300 SL Smog Problem - Mercedes-Benz Forum

 
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post #1 of 6 (permalink) Old 07-26-2006, 05:04 PM Thread Starter
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300 SL Smog Problem

I have a 1992 300 SL that will not pass California smog. The HC readings are too high. Have done a complete tune up and checked the emission systems. The car runs well and shows no signs of missing or burning oil. Any help would be appreciated.
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post #2 of 6 (permalink) Old 07-26-2006, 07:07 PM
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HC stands for hydrocarbons, which is unburned fuel. All engines produce some HC's. The test listed three things; HC, NOx, and CO. Hopefully they gave you a printout. Tell me those three numbers and I’ll be able to give you my best guesstimate. Also, tell me "which" test you blew high HC levels on – the low speed dyno, or the higher speed.

Hopefully it was "just" high HC, and the car wasn’t flagged as a "gross polluter". A "gross polluter" is a technical term for those of us in the Peoples Republic of California. Once the "gross polluter" flag is turn on for a particular car, instead of having your retest in two years, you get the pleasure of getting another test next year at a "California test only center". I absolutely HATE "test only centers". Lets hope you’re not on that road.

Excessive HC can be the result of incomplete combustion. Incomplete as in poor spark. On a 15 year car, poor spark can be due to something as simple as old plug wires or bad plugs. Timing that's too far advanced can cause it too . . . . . <caution, I’m wandering off into left field> BTW, if you could retard the timing, you can’t because the California smog tester’s are supposed to check the timing, but if you could, you could make the HC levels drop dramatically - of course, that causes NOx to rise, but there’s usually some leeway. </caution ended>

Anyway, rather than rambling forever, tell me the numbers and I’ll make my best internet prognosis (guess).
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post #3 of 6 (permalink) Old 07-27-2006, 03:31 PM Thread Starter
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Numbers

@706 RPM co2 14.3,o2 0.4,HC 276,CO 0.32
@2562 RPM co2 14.0,o2 0.5,HC 173, CO 0.38
I have replaced plugs, wires, rotor and cap, O2 sensor.
I still fall under the non dyno smog test.
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post #4 of 6 (permalink) Old 07-27-2006, 10:40 PM
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My car would not pass emissions (Washington)... I took the printout to my service center... they tweeked the fuel/air mixture to lean it out a little... and I passed with flying colors... my HC's dropped significantly. My car was not running poorly either or missing or burning any oil. Just another idea for ya
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post #5 of 6 (permalink) Old 07-27-2006, 11:13 PM
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Your HC numbers are VERY high. The high HC could be something as simple as a stuck EGR valve, but there are no NOx numbers to confirm it. If the EGR valve were stuck open the engine would have a lousy idle. You said it runs well, so forget the EGR. The CO numbers are high too. Not high like you’re going to fail (I think the state allows up to .7 on the low speed test), but high like it should be a lot lower for a car with all the smog equipment working. I think your main catalytic converter is toast. Before you get a quote from a dealer and pass out from the price of a “bolt in factory replacement”, you may want to consider having a shop cut out your old main cat and replace it with a universal main. Don’t bother with the pre-cat. Other than the visual, it has no effect on whether you pass smog. If I had to replace my main cats, I’d go with a pair from Random Technology. I don’t work there, have never been paid by them, or received free cats as compensation for an article. I have written articles where I said Random Technology makes the best cat on the market, and I still believe that to be true.

Be aware – aftermarket cats don’t last as long as the OEM parts. Six years from now you’re going to need new cats again.
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post #6 of 6 (permalink) Old 07-29-2006, 08:51 PM
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Hi, I also did not pass smog. I was told all pass except HC on low speed (15mph). On high speed I passed. could you tell me what could be wrong. Thanks!
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