thread and their contribution, I am sure, will lead to the solution of your problem.

it could be host of different part/s malfunction. as suggested
the OVP is a good start, the idle control valve is also a
candidate and so is the temperature sensor. vacuum leaks
could also trigger erratic behaviour. the oxygen sensor
should definitely be checked or replaced.

the cheapest way is to visit a trusted mechanic and hopefully
he could identify the culprit. good luck, cheers!

If the O2 sensor has not been replaced, appears to be original or in for a very long time, that is a good candidate for replacement too. On the 126's it is a real piece of cake, maybe 1/2 hour job.

for the emmisions it looks like you're running too rich, try adjuating the co a little it might even help out the idle problem. since its rich turn the screw counter clock wise (just a little about 1/8 turns at a time) just a though and a couple of pennies