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1981 380SL
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256 Posts
Discussion Starter #1
It stays at 10% no matter what I do with the mixture screw. I've set mixture for maximum vacuum for now. I've got a new O2 sensor installed.

Am I looking at a Lambda control box? I didn't see a troubleshooting chart in the FSM for this. If anyone has a procedure, I'm all ears. Hate throwing parts at a problem.
 

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2013 C300 4Matic, 98 SL500 SP1 Pano, 2014 CLA-250 4Matic
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292 Posts
Hornet,

I was fighting the same issue for quite some time and ultimately discovered a huge vacuum leak in the Air Guide Housing below the Throttle Body. There were approximately 7 large cracks in the housing that was allowing so much air into the system that the brain kept trying to add more fuel. The Air Guide Housing can't be inspected visually without the removal of the Throttle Body and Fuel Distributor. The only way I found the cracks was with a smoke test. The replacement Air Guide Housing is not to bad at around $80.00 if memory serves. If it were me I would start with the smoke test, then tackle any leaks you find. Once you have the vacuum perfect I think you will find that you can properly adjust the duty cycle.
 

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1981 380SL
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256 Posts
Discussion Starter #3
Doing a smoke test is a fine idea. Plus, I've got a nice Nicaraguan-wrapper torpedo beckoning.

I do have a contra-indication in this case because I can feed the computer both lean and rich voltages (using a voltage divider and feeding about 0.3VDC and 0.7VDC into the O2 sensor wire) and the ON-OFF ratio doesn't respond. Sounds like in your case the Lambda system was properly responding to a lean signal.
 

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Registered
1981 380SL
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256 Posts
Discussion Starter #4
Smoke tested. No leaks.

Looks like I'll just make sure the wiring to the computer is OK with my meter. If it is, it's computer time.
 
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