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Old 04-22-2008, 08:54 PM
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Join Date: Mar 2008
Posts: 98
Smog Shop Won't Adjust 420SEL Mixture

I think I've got my '87 420SEL running pretty darned good, and that it should pass the state smog inspection with a little tweaking of the mixture adjustment.

The EGR passages are cleaned out and the EGR works well; it has a rebuilt fuel distributor, a new O2 sensor, compression is excellent, no intake vacuum leaks, etc., etc. It really should smog.

I set the mixture adjustment "by the seat of my pants", i.e., until the car ran the best. (I probably should have made the "duty-cycle" adjustment with an ohmmeter. I can still do that, if necessary.)

I thought the smog shop would make the final mixture adjustment, but they didn't seem to want to, and the car failed the test. Probably they don't feel comfortable about doing it because they know little or nothing about Mercedes and are afraid they will screw it up.

I live in sort of a hick town, and I don't know of any smog shops that would want to make this adjustment. (I could always call a whole bunch of them and ask.) Nor do I know of any independent M-B mechanics who do smog inspections or have an exhaust gas analyzer.

Any suggestions? Should I take it to a dealer? Maybe I could get a M-B mechanic to talk to the smog shop on the phone and explain how to do it? Or maybe give the smog shop a written printout from the service manual?

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Old 04-22-2008, 09:25 PM
E150GT's Avatar
I'm a chicken
Join Date: Sep 2006
Posts: 1,148
Perhaps thats a good thing. You could screw it up so bad you'll wish you never knew what mixture was!

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Last edited by E150GT; 04-22-2008 at 09:38 PM.
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