I know that a "too rich" car can burn-up a catalytic converter because fuel is mixed-in with the exhaust and that this heats the catalyst thereby melting it. Suppose you had a car running very rich-- say at 4 or 5% CO. You can smell gas from the exhaust. How many miles of driving like that do you think it would take to burn-up the catalytic converter? 10 miles, 100 miles, 1000 miles, 10,000 miles?

I think the answer is that it can burn-up in a few hundred miles this rich. But another person disagrees and says it should take thousands. Anyone with any thoughts or experience with this care to comment? Thanks

If the mixture is over rich and there is no air injection, there is insufficient O2 in the exhaust to sustain a reaction in the converter, so I don't think any damage would be done. Since the early/mid eighties Merc engines have had TWCs (three way catalysts) and if there is an air pump it should only be active before the engine goes into closed loop.

Earlier engines (pre 02 sensor technology) with OC (oxdizing catalyst) had air injection at all times.

The type of catalyst is listed on the emission/tuneup decal in the engine compartment.

Duke

Here is a short story for you.
Friend of mine went on a servicecall. When he determined that the coilwire was bad ( and he did'nt have an extra one with him ), he played the good samaritan and exchanged his own ( good wire ), for the defective one.
He proceeded to drive home, noticing an increasing powerloss at the end of his 50 mile journey.
Next morning his engine would not start.
Total melt-down of the catalytic converter from the constant misfiring with the defective coilwire.
Remember, an O2 sensor will interpret a misfire ( plenty of oxygen ) as a lean fuelmixture, and just keep adding more fuel.

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2007 C 230 Sport.

This car recently had a new catalytic converter installed. Also, its fuel/air mixture is not yet optimized but is in a "sane" range for the car (Mercedes specifies .5 to 1.5 as I recall).

At 15 miles per hour we have the following readings:

HC 153 ppm
CO 1.08%
CO2 13.9%
O2 0%
No(x) 1885 ppm

At 25 miles per hour we have

HC 150 ppm
CO 1.16%
CO2 13.7%
O2 0%
No(x) 2407 ppm

What stands-out to me are two things: (1) the NOX is out-of-the-park high. (2) the O2 is at 0%.

This car had previously run for about 400 miles with a very rich mixture and a CO reading of about 5%. The question is, do we have a bad catalytic converter or is the lousy fuel/air mixture causing all of this problem (e.g. do we need to lean it out more to get the catalytic converter to reduce the NoX). Or, do we have some other problem. This has historically been a nice clean running car. The fuel system has been overhauled. It has a very slight manifold leak but historically this did not cause a lean miss at CO levels under .6. One other variable is if the fuel system work, comprised of a new fuel distributor and new injectors, is suspect. The car runs beautifully-- smooth idle, beautiful acceleration, quite and smooth as silk.

The catalytic converter was replaced 400 miles ago. At that time the car showed excellent emissions. After the emissions, it was richened to about 5% to troubleshoot a problem. So it ran for 400 miles at 5% CO and a very rich mixture.

In response to one question, the car has a 3-way cat. It seems pretty clear to me, after I look at this, that the catalytic converter is bad/defective. The higher CO level of about 1% should actually be decreasing NOX so I can't correlate CO with high NOX. If I were to reduce CO I would expect to increase NOX. So reducing CO doesn't seem to be the solution. The O2 of zero just means that it's running richer rather than leaner though 1% is within Mercedes specifications for the car (may not be stoichometric or however you spell it, but the manual states .5 to 1.5%). I will likely lean the car out more at some point but it's within spec. The car is running very well and the idle is smooth. The HC's are also high given the CO level, which further points to the catalytic converter.

Another post correlated the lack of oxygen with activation in the catalytic converter. I wonder if that necessarily applies here. You could smell the gas in the exchaust, it was definitely working its way through the catalytic converter. The catalytic converter was certainly hot. In the end, to me this adds-up to gas, heat, and a material (catalyst) ready to melt in response to all of this. Whether or not there is an actual chemical reaction around O2 would seem to be an interesting point to understand but I'm not sure if it relates to melting (???)= heat+gas+material

The other cause of high NOX is an engine too hot or one that has carbon deposits. I doubt the engine is running at too high a temperature or has a carbon-cylinder issue. Temperatures seem fine and if it were running so hot, I'd think I'd be having a lean miss and lower CO, none of which is the case. The car's cooling system was recently gone through with a fine tooth comb. I put in a new Behr radiator, new fan clutch, and the water pump is only 2 years old. I've tested the engine temperature with an infrared thermometer and it's right-on.

I'm open to all opinions. The mystery is going to be solved soon since I think I'm going to put another cat on the car.

Can you enlighten us on the year, model & engine of this car ?
When you say M-B spec's are 0.5 - 1.5 % CO, are these readings before or after the cat ?
Other than that, I'm inclined to agree with you on the cat problem.

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2007 C 230 Sport.

Sorry, I can't believe I forgot to mention that. It's a 1984 380 SL with 125,000 miles. I've owned the car for about 10 years. The 1% CO, which I know is high by state emissions standards, was measured after the cat at the back of the car (stuck the probe into the tailpipe). According to the Mercedes engine manual I have (I have the documentation set for the car), CO can be in the range of 0.5% to 1.5% at an oil temperature of about 80 degrees. That's where I have that number. The car, historically (over the past 10 years), ran at CO levels below .5%, usually .2, .3, or .4%. A new fuel distributor and injectors were installed along with the catalytic converter.

I agree with you, and will confirm and share the results, that the catalytic converter was either defective (one possibility) or was damaged by the fuel/air mixture. If it is the latter case (and we will never know), then this confirms that a fuel/air mixture that is off in the car (very high) can very rapidly destroy a catalytic converter. Rapidly means a few hundred miles, not months or years or 1000's of miles. Again, this is all guesswork so we may be wrong.