I've got an 83 380SL. It failed smog on low and high idle for CO and HC. Plugs are less than 10k old but are coated with fluffy black carbon. Will change those. Back probed the O2 sensor with and without an induced air leak (by disconnecting the vacuum booster). It was not varying or hunting much from about 300mV so changed that. Now the sensor bounces arround between 200~750mV and stays down arround 220 with the above mentioned air leak. I assume the engine goes to closed loop after warm up.

Ignition was 3 degrees off of engine specified 0 (TDC) so I set the timing right. No big difference here.

The cat is about the same temp (+/- 10%) from end to end after a long warm up at idle. Doesn't seem plugged (car has plenty of power). I guess I could take a shot at smog again but it may be a good time to check how rich the car is running and also if the cat really needs replacing.

Engine has no vacuum leaks and the engine doesn't missfire but at idle the engine occasionaly revs up a bit. I took off the idle control valve and cleaned it with carb cleaner (not the electrical part of course). Looking under the sensor plate with a flex light I could see the throttle and it needed cleaning. With a 1/2 cans worth of carb cleaner and a clean rag I did my best to remove the carbon from the throttle plate and throat--delicately using a screw driver and very long nose pliers to pull and push the rag). Now the idle is stable.

I would like to at least check that the richness control is working correctly. Could someone tell me how to hook up my meter and what to expect? I don't want to mess with the mixture control screw if it's not needed. My multi meter measures duty cycle, freq and dwell. I don't have an analog meter. I searched this forum for the correct meter hookup after moving on from the MB shopCD. MB uses their tool and targets 40-60% on/off ratio--not to deviate by more that +/-10% at 2500RPM.
CW on the mixture screw is richer and
CCW is leaner.

I probbed the car's diagnostic (terminal 3) and got either nothing, 10 or 95 for the duty cycle with one probe on terminal 3 and the other one put to battery positive, ground or left unconnected. Also did the same three connections with the probes reversed. For whatever it's worth I measured 12V stable at terminal 3 with the meter set to DCV and car warmed up and idling.

Any help would be much appreciated.

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1972 450SL
1982 300D Turbo