O2 sensor, pollution

The Oxygen Sensor of my 230E 1992 has 90 kmiles, and the HC emission reading is 74 ppm while idlling. I consider it a bit high because where I live the
standard is 100 ppm, max value, and I also have a 1998 E240 with 28 kmiles, which is reading only 4 ppm on HCs. Both cars have very low CO readings though. The 230E has a slight unstable idle too, warm or hot. Besides, we here in my city have a very severe pollution condition right now, and the litte voice in my mind is telling me to do anything to help. Under this circumstances, is it worth to change the sensor? Do I need more information to know wether it is working properly or not? Thanks MMansilla
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Maybe. But I'm wondering if you have a leaky intake manifold. This can cause a rough idle and excessive hydrocarbon in the exhaust. You can try to spray some carb cleaner around the intake manifold while the engine is running - you have a leak if the engine idle changes.
Josh
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I agree that the HC problem may arise from a vacuum leak that causes the motor to misfire at idle. A cracked vacuum hose or a leaking fuel injector "O" ring or loose intake manifold gasket could be the culprit. Listen for a hissing sound near the intake manifold. If nothing found one can VERY CAREFULLY apply some propane to suspected areas - the leak is discovered when the motor's idle improves.
Alternatively, one could test the HC at engine speeds above idle to see if the HC falls - a vacuum leak is significant at idle but tiny at say 2,000 rpm, so the HC % should be about normal then.
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I forgot to mention, I have long suspected a vacuum leak and tried that carb cleaner test suggested by Josh, with no change in idle speed. I'm not sure anyway, the hoses in the intake manifold looks hardened and brittle, maybe a small fissure is leaking there. I will try with propane, it is clear from your posts that I have to discard a leak before replacing the sensor. Do I have to use those cans of propane for camping lamps?
With respect to the hidrocarbon readings at higher speeds, at 2500 rpm the 230E reads 40 ppm, almost half of its value at idle, however far from the low reading of the newer E240.
Thanks MMansilla

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Propane from a camp stove is exactly what I had in mind but, again, realize that can explode so keep your face away and don't be overly generous, apply only to suspect sites.
Given your additional data, IMHO, the O2 sensor is not the problem.
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A appreciate your help and concern. Thank you. I got my can of propane today. I'm now thinking how to get the gas out in the safest way. We'll see.

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I did the propane test. I sprayed gas at air hoses, intake gasket, fuel dist gasket, injectors seals, everywhere. Nothing. To verify the method, I applied a bit of propane on the air throttle plate, and the engine speeds up instantly and notoriously. Anyway, I have decided to replace all the (cheap) rubber hoses of the intake. If this doesnt help with the emissions/idle it can be considered as preventive maintenance, they may be good but look ugly.
If there is no change, I will try to chase the elusive leak with a multimeter connected to the sensor wires, as described in this link:
http://www.mercedesshop.com/Wikka/M103VacuumLeak
If I can't find one, I will try to test the O2 sensor at the same time, inducing a vacuum leak.
Thanks for all your help. MMansilla
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Interesting that nothing was found. Before undertaking hose replacement which can be costly I suggest an alternative.
The ignition wires are 14 years old, have you checked their ohm resistance?
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Also, since you first asked about the O2 sensor, might as well replace it. These are relatively cheap to replace *and* on a car of this age (assuming that it is original) can be prone to give an incorrect reading.
Josh
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Hey, you both were right. There was a leak. I found it while replacing the hoses (they were already replaced when I saw your post TG). It was at the idle hoses connected to the intake, but it was at the bottom of a hose's edge, so the propane couldn't reach it as I applied it from above (I must have considered that the hoses receive much more heat underneath) When I removed them I saw a clear gap, 3 mm wide. As soon as I can will review the emissions, the idle improved a lot, but it has yet this tiny oscillation. So yes, I'm still thinking about replacing the O2 sensor, it is original, not so expensive, easy DIY.
Thanks MMansilla
PS: Spark plugs, wires, dist cap, rotor, engine and transmission mounts, injectors with seals, all new.

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