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Failed smog, check mixture setting on 83 SL
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 Last edited by erubin; 02-10-2006 at 07:01 AM. |
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Thank you for your responses. The systems you refer me to are for newer cars. Mine is an 83 380SL so I it may be different particularly with KOEO.
Digital meter hooked up as follows: positive lead to #3 term and neg lead to ground (available at term 4). edit:< Here is what i got (different than last night): KOEO: 0Vdc on terminal 3 and ground; 0% duty cycle Cold idle: batt voltage btwn term 3 and ground; 26% duty cycle 2 min warm up (below 80C): 13% duty cycle Warmed up and idling at 1200RPM: 26% duty cycle In my car it seems that with KOEO terminal 3 gives useless info (is this a problem?). BTW, i am using a digital meter. After the work i did yesterday cleaning the ICV and throttle plate the engine idles too high at 1200RPM and doesn't go back down. That's weird because yesterday after cleaning everything I got the idle perfect, something happened overnight. With the car running at this high idle and in closed loop I back probed the O2 sensor and got values between 300 and 600mV. I measured the cat temp: inlet 400C outlet 400C Smog test results (done before replacing the defective O2 sensor) showed 3.89% O2 at 15mph and 4.23% at 25mph.
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1972 450SL 1982 300D Turbo Last edited by erubin; 02-12-2006 at 04:37 AM. |
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Answer
Emission testing and troubleshooting Emission test failures
Emission testing and troubleshooting Emission test failures |
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By disconnecting and connecting the ICV a few times i got the engine to idle at about 750~800. The duty cycle was at 22%. I went ahead and started backing out the mixture screw (3mm allen) and the duty cycle climbed up to 36% (leaner). If i continue CCW on the screw to make it even leaner it doesn't budge from 36% but if i go far enough I stall the engine. When I raise the RPM to 2500 the duty cycle doesn't change more than 0.2%. Any suggestions please?
Update: I took the car out for a hard drive. Immediately after returning I measured the duty cycle as 31% at idle and 37% at 25000 RPM (no load). Based on what I have read this is not acceptable because the difference is greater than 10%. Should I turn the screw richer to get it within 10%. I don't think it will let me go any leaner. What is more important the absolute duty cycle at idle or respecting the +/-10% between idle and 2500?
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1972 450SL 1982 300D Turbo Last edited by erubin; 02-10-2006 at 11:33 PM. |
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what was your final repair to lower the idle?
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Thanks, Juan |
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